Maxim MAX1993 Quick-pwm step-down controller Datasheet

19-2661; Rev 1; 9/05
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
♦
♦
♦
♦
♦
♦
♦
♦
♦
Inductor Saturation Protection
Accurate Current Limit
Ultra-High Efficiency
Quick-PWM with 100ns Load-Step Response
MAX1992
1.8V/2.5V Fixed or 0.7V to 5.5V Adjustable
Output Range
MAX1993
External Reference Input
Dynamically Selectable Output Voltage
(0.7V to 5.5V)
Optional Power-Good and Fault Blanking
During Transitions
±1% VOUT Accuracy Over Line and Load
2V to 28V Battery Input Range (VIN)
200/300/450/600kHz Switching Frequency
Overvoltage/Undervoltage Protection Option
1.7ms Digital Soft-Start
Drives Large Synchronous Rectifier FETs
2V ±0.7% Reference Output
Power-Good Window Comparator
Ordering Information
TEMP RANGE
PIN-PACKAGE
MAX1992ETG
PART
-40°C to +85°C
24 Thin QFN 4mm × 4mm
MAX1992ETG+
-40°C to +85°C
24 Thin QFN 4mm × 4mm
MAX1993ETG
-40°C to +85°C
24 Thin QFN 4mm × 4mm
MAX1993ETG+
-40°C to +85°C
24 Thin QFN 4mm × 4mm
+ Denotes lead-free package.
Pin Configurations
Active Termination Buses (MAX1993)
CPU/Chipset/GPU with Dynamic Voltage Cores
(MAX1993)
BST
LX
DH
V+
SKIP
15
14
13
12
CSN
PGND
20
11
CSP
AGND
21
VCC
22
MAX1992
10
OUT
9
FB
SHDN
23
8
N.C.
OVP/UVP
24
7
N.C
1
2
3
4
5
6
REF
DDR Memory Termination (MAX1993)
16
ILIM
1.8V and 2.5V Supplies
17
19
N.C.
Core/IO Supplies as Low as 0.7V
18
VDD
TON
Notebook Computers
DL
TOP VIEW
Applications
PGOOD
Single-stage buck conversion allows the MAX1992/
MAX1993 to directly step down high-voltage batteries for
the highest possible efficiency. Alternatively, two-stage
conversion (stepping down from another system supply
rail instead of the battery) at the maximum switching frequency allows the minimum possible physical size.
The MAX1992 powers the CPU core, chipset, DRAM, or
other supply rails as low as 0.7V. The MAX1993 powers
chipsets and graphics processor cores, which require
dynamically adjustable output voltages. The MAX1993
provides a tracking input that can be used for active termination buses. The MAX1992/MAX1993 are available in
a 24-pin thin QFN package with optional overvoltage and
undervoltage protection.
For dual step-down PWM controllers with inductor saturation protection, external reference input voltage, and
dynamically selectable output voltages, refer to the
MAX1540/MAX1541 data sheet.
♦
♦
♦
♦
♦
LSAT
The MAX1992/MAX1993 pulse-width modulation (PWM)
controllers provide high-efficiency, excellent transient
response, and high DC output accuracy. The devices
step down high-voltage batteries to generate lowvoltage CPU core or chipset/RAM supplies in notebook
computers.
Maxim’s proprietary Quick-PWM™ quick-response, constant on-time PWM control scheme handles wide
input/output voltage ratios with ease and provides 100ns
“instant-on” response to load transients, while maintaining
a relatively constant switching frequency. Efficiency is
enhanced by the ability to drive very large synchronousrectifier MOSFETs. Current sensing to ensure reliable
overload and inductor saturation protection is available
using an external current-sense resistor in series with the
output. Alternatively, the controller can sense the current
across the synchronous rectifier alone or use lossless
inductor sensing for lowest power dissipation.
Features
THIN QFN
4mm x 4mm
A "+" SIGN WILL REPLACE THE FIRST PIN INDICATOR ON LEAD-FREE PACKAGES.
Quick-PWM is a trademark of Maxim Integrated Products, Inc.
Pin Configurations continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX1992/MAX1993
General Description
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
ABSOLUTE MAXIMUM RATINGS (Note 1)
V+ to AGND............................................................-0.3V to +30V
VCC to AGND............................................................-0.3V to +6V
VDD to PGND............................................................-0.3V to +6V
PGOOD, ILIM, SKIP, SHDN to AGND ......................-0.3V to +6V
REFIN, FB, CSP to AGND.........................................-0.3V to +6V
GATE, OD to GND (MAX1993 only) .........................-0.3V to +6V
TON, OVP/UVP, LSAT to AGND .................-0.3V to (VCC + 0.3V)
REF, OUT to AGND ....................................-0.3V to (VCC + 0.3V)
FBLANK to GND (MAX1993 only) ..............-0.3V to (VCC + 0.3V)
DL to PGND................................................-0.3V to (VDD + 0.3V)
CSN to AGND............................................................-2V to +30V
DH to LX .....................................................-0.3V to (BST + 0.3V)
LX to AGND ...............................................................-2V to +30V
BST to LX..................................................................-0.3V to +6V
AGND to PGND (MAX1992 only) ..........................-0.3V to +0.3V
REF Short Circuit to AGND.........................................Continuous
Continuous Power Dissipation (TA = +70°C)
24-Pin 4mm x 4mm Thin QFN
(derated 20.8mW/°C above +70°C)...........................1667mW
Operating Temperature Range
MAX199_ETG ..................................................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Note 1: For the MAX1993, AGND and PGND refer to a single pin designated GND.
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V+ = 15V, VCC = VDD = SHDN = 5V, SKIP = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
PWM CONTROLLER
Input Voltage Range
Output Voltage Accuracy
(MAX1992 Fixed)
Feedback Voltage Accuracy
(MAX1992 Adjustable)
Feedback Voltage Accuracy
(MAX1993)
VIN
VBIAS
VCC, VDD
VOUT
MAX1992
V+ = 4.5V to 28V,
SKIP = VCC
(Note 2)
FB = VCC
1.782
1.8
1.818
0.693
0.7
0.707
REFIN = 0.35 × REF
0.693
0.7
0.707
REFIN = REF
1.980
2
2.020
VFB
MAX1993
V+ = 4.5V to 28V,
SKIP = VCC
(Note 2)
0.1
VCC = 4.5V to 5.5V, V+ = 4.5V to 28V
IFB
MAX1992
%
0.25
%
-0.1
+0.1
µA
0.7
5.5
V
FB = GND
90
190
FB = VCC or adjustable
70
145
270
400
800
1400
10
25
Ω
0.3
0.4
V
RDISCHARGE
0.2
tSS
V
V
ILOAD = 0 to 3A, SKIP = VCC
ROUT
V
V
OUT Synchronous Rectifier
Discharge Mode Turn-On Level
2
5.5
2.525
MAX1993
Soft-Start Ramp Time
4.5
2.5
Output Adjust Range
OUT Discharge Mode
On-Resistance
28
2.475
MAX1992 V+ = 4.5V to 28V, SKIP = VCC
(Note 2)
Line Regulation Error
OUT Input Resistance
2
FB = GND
VFB
Load Regulation Error
FB Input Bias Current
Battery voltage, V+
Rising edge on SHDN to full current limit
1.7
_______________________________________________________________________________________
350
kΩ
ms
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
MAX1992/MAX1993
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, VCC = VDD = SHDN = 5V, SKIP = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
TON = GND (600kHz)
On-Time
tON
Minimum Off-Time
tOFF(MIN)
Quiescent Supply Current (VCC)
ICC
Quiescent Supply Current (VDD)
IDD
Quiescent Supply Current (V+)
IV+
V+ = 15V,
VOUT = 1.5V
(Note 3)
MIN
TYP
MAX
170
194
219
TON = REF (450kHz)
213
243
273
TON = open (300kHz)
316
352
389
TON = VCC (200kHz)
461
516
571
(Note 3)
400
500
FB forced above the regulation point,
LSAT = GND
0.55
0.85
UNITS
ns
ns
mA
FB forced above the regulation point,
VLSAT > 0.5V
1
<1
5
µA
25
40
µA
SHDN = GND
<1
7
µA
Shutdown Supply Current (VDD)
SHDN = GND
<1
5
µA
Shutdown Supply Current (V+)
SHDN = GND, V+ = 28V,
VCC = VDD = 0 or 5V
<1
5
µA
Shutdown Supply Current (VCC)
FB forced above the regulation point
REFERENCE
Reference Voltage
VREF
Reference Load Regulation
REF Lockout Voltage
ΔVREF
VCC = 4.5V to 5.5V,
IREF = 0
TA = +25°C to +85°C
1.986
2
2.014
TA = 0°C to +85°C
1.983
2
2.017
IREF = -10µA to 50µA
REFIN Voltage Range
REFIN Input Bias Current
-0.01
VREF(UVLO) Rising edge, hysteresis = 350mV
+0.01
1.95
0.7
IREFIN
V
V
V
VREF
V
0.01
0.05
µA
16
20
%
FAULT DETECTION
With respect to error comparator threshold,
OVP/UVP = VCC
Overvoltage Trip Threshold
Overvoltage Fault Propagation
Delay
tOVP
Output Undervoltage Protection
Trip Threshold
Output Undervoltage Protection
Blanking Time
Output Undervoltage Fault
Propagation Delay
tBLANK
12
FB forced 2% above trip threshold
10
With respect to error comparator threshold,
OVP/UVP = VCC
65
From rising edge of SHDN
10
tUVP
70
µs
75
%
35
ms
10
µs
PGOOD Lower Trip Threshold
With respect to error comparator threshold,
hysteresis = 1%
-13
-10
-7
%
PGOOD Upper Trip Threshold
With respect to error comparator threshold,
hysteresis = 1%
+7
+10
+13
%
PGOOD Propagation Delay
tPGOOD
FB forced 2% beyond PGOOD trip
threshold
10
µs
_______________________________________________________________________________________
3
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, VCC = VDD = SHDN = 5V, SKIP = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
PGOOD Output Low Voltage
IPGOOD
Fault Blanking Time
tFBLANK
VCC Undervoltage Lockout
Threshold
MIN
TYP
ISINK = 4mA
PGOOD Leakage Current
Thermal Shutdown Threshold
CONDITIONS
TSHDN
FB = REF (PGOOD high impedance),
PGOOD forced to 5.5V
MAX
UNITS
0.3
V
1
µA
FBLANK = VCC
120
218
320
FBLANK = open
80
140
205
FBLANK = REF
35
63
95
Hysteresis = 15°C
160
Rising edge, PWM disabled below this level
VUVLO(VCC)
hysteresis = 20mV
4.1
4.25
µs
°C
4.4
V
V
CURRENT LIMIT
ILIM Adjustment Range
Current-Limit Input Range
0.25
2.00
CSP
0
2.7
CSN
-0.3
+28.0
-0.5
+0.5
µA
55
mV
CSP/CSN Input Current
Valley Current-Limit Threshold
(Fixed)
VLIM(VAL)
VCSP - VCSN, ILIM = VCC
Valley Current-Limit Threshold
(Adjustable)
VLIM(VAL)
VCSP - VCSN
Current-Limit Threshold
(Negative)
VNEG
Current-Limit Threshold
(Zero Crossing)
VZX
ILIM Saturation Fault Sink Current
IILIM(LSAT)
50
VILIM = 250mV
15
25
35
VILIM = 2.00V
170
200
230
-75
-60
-45
VCSP - VCSN, SKIP = ILIM = VCC,
TA = +25°C
With respect to valley current-limit
threshold, VCSP - VCSN, SKIP = GND,
ILIM = VCC
With respect to
valley current-limit
threshold,
ILIM = VCC
Inductor Saturation Current-Limit
Threshold
45
2.5
mV
mV
mV
LSAT = VCC
180
200
220
LSAT = open
157
175
193
LSAT = REF
135
150
165
4
6
8
µA
0.1
µA
Ω
VCSP - VCSN > inductor saturation current
limit, 0.25V < VILIM < 2.0V
VCSP - VCSN < inductor saturation current
limit
ILIM Leakage Current
V
%
GATE DRIVERS
DH Gate Driver On-Resistance
RDH
DL Gate Driver On-Resistance
RDL
DH Gate Driver Source/Sink
Current
IDH
DL Gate Driver Source Current
4
BST - LX forced to 5V
1.5
5
DL, high state
1.5
5
DL, low state
0.6
3
DH forced to 2.5V, BST - LX forced to 5V
IDL(SOURCE) DL forced to 2.5V
Ω
1
A
1
A
_______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
MAX1992/MAX1993
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, VCC = VDD = SHDN = 5V, SKIP = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
DL Gate Driver Sink Current
Dead Time
SYMBOL
IDL(SINK)
tDEAD
CONDITIONS
MIN
TYP
DL forced to 2.5V
3
DL rising
35
DH rising
26
MAX
UNITS
A
ns
INPUTS AND OUTPUTS
GATE = VCC
10
25
Ω
OD Leakage Current
GATE = GND, OD forced to 5.5V
1
200
nA
Logic Input Threshold
SHDN, SKIP, GATE
rising edge, hysteresis = 225mV
1.7
2.20
V
Logic Input Current
SHDN, SKIP, GATE
+1
µA
Dual Mode™ Threshold Voltage
MAX1992 FB
OD On-Resistance
ROD
TON, OVP/UVP,
LSAT, FBLANK
Four-Level Input Logic Levels
1.20
-1
High
1.9
2.0
2.1
Low
0.05
0.1
0.15
High
VCC 0.4V
Open
3.15
3.85
REF
1.65
2.35
Low
TON, OVP/UVP, LSAT,
FBLANK forced to GND or VCC
Four-Level Logic Input Current
V
V
0.5
-3
+3
µA
MAX
UNITS
ELECTRICAL CHARACTERISTICS
(V+ = 15V, VCC = VDD = SHDN = 5V, SKIP = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 4)
PARAMETER
SYMBOL
CONDITIONS
MIN
PWM CONTROLLER
Input Voltage Range
Output Voltage Accuracy
(MAX1992 Fixed)
Feedback Voltage Accuracy
(MAX1992 Adjustable)
Feedback Voltage Accuracy
(MAX1993)
VIN
Battery voltage, V+
VBIAS
VCC, VDD
VOUT
MAX1992,
V+ = 4.5V to 28V,
SKIP = VCC
(Note 2)
2
28
4.5
5.5
FB = GND
2.462
2.538
FB = VCC
1.773
1.827
0.689
0.711
REFIN = 0.35 × REF
0.689
0.711
REFIN = REF
1.970
2.030
170
219
V
VFB
MAX1992, V+ = 4.5V to 28V, SKIP = VCC
(Note 2)
VFB
MAX1993,
V+ = 4.5V to 28V,
SKIP = VCC
(Note 2)
On-Time
tON
V
V
TON = GND (600kHz)
V+ = 15V,
VOUT = 1.5V
(Note 3)
V
TON = REF (450kHz)
213
273
TON = open (300kHz)
316
389
TON = VCC (200kHz)
461
571
ns
Dual Mode is a trademark of Maxim Integrated Products, Inc.
_______________________________________________________________________________________
5
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, VCC = VDD = SHDN = 5V, SKIP = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 4)
PARAMETER
Minimum Off-Time
Quiescent Supply Current (VCC)
SYMBOL
tOFF(MIN)
ICC
Quiescent Supply Current (VDD)
IDD
Quiescent Supply Current (V+)
IV+
MAX
UNITS
(Note 3)
CONDITIONS
MIN
500
ns
FB forced above the regulation point,
LSAT = GND
0.85
mA
FB forced above the regulation point,
VLSAT > 0.5V
1.0
FB forced above the regulation point
5
µA
40
µA
Shutdown Supply Current (VCC)
SHDN = GND
7
µA
Shutdown Supply Current (VDD)
SHDN = GND
5
µA
Shutdown Supply Current (V+)
SHDN = GND, V+ = 28V,
VCC = VDD = 0 or 5V
5
µA
1.980
2.020
V
0.7
VREF
V
REFERENCE
Reference Voltage
VREF
VCC = 4.5V to 5.5V, IREF = 0
REFIN Voltage Range
FAULT DETECTION
Overvoltage Trip Threshold
With respect to error comparator threshold,
OVP/UVP = VCC
10
20
%
Output Undervoltage Protection
Trip Threshold
With respect to error comparator threshold,
OVP/UVP = VCC
65
75
%
PGOOD Lower Trip Threshold
With respect to error comparator threshold,
hysteresis = 1%
-14
-6
%
PGOOD Upper Trip Threshold
With respect to error comparator threshold,
hysteresis = 1%
+6
+14
%
Rising edge, PWM disabled below this level
hysteresis = 20mV
4.1
4.4
V
VCC Undervoltage Lockout
Threshold
VUVLO(VCC)
CURRENT LIMIT
Current-Limit Input Range
CSP
0
2.7
CSN
-0.3
+28.0
V
Valley Current-Limit Threshold
(Fixed)
VLIM(VAL)
VCSP - VCSN, ILIM = VCC
35
65
mV
Valley Current-Limit Threshold
(Adjustable)
VLIM(VAL)
VCSP - VCSN, VILIM = 2.00V
160
240
mV
6
_______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
(V+ = 15V, VCC = VDD = SHDN = 5V, SKIP = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 4)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
1.20
2.20
V
INPUTS AND OUTPUTS
Logic Input Threshold
SHDN, SKIP, GATE
rising edge, hysteresis = 225mV
Dual Mode Threshold Voltage
MAX1992 FB
TON, OVP/UVP,
LSAT, FBLANK
Four-Level Input Logic Levels
High
1.9
2.1
Low
0.05
0.15
High
VCC 0.4V
Open
3.15
3.85
REF
1.65
2.35
Low
V
V
0.5
Note 2: When the inductor is in continuous conduction, the output voltage has a DC regulation level higher than the error comparator threshold by 50% of the output ripple. In discontinuous conduction (SKIP = GND, light load), the output voltage has a
DC regulation level higher than the trip level by approximately 1.5% due to slope compensation.
Note 3: On-time and off-time specifications are measured from 50% point to 50% point at the DH pin with LX = GND, VBST = 5V,
and a 250pF capacitor connected from DH to LX. Actual in-circuit times can differ due to MOSFET switching speeds.
Note 4: Specifications to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
(MAX1992 Circuit of Figure 1, MAX1993 Circuit of Figure 9, VIN = 12V, VDD = VCC = 5V, SKIP = VCC, TON = open, TA = +25°C,
unless otherwise noted.)
2.5V OUTPUT VOLTAGE
vs. LOAD CURRENT
VIN = 7V
VIN = 12V
70
VIN = 20V
SKIP = GND
SKIP = VCC
MAX1992 toc02
2.52
VIN = 20V
2.51
2.50
VIN = 7V
1
LOAD CURRENT (A)
80
VIN = 7V
70
VIN = 12V
VIN = 20V
60
2.47
0.1
90
2.48
50
0.01
SKIP = GND
SKIP = VCC
2.53
2.49
60
100
EFFICIENCY (%)
80
SKIP = GND
SKIP = VCC
2.54
OUTPUT VOLTAGE (V)
90
EFFICIENCY (%)
2.55
MAX1992 toc01
100
EFFICIENCY vs. LOAD CURRENT
(VOUT = 1.8V)
MAX1992 toc03
EFFICIENCY vs. LOAD CURRENT
(VOUT = 2.5V)
10
50
0
1
2
3
LOAD CURRENT (A)
4
5
0.01
0.1
1
10
LOAD CURRENT (A)
_______________________________________________________________________________________
7
MAX1992/MAX1993
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics (continued)
(MAX1992 Circuit of Figure 1, MAX1993 Circuit of Figure 9, VIN = 12V, VDD = VCC = 5V, SKIP = VCC, TON = open, TA = +25°C,
unless otherwise noted.)
1.8V OUTPUT VOLTAGE
vs. LOAD CURRENT
VIN = 20V
1.81
VIN = 7V
150
100
1
2
3
MAX1992 toc06
MAX1992 toc05
200
320
4A LOAD
280
NO LOAD
240
0
5
4
200
0
1
2
3
5
4
0
4
8
12
16
20
24
LOAD CURRENT (A)
LOAD CURRENT (A)
INPUT VOLTAGE (V)
SWITCHING FREQUENCY
vs. TEMPERATURE
MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
MAXIMUM OUTPUT CURRENT
vs. TEMPERATURE
280
5.1
MAXIMUM IOUT (A)
4A LOAD
320
5.2
MAXIMUM IOUT (A)
360
5.2
28
MAX1992 toc09
5.5
MAX1992 toc07
400
MAX1992 toc08
0
4.9
4.6
NO LOAD
5.0
4.9
4.3
200
4.8
4.0
-40
-15
10
35
85
60
0
4
8
12
16
20
24
INPUT VOLTAGE (V)
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
(FORCED-PWM MODE)
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
(SKIP MODE)
7
6
5
IIN
4
SKIP = AGND
1.2
SUPPLY CURRENT (mA)
IBIAS
8
1.5
MAX1992 toc10
9
3
2
0.9
0.6
IBIAS
8
12
16
20
INPUT VOLTAGE (V)
24
60
85
2.010
2.006
2.002
1.998
IIN
0
4
35
1.994
SKIP = VCC
0
0
10
REFERENCE LOAD REGULATION
0.3
1
-15
TEMPERATURE (°C)
TEMPERATURE (°C)
10
-40
28
MAX1992 toc12
240
REFERENCE VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
250
360
50
1.80
8
300
400
SWITCHING FREQUENCY (kHz)
1.83
SKIP = GND
SKIP = VCC
350
MAX1992 toc11
OUTPUT VOLTAGE (V)
1.84
400
SWITCHING FREQUENCY (kHz)
MAX1992 toc04
SKIP = GND
SKIP = VCC
1.82
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
SWITCHING FREQUENCY
vs. LOAD CURRENT
1.85
SUPPLY CURRENT (mA)
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
28
1.990
0
4
8
12
16
20
INPUT VOLTAGE (V)
24
28
-20
0
20
40
IREF (μA)
_______________________________________________________________________________________
60
80
100
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
(MAX1992 Circuit of Figure 1, MAX1993 Circuit of Figure 9, VIN = 12V, VDD = VCC = 5V, SKIP = VCC, TON = open, TA = +25°C,
unless otherwise noted.)
ILIM SATURATION FAULT CURRENT
vs. ILIM VOLTAGE
STARTUP WAVEFORMS
(HEAVY LOAD)
MAX1992 toc14
MAX1992 toc13
7
6
MAX1992 toc15
5V
5V
A
0
A
0
4A
5
IILIM(LSAT) (μA)
STARTUP WAVEFORMS
(LIGHT LOAD)
B
2A
0
0
2V
C
3V
2A
4
3
2
0
1
5V
B
C
0
D
D
0
0
0
0
0.4
0.8
1.2
1.6
2.0
VILIM (V)
400μs/div
200μs/div
A. SHDN = 0 TO 5V; 5V/div
B. INDUCTOR CURRENT: 2A/div
C. OUTPUT VOLTAGE (VOUT): 2V/div
D. PGOOD: 5V/div, 0.7Ω LOAD
A. SHDN = 0 TO 5V; 5V/div
B. INDUCTOR CURRENT: 2A/div
C. OUTPUT VOLTAGE (VOUT): 2V/div
D. PGOOD: 5V/div, 100Ω LOAD
SHUTDOWN WAVEFORMS
(DISCHARGE MODE ENABLED)
SHUTDOWN WAVEFORMS
(DISCHARGE MODE DISABLED)
LOAD TRANSIENT
(FORCED-PWM OPERATION)
MAX1992 toc17
MAX1992 toc16
MAX1992 toc18
5V
A
B
A
0
4A
A
0
2.6V
B
0
B
2.5V
5V
C
C
0
2.5V
12V
5V
20ms/div
A. SHDN = 5V TO 0; 5V/div
B. INDUCTOR CURRENT: 2A/div
C. DL: 5V/div
D. OUTPUT VOLTAGE (VOUT): 2V/div
E. PGOOD: 5V/div, 100Ω LOAD, OVP/UVP = OPEN OR G
C
0
D
D
E
2.4V
5A
D
E
0
1.0ms/div
A. SHDN = 5V TO 0; 5V/div
B. INDUCTOR CURRENT: 2A/div
C. DL: 5V/div
D. OUTPUT VOLTAGE (VOUT): 2V/div
E. PGOOD: 5V/div, 100Ω LOAD, OVP/UVP = VCC OR REF
0
20μs/div
A. LOAD: IOUT = 0.2A TO 4A; 5A/div
B. 2.5V OUTPUT: 100mV/div
C. INDUCTOR CURRENT: 5A/div
D. LX: 10V/div, SKIP = VCC
_______________________________________________________________________________________
9
MAX1992/MAX1993
Typical Operating Characteristics (continued)
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
Typical Operating Characteristics (continued)
(MAX1992 Circuit of Figure 1, MAX1993 Circuit of Figure 9, VIN = 12V, VDD = VCC = 5V, SKIP = VCC, TON = open, TA = +25°C,
unless otherwise noted.)
LOAD TRANSIENT
(PULSE-SKIPPING OPERATION)
OUTPUT OVERLOAD WAVEFORMS
(UVP DISABLED)
MAX1992 toc19
OUTPUT OVERLOAD WAVEFORMS
(UVP ENABLED)
MAX1992 toc20
MAX1992 toc21
10A
4A
10A
A
0
2.6V
A
0
2.5V
B
0
5V
C
0
5A
0
B
2.5V
2.4V
5A
2.5V
C
0
5A
0
0
12V
5V
B
C
D
0
5V
D
D
0
0
A
20μs/div
E
0
40μs/div
20μs/div
A. LOAD CURRENT (0 TO 250mΩ): 10A/div
B. 2.5V OUTPUT: 2V/div
C. INDUCTOR CURRENT: 5A/div
D. PGOOD: 5V/div, OVP/UVP = OPEN OR GND
A. LOAD: IOUT = 0.2A TO 4A; 5A/div
B. 2.5V OUTPUT: 100mV/div
C. INDUCTOR CURRENT: 5A/div
D. LX: 10V/div, SKIP = GND
INDUCTOR SATURATION PROTECTION
(LSAT DISABLED)
A. LOAD CURRENT (0 TO 250mΩ): 10A/div
B. 2.5V OUTPUT: 2V/div
C. DL: 5V/div
D. INDUCTOR CURRENT: 5A/div
E. PGOOD: 5V/div, OVP/UVP = VCC OR REF
INDUCTOR SATURATION PROTECTION
(ΔVILIM = 200mV)
MAX1992 toc22
INDUCTOR SATURATION PROTECTION
(ΔVILIM = 400mV)
MAX1992 toc23
5A
MAX1992 toc24
5A
5A
A
0
A
0
A
0
2.5V
2.5V
B
2.5V
B
0.67V
C
0.67V
C
7.5A
0.67V
7.5A
7.5A
A. LOAD CURRENT: IOUT = 0 TO 5A; 5A/div
B. 2.5V OUTPUT: 200mV/div
C. VILIM: 100mV/div
D. INDUCTOR CURRENT: 5A/div;
LSAT = AGND; L = 3.3μH, 3.5A
10
D
D
E
0
0
20μs/div
C
5V
0.47V
D
0
B
20μs/div
A. LOAD CURRENT: IOUT = 0 TO 5A; 5A/div
B. 2.5V OUTPUT: 200mV/div
C. VILIM: 200mV/div
D. INDUCTOR CURRENT: 5A/div,
LSAT = REF; L = 3.3μH, 3.5A
20μs/div
A. LOAD CURRENT: IOUT = 0 TO 5A; 5A/div
B. 2.5V OUTPUT: 1V/div
C. PGOOD: 5V/div
D. VILIM: 400mV/div
E. INDUCTOR CURRENT: 5A/div,
LSAT = REF; L = 3.3μH, 3.5A
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
(MAX1992 Circuit of Figure 1, MAX1993 Circuit of Figure 9, VIN = 12V, VDD = VCC = 5V, SKIP = VCC, TON = open, TA = +25°C,
unless otherwise noted.)
MAX1993 DYNAMIC OUTPUT VOLTAGE
TRANSITION (CREFIN = 1nF)
MAX1993 DYNAMIC OUTPUT VOLTAGE
TRANSITION (CREFIN = 100pF)
MAX1992 toc25
MAX1992 toc26
5V
A
0
1.5V
5V
A
0
1.5V
B
1.5V
C
1.0V
B
1.5V
C
1.0V
D
5V
2.5A
D
5V
5A
E
0
E
0
-5A
-2.5A
100μs/div
40μs/div
A. VGATE = 0 TO 5V; 5V/div
B. OUTPUT = 1.5V TO 1.0V; 0.5V/div
C. VREFIN: 0.5V/div
D. PGOOD: 5V/div
E. INDUCTOR CURRENT: 2.5A/div
100mA LOAD, SKIP = GND, CIRCUIT OF FIGURE 9
A. VGATE = 0 TO 5V; 5V/div
B. OUTPUT = 1.5V TO 1.0V; 0.5V/div
C. VREFIN: 0.5V/div
D. PGOOD: 5V/div
E. INDUCTOR CURRENT: 2.5A/div
100mA LOAD, SKIP = GND, CIRCUIT OF FIGURE 9
Pin Description
PIN
MAX1992
MAX1993
NAME
FUNCTION
1
1
TON
On-Time Selection Control Input. This four-level logic input sets the K-factor value
used to determine the DH on-time (see the On-Time One-Shot section). Connect to
analog ground (AGND or GND), REF, VCC, or leave TON unconnected to select the
following nominal switching frequencies:
VCC = 200kHz
Open = 300kHz
REF = 450kHz
AGND = 600kHz
2, 7, 8
—
N.C.
No Connection. Not internally connected.
—
2
FBLANK
Fault Blanking Control Input. This four-level logic input enables or disables fault
blanking, and sets the minimum forced-PWM operation time (tFBLANK). When fault
blanking is enabled, PGOOD, OVP protection, and UVP protection are blanked for the
selected time period after a transition is detected on GATE. Additionally, the controller
enters forced-PWM mode for the duration of tFBLANK anytime GATE changes states.
Connect FBLANK as follows:
VCC = 140µs (min) tFBLANK, fault blanking enabled
Open = 90µs (min) tFBLANK, fault blanking enabled
REF = 40µs (min) tFBLANK, fault blanking enabled
AGND = 90µs (min) tFBLANK, fault blanking disabled
______________________________________________________________________________________
11
MAX1992/MAX1993
Typical Operating Characteristics (continued)
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
MAX1992/MAX1993
Pin Description (continued)
PIN
MAX1992
3
4
5
3
4
5
NAME
LSAT
FUNCTION
Inductor Saturation Control Input. This four-level logic input sets the inductor current
saturation limit as a multiple of the valley current-limit threshold set by ILIM, or
disables the function if not required. Connect LSAT to the following pins to set the
saturation current limit:
VCC = 2 × ILIM(VAL)
Open = 1.75 × ILIM(VAL)
REF = 1.5 × ILIM(VAL)
AGND = disable LSAT protection
See the Inductor Saturation Limit and Setting the Current Limit sections.
PGOOD
Open-Drain Power-Good Output. PGOOD is low when the output voltage is more than
10% (typ) above or below the normal regulation point, during soft-start, and in
shutdown. After the soft-start circuit has terminated, PGOOD becomes high
impedance if the output is in regulation. For the MAX1993, PGOOD is
blanked—forced high-impedance state—when FBLANK is enabled and the controller
detects a transition on GATE.
ILIM
Valley Current-Limit Threshold Adjustment. The valley current-limit threshold defaults
to 50mV if ILIM is tied to VCC. In adjustable mode, the valley current-limit threshold
across CSP and CSN is precisely 1/10th the voltage seen at ILIM over a 250mV to
2.5V range. The logic threshold for switchover to the 50mV default value is
approximately VCC - 1V. When the inductor saturation protection threshold is
exceeded, ILIM sinks 6µA. See the Current-Limit Protection (ILIM) section.
6
6
REF
2.0V Reference Voltage Output. Bypass REF to analog ground with a 0.1µF or greater
ceramic capacitor. The reference can source up to 50µA for external loads. Loading
REF degrades output voltage accuracy according to the REF load regulation error.
The reference is disabled when the MAX1992/MAX1993 is shut down.
—
7
REFIN
External Reference Input. REFIN sets the feedback regulation voltage (VFB = VREFIN)
of the MAX1993.
—
8
OD
Open-Drain Output. Controlled by GATE.
FB
Feedback Input.
MAX1992: Connect to VCC for a +1.8V fixed output or to AGND for a +2.5V fixed
output. For an adjustable output (0.7V to 5.5V), connect FB to a resistive divider from
the output voltage. The FB regulation level is +0.7V.
MAX1993: The FB regulation level is set by the voltage at REFIN.
OUT
Output Voltage Sense. Connect directly to the positive terminal of the output
capacitors as shown in the standard application circuits (Figures 1 and 9). OUT
senses the output voltage to determine the on-time for the high-side switching
MOSFET. For the MAX1992, OUT also serves as the feedback input when using the
preset internal output voltages as shown in Figure 7. When discharge mode is
enabled by OVP/UVP, the output capacitor is discharged through an internal 10Ω
resistor connected between OUT and ground.
CSP
Positive Current-Sense Input. Connect to the positive terminal of the current-sense
element. Figure 10 and Table 7 describe several current-sensing options. The PWM
controller does not begin a cycle unless the current sensed is less than the valley
current-limit threshold programmed at ILIM.
9
10
11
12
MAX1993
9
10
11
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
PIN
MAX1992
MAX1993
NAME
FUNCTION
12
12
CSN
Negative Current-Sense Input. Connect to the negative terminal of the current-sense
element. Figure 10 and Table 7 describe several current-sensing options. The PWM
controller does not begin a cycle unless the current sensed is less than the valley
current-limit threshold programmed at ILIM.
13
13
SKIP
Pulse-Skipping Control Input. Connect SKIP to VCC for low-noise, forced-PWM mode
or connect SKIP to analog ground (AGND or GND) to enable pulse-skipping
operation.
14
14
V+
Battery Voltage-Sense Connection. The controller only uses V+ to set the on-time oneshot timing. The DH on-time is inversely proportional to input voltage over a range of
2V to 28V.
15
15
DH
High-Side Gate-Driver Output. DH swings from LX to BST.
16
16
LX
Inductor Connection. Connect LX to the switched side of the inductor. LX serves as
the lower supply rail for the DH high-side gate driver.
17
17
BST
Boost Flying Capacitor Connection. Connect to an external capacitor and diode as
shown in Figure 6. An optional resistor in series with BST allows the DH pullup current
to be adjusted.
18
18
DL
Low-Side Gate-Driver Output. DL swings from PGND to VDD (MAX1992) or GND to
VDD (MAX1993).
19
19
VDD
Supply Voltage Input for the DL Gate Driver. Connect to the system supply voltage
(+4.5V to +5.5V). Bypass VDD to PGND with a 1µF or greater ceramic capacitor.
20
—
PGND
Power Ground. Ground connection for the DL low-side gate driver.
Analog and Power Ground. AGND and PGND connect together internally. Connect
backside pad to GND.
—
20
GND
21
—
AGND
Analog Ground. Connect backside pad to AGND.
—
21
GATE
Buffered N-Channel MOSFET Gate Input. A logic low on GATE turns off the internal
MOSFET so OD appears as a high impedance. A logic high on GATE turns on the
internal MOSFET, pulling OD to ground.
22
22
VCC
Analog Supply Input. Connect to the system supply voltage (+4.5V to +5.5V) through
a series 20Ω resistor. Bypass VCC to analog ground with a 1µF or greater ceramic
capacitor.
______________________________________________________________________________________
13
MAX1992/MAX1993
Pin Description (continued)
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
MAX1992/MAX1993
Pin Description (continued)
PIN
MAX1992
MAX1993
23
23
24
24
NAME
FUNCTION
SHDN
Shutdown Control Input. Connect to VCC for normal operation. Connect to analog
ground to put the controller into its 1µA shutdown state. When discharge mode is
enabled by OVP/UVP, the output is discharged through a 10Ω resistor between OUT
and ground, and DL is forced high after VOUT drops below 0.3V. When discharge
mode is disabled by OVP/UVP, OUT remains a high-impedance input and DL is
forced low, so LX also appears as a high-impedance input. A rising edge on SHDN
clears the fault-protection latch.
OVP/UVP
Overvoltage/Undervoltage Protection and Discharge Mode Control Input. This fourlevel logic input selects between various output fault-protection options (Table 6) by
selectively enabling OVP protection and UVP protection. When enabled, the OVP limit
defaults at 116% of the nominal output voltage, and the UVP limit defaults at 70% of
the nominal output voltage. Discharge mode is enabled when UVP protection is also
enabled. Connect OVP/UVP to the following pins for the desired function:
VCC = enable OVP and discharge mode, enable UVP
Open = enable OVP and discharge mode, disable UVP
REF = disable OVP and discharge mode, enable UVP
AGND = disable OVP and discharge mode, and UVP
See the Fault Protection and Shutdown and Output Discharge sections.
Table 1. Component Selection for Standard Applications
VOUT = 2.5V AT 5A
(FIGURE 1)
VOUT = 1.8V AT 5A
VOUT = 1.0V / 1.5V AT 4A
(FIGURE 9)
VIN = 7V to 24V,
TON = OPEN (300kHz)
VIN = 7V to 24V,
TON = OPEN (300kHz)
VIN = 4.5V to 5.5V,
TON = GND (600kHz)
MAX1992
FB = AGND
FB = VCC
Not recommended
MAX1993
Adjustable FB,
REFIN = REF
FB = OUT,
VREFIN = 1.8V
FB = OUT,
VREFIN = 1.0V / 1.5V
CIN, input capacitor
10µF, 25V
Taiyo Yuden TMK432BJ106KM
10µF, 25V
Taiyo Yuden TMK432BJ106KM
100µF, 10V
Sanyo POSCAP 10TPA100M
COUT, output capacitor
220µF, 4V, 15mΩ
Sanyo POSCAP 4TPE220MF
220µF, 4V, 15mΩ
Sanyo POSCAP 4TPE220MF
220µF, 6V, 12mΩ
Sanyo POSCAP 6TPD220M
NH high-side MOSFET
Fairchild Semiconductor
1/2 FDS6982A
Fairchild Semiconductor
1/2 FDS6982A
Fairchild Semiconductor
1/2 FDS6982S
NL low-side MOSFET
Fairchild Semiconductor
1/2 FDS6982A
Fairchild Semiconductor
1/2 FDS6982A
Fairchild Semiconductor
1/2 FDS6982S
DL Schottky rectifier
(optional)
Nihon EP10QS03L
1A, 30V, 0.45Vf
Nihon EP10QS03L
1A, 30V, 0.45Vf
Nihon EP10QS03L
1A, 30V, 0.45Vf
L1 inductor
4.3µH
Sumida CDEP105(L)
3.2µH
Sumida CDEP105(L)
1.4µH
Sumida CDEP105(L)
RSENSE
15mΩ ±1% 0.5W resistor
IRC LR2010-01-R015F or
Dale WSL-2010-R015F
15mΩ ±1% 0.5W resistor
IRC LR2010-01-R015F or
Dale WSL-2010-R015F
15mΩ ±1% 0.5W resistor
IRC LR2010-01-R015F or
Dale WSL-2010-R015F
COMPONENT
14
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
MAX1992/MAX1993
Table 2. Component Suppliers
SUPPLIER
PHONE
WEBSITE
Central Semiconductor
631-435-1110 (USA)
www.centralsemi.com
Coilcraft
800-322-2645 (USA)
www.coilcraft.com
Coiltronics
561-752-5000 (USA)
www.coiltronics.com
Fairchild Semiconductor
888-522-5372 (USA)
www.fairchildsemi.com
International Rectifier
310-322-3331 (USA)
www.irf.com
Kemet
408-986-0424 (USA)
www.kemet.com
714-373-7366 (USA)
www.panasonic.com
65-231-3226 (Singapore)
408-749-9714 (USA)
www.secc.co.jp
Panasonic
Sanyo
Siliconix (Vishay)
203-268-6261 (USA)
www.vishay.com
Sumida
408-982-9660 (USA)
www.sumida.com
03-3667-3408 (Japan)
408-573-4150 (USA)
www.t-yuden.com
847-803-6100 (USA)
81-3-5201-7241 (Japan)
www.component.tdk.com
858-675-8013 (USA)
www.tokoam.com
Taiyo Yuden
TDK
TOKO
C1
1μF
R1
20Ω
R2
100kΩ
VDD
VCC
LSAT
POWER GOOD
DBST
CMPSH-3
+5V BIAS
SUPPLY
C2
1μF
V+
PGOOD
BST
SHDN
DH
CIN
10μF
INPUT (VIN)*
7V TO 20V
NH
ON OFF
FLOAT
(300kHz)
RSENSE
15mΩ
OUTPUT (VOUT)
2.5V
LX
TON
SKIP
MAX1992
CREF
0.22μF
NL
DL
DL
COUT
220μF
PGND
REF
R3
100kΩ
CILIM
470pF
L1
4.3μH
CBST
0.1μF
AGND
CSP
ILIM
R4
49.9kΩ
CSN
OUT
FB
OVP/UVP
POWER GROUND
ANALOG GROUND
*LOWER INPUT VOLTAGES REQUIRE
ADDITIONAL INPUT CAPACITANCE.
SEE TABLE 1 FOR COMPONENT SPECIFICATIONS.
BOLD LINES INDICATE HIGH CURRENT TRACES.
Figure 1. MAX1992 Standard Application Circuit
______________________________________________________________________________________
15
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
Detailed Description
The MAX1992/MAX1993 buck controllers are ideal for
low-voltage power supplies for notebook computers.
Maxim’s proprietary Quick-PWM pulse-width modulator
in the MAX1992/MAX1993 is designed for handling fast
load steps while maintaining a relatively constant operating frequency and inductor operating point over a
wide range of input voltages. The Quick-PWM architecture circumvents the poor load-transient timing problems of fixed-frequency current-mode PWMs while
avoiding the problems caused by widely varying
switching frequencies in conventional constant-on-time
and constant-off-time PWM schemes.
See Table 1 for component selections and Table 2 for a
list of component suppliers.
+5V Bias Supply (VCC and VDD)
The MAX1992/MAX1993 require an external 5V bias
supply in addition to the battery. Typically, this 5V bias
supply is the notebook’s 95%-efficient 5V system supply. Keeping the bias supply external to the IC improves
efficiency and eliminates the cost associated with the 5V
linear regulator that would otherwise be needed to supply the PWM circuit and gate drivers. If stand-alone
capability is needed, the 5V supply can be generated
with an external linear regulator such as the MAX1615.
The 5V bias supply must provide VCC (PWM controller)
and VDD (gate-drive power), so the maximum current
drawn is:
IBIAS = ICC + fSW (QG(LOW) + QG(HIGH))
= 2mA to 20mA (typ)
where ICC is 550µA (typ), fSW is the switching frequency,
and Q G(LOW) and Q G(HIGH) are the MOSFET data
sheet’s total gate-charge specification limits at VGS = 5V.
The V+ battery input and 5V bias inputs (VCC and VDD)
can be connected together if the input source is a fixed
4.5V to 5.5V supply. If the 5V bias supply is powered
up prior to the battery supply, the enable signal (SHDN
going from low to high) must be delayed until the battery voltage is present in order to ensure startup.
Free-Running Constant-On-Time PWM
Controller with Input Feed Forward
The Quick-PWM control architecture is a pseudofixedfrequency, constant on-time, current-mode regulator
with voltage feed forward (Figure 2). This architecture
relies on the output filter capacitor’s ESR to act as a
current-sense resistor, so the output ripple voltage provides the PWM ramp signal. The control algorithm is
simple: the high-side switch on-time is determined solely by a one-shot whose pulse width is inversely proportional to input voltage and directly proportional to
output voltage. Another one-shot sets a minimum offtime (400ns typ). The on-time one-shot is triggered if
the error comparator is low, the low-side switch current
is below the valley current-limit threshold, and the minimum off-time one-shot has timed out.
On-Time One-Shot (TON)
The heart of the PWM core is the one-shot that sets the
high-side switch on-time. This fast, low-jitter, adjustable
one-shot includes circuitry that varies the on-time in
response to battery and output voltage. The high-side
switch on-time is inversely proportional to the battery
voltage as measured by the V+ input and is proportional
to the output voltage:
On-time = K (VOUT + 0.075V) / VIN
Table 3. Approximate K-Factor Errors
TON SETTING
(kHz)
TYPICAL
K-FACTOR (µs)
K-FACTOR
ERROR (%)
MINIMUM VIN
AT VOUT = 2.5V
(h = 1.5) (V)
TYPICAL
APPLICATION
COMMENTS
200
(TON = VCC)
5.0
±10
3.14
4-cell Li+ notebook
Use for absolute best efficiency
300
(TON = open)
3.3
±10
3.47
4-cell Li+ notebook
Considered mainstream by
current standards
450
(TON = REF)
2.2
±12.5
4.13
3-cell Li+ notebook
Useful in 3-cell systems for
lighter loads than the CPU core
or where size is key
600
(TON = GND)
1.7
±12.5
5.61
+5V input
16
Good operating point for
compound buck designs or
desktop circuits
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
TON
tOFF(MIN)
ON-TIME
COMPUTE
Q
OUT
TRIG
MAX1992
MAX1993
1-SHOT
S
tON
Q
TRIG
MAX1992/MAX1993
V+
1-SHOT
BST
DH
Q
LX
R
ILIM
ERROR
AMP
R
Q
S
VDD
1.14 x
INTREF
S
DL
Q
ENABLE
OVP
OVP/UVP
R
FAULT
LATCH
QUAD
LEVEL
DECODE
PGND
SATURATION
LIMIT
CSP
CSN
QUAD
LEVEL
DECODE
BLANK
LSAT
VCC - 1.0V
20ms
TIMER
ENABLE
UVP
R
9R
SKIP
CSP
0.7 x
INTREF
ILIM
POR
CSP
CURRENT
LIMIT
ZERO CROSSING
CSN
CSN
0.5V
REF
INTREF
*OD
*GATE
VCC
0.9 x
INTREF
1.1 x
INTREF
13R
MAX1993
ONLY
2.0V
REF
AGND
0.7V
MAX1992 vs. MAX1993
INTERNAL OPTION
PGOOD
7R
*REFIN
*FBLANK
FB
MAX1992
FB DECODE
(FIGURE 7)
BLANK
FBLANK
DECODE
AND TIMER
*MAX1993 ONLY. IN THE MAX1993, AGND AND PGND
ARE INTERNALLY CONNECTED AND CALLED GND.
OUT
DISCHARGE
LOGIC
SHDN
Figure 2. MAX1992/MAX1993 Functional Diagram
______________________________________________________________________________________
17
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
where K (switching period) is set by the TON pin-strap
connection (Table 3), and 0.075V is an approximation to
accommodate the expected drop across the low-side
MOSFET switch. This algorithm results in a nearly constant switching frequency despite the lack of a fixed-frequency clock generator. The benefits of a constant
switching frequency are twofold: 1) the frequency can
be selected to avoid noise-sensitive regions such as the
455kHz IF band; 2) the inductor ripple-current operating
point remains relatively constant, resulting in easy design
methodology and predictable output voltage ripple.
The on-time one-shot has good accuracy at the operating points specified in the Electrical Characteristics
(approximately ±12.5% at 600kHz and 450kHz and
±10% at 200kHz and 300kHz). On-times at operating
points far removed from the conditions specified in the
Electrical Characteristics can vary over a wider range.
For example, the 600kHz setting typically runs approximately 10% slower with inputs much greater than 5V
due to the very short on-times required.
The constant on-time translates only roughly to a constant
switching frequency. The on-times guaranteed in the
Electrical Characteristics are influenced by resistive losses and by switching delays in the high-side MOSFET.
Resistive losses—including the inductor, both MOSFETs,
output capacitor ESR, and PC board copper losses in the
output and ground—tend to raise the switching frequency
as the load increases. The dead-time effect increases the
effective on-time, reducing the switching frequency as
one or both dead times are added to the effective ontime. It occurs only in PWM mode (SKIP = VCC) and
during dynamic output voltage transitions when the
inductor current reverses at light or negative load
currents. With reversed inductor current, the inductor’s
EMF causes LX to go high earlier than normal, extending
the on-time by a period equal to the DH-rising dead time.
For loads above the critical conduction point, where the
dead-time effect is no longer a factor, the actual switching frequency is:
fSW =
VOUT + VDROP1
t ON ( VIN + VDROP2 )
where VDROP1 is the sum of the parasitic voltage drops
in the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; VDROP2 is
the sum of the resistances in the charging path, including the high-side switch, inductor, and PC board resistances; and t ON is the on-time calculated by the
MAX1992/MAX1993.
18
Automatic Pulse-Skipping Mode
(SKIP = GND)
In skip mode (SKIP = GND), an inherent automatic
switchover to PFM takes place at light loads (Figure 3).
This switchover is affected by a comparator that truncates the low-side switch on-time at the inductor current’s zero crossing. The zero-crossing comparator
differentially senses the inductor current across the current-sense resistor (CSP to CSN). Once VCSP - VCSN
drops below 5% of the current-limit threshold (2.5mV
for the default 50mV current-limit threshold), the comparator forces DL low (Figure 2). This mechanism causes the threshold between pulse-skipping PFM and
nonskipping PWM operation to coincide with the
boundary between continuous and discontinuous
inductor-current operation (also known as the critical
conduction point). The load-current level at which
PFM/PWM crossover occurs, ILOAD(SKIP), is equal to
one-half the peak-to-peak ripple current, which is a
function of the inductor value (Figure 3). This threshold
is relatively constant, with only a minor dependence on
battery voltage:
K ⎞ ⎛ V − VOUT ⎞
⎛V
ILOAD(SKIP) ≈ ⎜ OUT ⎟ ⎜ IN
⎟
⎝ 2L ⎠ ⎝
VIN
⎠
where K is the on-time scale factor (Table 3). For example, in the standard application circuit (K = 3.3µs, VOUT =
2.5V, VIN = 12V, and L = 4.3µH), the pulse-skipping
switchover occurs at:
⎛ 2.5V × 3.3μs ⎞ ⎛ 12V − 2.5V ⎞
⎟ = 0.76A
⎜
⎟⎜
⎠
12V
⎝ 2 × 4.3μH ⎠ ⎝
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used. The switching waveforms can appear noisy and asynchronous
when light loading causes pulse-skipping operation,
but this is a normal operating condition that results in
high light-load efficiency. Trade-offs in PFM noise vs.
light-load efficiency are made by varying the inductor
value. Generally, low inductor values produce a broader efficiency vs. load curve, while higher values result in
higher full-load efficiency (assuming that the coil resistance remains fixed) and less output voltage ripple.
Penalties for using higher inductor values include larger
physical size and degraded load-transient response
(especially at low input voltage levels).
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
Forced-PWM mode is most useful for reducing audiofrequency noise, improving load-transient response,
and providing sink-current capability for dynamic output voltage adjustment. The MAX1993 uses forcedPWM operation during all dynamic output voltage
transitions (GATE transition detected) in order to ensure
fast, accurate transitions. Because forced-PWM operation disables the zero-crossing comparator, the inductor current reverses under light loads, quickly
discharging the output capacitors. FBLANK determines
how long the MAX1993 maintains forced-PWM operation—140µs (FBLANK = VCC), 90µs (FBLANK = open
or AGND), or 40µs (FBLANK = REF).
VIN - VOUT
L
IPEAK
INDUCTOR CURRENT
Forced-PWM Mode (SKIP = VCC)
The low-noise forced-PWM mode (SKIP = VCC) disables the zero-crossing comparator, which controls the
low-side switch on-time. This forces the low-side gatedrive waveform to constantly be the complement of the
high-side gate-drive waveform, so the inductor current
reverses at light loads while DH maintains a duty factor
of VOUT/VIN. Forced-PWM mode keeps the switching
frequency fairly constant. However, forced-PWM operation comes at a cost: the no-load 5V bias current
remains between 2mA and 20mA, depending on the
external MOSFETs and switching frequency.
ΔI
=
Δt
MAX1992/MAX1993
DC output accuracy specifications refer to the threshold of the error comparator. When the inductor is in
continuous conduction, the MAX1992/MAX1993 regulate the valley of the output ripple, so the actual DC output voltage is higher than the trip level by 50% of the
output ripple voltage. In discontinuous conduction
(SKIP = GND and IOUT < ILOAD(SKIP)), the output voltage has a DC regulation level higher than the errorcomparator threshold by approximately 1.5% because
of slope compensation.
ILOAD = IPEAK/2
0
ON-TIME
TIME
Figure 3. Pulse-Skipping/Discontinuous Crossover Point
MAX1992
MAX1993
CREF
REF
RA
TO VALLEY
CURRENT-LIMIT
COMPARATOR
(FIGURE 2)
CILIM
ILIM
6μA
RB
FROM LSAT
COMPARATOR
AND LOGIC
(FIGURE 2)
Figure 4. Adjustable Current-Limit Threshold
Current-Limit Protection (ILIM)
IPEAK
ILOAD
INDUCTOR CURRENT
Valley Current Limit
The current-limit circuit employs a unique “valley” current-sensing algorithm that uses a current-sense resistor between CSP and CSN as the current-sensing
element (Figure 10). If the magnitude of the currentsense signal is above the valley current-limit threshold,
the PWM controller is not allowed to initiate a new cycle
(Figure 5). The actual peak current is greater than the
valley current-limit threshold by an amount equal to the
inductor ripple current. Therefore, the exact currentlimit characteristic and maximum load capability are a
function of the current-sense resistance, inductor value,
and battery voltage. When combined with the undervoltage protection circuit, this current-limit method is
effective in almost every circumstance.
ILIMIT
( LIR2 )
ILIM(VAL) = ILOAD(MAX) 1-
0
TIME
Figure 5. “Valley” Current-Limit Threshold Point
______________________________________________________________________________________
19
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
In forced-PWM mode, the MAX1992/MAX1993 also
implement a negative current limit to prevent excessive
reverse inductor currents when VOUT is sinking current.
The negative current-limit threshold is set to approximately 120% of the positive current limit and tracks the
positive current limit when ILIM is adjusted.
The current-limit threshold is adjusted with an external
resistor-divider at ILIM. A 2µA to 20µA divider current is
recommended for accuracy and noise immunity. The
current-limit threshold adjustment range is from 25mV
to 200mV. In the adjustable mode, the current-limit
threshold voltage is precisely 1/10th the voltage seen at
ILIM. The threshold defaults to 50mV when ILIM is connected to VCC. The logic threshold for switchover to the
50mV default value is approximately VCC - 1V.
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors do not corrupt the differential current-sense signals seen by CSP and CSN.
Place the IC close to the sense resistor with short,
direct traces, making a Kelvin-sense connection to the
current-sense resistor.
Inductor Saturation Limit
The LSAT connection selects an upper current-sense
limit as the inductor saturation threshold or disables the
inductor saturation protection feature altogether (LSAT
= GND). When enabled, the inductor saturation threshold is set as a multiple of the positive valley current-limit
threshold (Table 4) and tracks the valley current limit
when ILIM is adjusted. The inductor saturation threshold should be selected to give sufficient headroom
above the peak inductor current so switching noise
does not accidentally trip the saturation protection.
Selecting too high a threshold can cause an inductor
saturation to go undetected. For an inductor with a low
LIR (the ratio of the inductor ripple current to the
designed maximum load current) of approximately
20%, the lowest saturation threshold of 1.5 x ILIM(VAL)
(LSAT = REF) may be acceptable. When using an
inductor with a higher LIR, increase the inductor saturation threshold accordingly.
When inductor saturation is enabled, the MAX1992/
MAX1993 continuously monitor the inductor current
through the voltage across the current-sense resistor.
When the inductor saturation threshold is exceeded, the
MAX1992/MAX1993 immediately turn off the high-side
gate driver and enable a 6µA discharge current on ILIM
(Figure 4) at the beginning of the next DH on-time.
20
This reduces the voltage on ILIM by ΔVILIM where:
⎛ R × RB ⎞
ΔVILIM = −⎜ A
⎟IILIM (LSAT)
⎝ RA + RB ⎠
where IILIM(LSAT) is 6µA ILIM saturation fault sink current (see the Electrical Characteristics table). When
using the default 50mV valley current-limit threshold
(ILIM = VCC), the ILIM saturation fault sink current does
not lower the current-limit threshold (see Figure 2).
If the inductor current remains below the saturation
threshold during the next cycle, the ILIM discharge current is disabled, and the ILIM voltage returns to its original set point. The inductor should not remain in
saturation once the controller reduces the valley current
limit. However, if the inductor remains in saturation, the
output voltage may drop low enough to trip the undervoltage fault protection (UVP enabled), causing the
MAX1992/MAX1993 to shut down and latch off. Adding
a capacitor from ILIM to GND slows the ILIM voltage
change by the time constant τ = (RA//RB) x CILIM. A
suitable time constant is between 5 to 10 switching
cycles. If the inductor saturation occurs only during a
short load transient, the time constant allows the power
supply to recover before the output voltage drops
below the output undervoltage threshold.
Set ΔVILIM to be at least 30% (LIR) of the ILIM set voltage. Calculate RA and RB using the equations below:
RA =
⎛ ΔV
⎞
ILIM
⎜
⎟
IILIM(LSAT) ⎝ VILIM(SET) ⎠
VREF
⎛ ΔV
⎞
ILIM
with ⎜
⎟ set at 30%
⎝ VILIM(SET) ⎠
RB =
RA
⎛ V
⎞
REF
− 1⎟
⎜
⎝ VILIM(SET) ⎠
Inductor saturation works best using a current-sense
resistor in series with the inductor. A low-side currentsense resistor configuration can sense the saturation
Table 4. LSAT Configuration Table
LSAT
INDUCTOR SATURATION THRESHOLD
VCC
2.00 × ILIM(VAL)
Open
1.75 × ILIM(VAL)
REF
1.50 × ILIM(VAL)
GND
Disabled
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
MOSFET Gate Drivers (DH, DL)
The DH and DL drivers are optimized for driving moderately sized high-side and larger low-side power
MOSFETs. This is consistent with the low duty factor seen
in notebook applications, where a large VIN - VOUT differential exists. An adaptive dead-time circuit monitors the
DL output and prevents the high-side MOSFET from turning on until DL is off. A similar adaptive dead-time circuit
monitors the DH output, preventing the low-side MOSFET
from turning on until DH is off. There must be a low-resistance, low-inductance path from the DL and DH drivers to
the MOSFET gates in order for the adaptive dead-time
circuits to work properly; otherwise, the sense circuitry in
the MAX1992/MAX1993 interpret the MOSFET gates as
“off” while charge actually remains. Use very short, wide
traces (50 mils to 100 mils wide if the MOSFET is 1in from
the driver).
The internal pulldown transistor that drives DL low is
robust, with a 0.6Ω (typ) on-resistance. This helps prevent
DL from being pulled up because of capacitive coupling
from the drain to the gate of the low-side MOSFETs when
the inductor node (LX) quickly switches from ground to
VIN. Applications with high-input voltages and long inductive driver traces can require additional gate-to-source
capacitance to ensure that fast-rising LX edges do not
pull up the low-side MOSFETs gate, causing shootthrough currents. The capacitive coupling between LX
and DL created by the MOSFET’s gate-to-drain capacitance (CRSS), gate-to-source capacitance (CISS - CRSS),
and additional board parasitics should not exceed the
following minimum threshold.
⎛C
⎞
VGS( TH) < VIN ⎜ RSS ⎟
⎝ CISS ⎠
Lot-to-lot variation of the threshold voltage can cause
problems in marginal designs. Alternatively, adding a
resistor of less than 10Ω in series with BST can remedy
the problem by increasing the turn-on time of the highside MOSFET without degrading the turn-off time
(Figure 6).
MAX1992/MAX1993
current only at the start of the off-cycle. See Setting the
Current Limit section for various current-sense configurations (Figure 10) and LSAT recommendations.
CBYP
MAX1992
MAX1993
VDD
BST
(RBST)*
DBST
INPUT (VIN)
CBST
DH
NH
L
LX
VDD
DL
NL
(CNL)*
PGND
(RBST)* OPTIONAL—THE RESISTOR LOWERS EMI BY DECREASING
THE SWITCHING NODE RISE TIME.
(CNL)* OPTIONAL—THE CAPACITOR REDUCES LX TO DL CAPACITIVE
COUPLING THAT CAN CAUSE SHOOT-THROUGH CURRENTS.
Figure 6. Optional Gate Driver Circuitry
POR, UVLO, and Soft-Start
Power-on reset (POR) occurs when VCC rises above
approximately 2V, resetting the fault latch and soft-start
counter, powering up the reference, and preparing the
PWM for operation. Until VCC reaches 4.25V (typ), VCC
undervoltage lockout (UVLO) circuitry inhibits switching. The controller inhibits switching by pulling DH low
and holding DL low when OVP and shutdown discharge are disabled or forcing DL high when OVP and
shutdown discharge are enabled (Table 6). When VCC
rises above 4.25V, the controller activates the PWM
controller and initializes soft-start.
Soft-start allows a gradual increase of the internal currentlimit level during startup to reduce the input surge currents. The MAX1992/MAX1993 divide the soft-start period
into five phases. During the first phase, the controller limits the current limit to only 20% of the full current limit. If
the output does not reach regulation within 425µs, softstart enters the second phase, and the current limit is
increased by another 20%. This process repeats until the
maximum current limit is reached after 1.7ms or when the
output reaches the nominal regulation voltage, whichever
occurs first (see soft-start waveforms in the Typical
Operating Characteristics). Adding a capacitor in parallel
with the external ILIM resistors creates a continuously
adjustable analog soft-start function.
______________________________________________________________________________________
21
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
Power-Good Output (PGOOD)
Shutdown and Output Discharge
PGOOD is the open-drain output for a window comparator that continuously monitors the output. PGOOD
is actively held low in shutdown and during soft-start.
After the digital soft-start terminates, PGOOD becomes
high impedance as long as the output voltage is within
±10% of the nominal regulation voltage set by FB.
When the output voltage drops 10% below or rises 10%
above the nominal regulation voltage, the MAX1992/
MAX1993 pull PGOOD low. Any fault condition forces
PGOOD low until the fault latch is cleared by toggling
SHDN or cycling VCC power below 1V. For logic level
output voltages, connect an external pullup resistor
between PGOOD and VCC. A 100kΩ resistor works well
in most applications.
Note that the PGOOD window detector is completely
independent of the overvoltage and undervoltage protection fault detectors.
When output discharge is enabled (OVP/UVP = VCC or
open) and SHDN is pulled low, or the output undervoltage fault latch is set (OVP/UVP = V CC or REF), the
MAX1992/MAX1993 discharge the output through an
internal 10Ω switch to ground. While the output is discharging, DL is forced low and the PWM controller is
disabled, but the reference remains active to provide
an accurate threshold. Once the output voltage drops
below 0.3V, the MAX1992/MAX1993 shut down the reference and pull DL high, effectively clamping the output and LX switching node to ground.
When output discharge is disabled (OVP/UVP = REF or
GND), the controller does not actively discharge the output, and the DL driver remains low. Under these conditions, the output discharge rate is determined by the load
current and output capacitance.
The controller detects and latches the discharge mode
state set by OVP/UVP on startup.
Fault Blanking (MAX1993 FBLANK)
The MAX1993 automatically enters forced-PWM operation during all dynamic output voltage transitions (GATE
transition detected) in order to ensure fast, accurate
transitions. FBLANK determines how long the MAX1993
maintains forced-PWM operation (Table 5)—at least
140µs (FBLANK = V CC ), 90µs (FBLANK = open or
GND), or 40µs (FBLANK = REF).
When fault blanking is enabled (FBLANK = VCC, open,
or REF), the MAX1993 also disables the overvoltage
and undervoltage fault protection and forces PGOOD
to a high-impedance state during the transition period
selected by FBLANK (Table 5). This prevents fault protection from latching off the controller and the PGOOD
signal from going low when the output voltage change
(ΔVOUT) cannot occur as fast as the REFIN voltage
change (ΔVREFIN).
Table 5. FBLANK Configuration Table
FBLANK
FAULT BLANKING
MINIMUM FORCEDPWM DURATION (µs)
VCC
Enabled
140
22
Open
Enabled
90
REF
Enabled
40
GND
Disabled
90
Fault Protection
The MAX1992/MAX1993 provide over/undervoltage
fault protection. Drive OVP/UVP to enable and disable
fault protection as shown in Table 6. Once activated,
the controller continuously monitors the output for
undervoltage and overvoltage fault conditions.
Overvoltage Protection (OVP)
When the output voltage rises above 116% of the nominal regulation voltage and OVP is enabled (OVP/UVP =
VCC or open), the OVP circuit sets the fault latch, shuts
down the PWM controller, and immediately pulls DH low
and forces DL high. This turns on the synchronous rectifier MOSFET with 100% duty, rapidly discharging the output capacitor and clamping the output to ground. Note
that immediately latching DL high can cause the output
voltage to go slightly negative due to energy stored in
the output LC at the instant the OVP occurs. If the load
cannot tolerate a negative voltage, place a power
Schottky diode across the output to act as a reversepolarity clamp. If the condition that caused the overvoltage persists (such as a shorted high-side MOSFET), the
battery fuse blows. OVP is ignored when transitions are
detected on GATE (MAX1993 only, FBLANK enabled).
Toggle SHDN or cycle VCC power below 1V to clear the
fault latch and restart the controller.
OVP is disabled when OVP/UVP is connected to REF or
GND (Table 6).
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
OVP/UVP SHDN DISCHARGE*
OVP PROTECTION
THERMAL PROTECTION
Yes.
Enabled.
DL forced high when Discharge sequence activated;
DL forced high when shut down.
shut down.
Enabled.
DH pulled low and
DL forced high.
Enabled.
Discharge sequence activated;
DL forced high when shut down.
Open
Yes.
DL forced high when
shut down.
Disabled.
Enabled.
DH pulled low and
DL forced high.
Enabled.
Discharge sequence activated;
DL forced high when shut down.
REF
No.
DL forced low when
shut down.
Enabled.
Discharge sequence activated;
DL forced high when shut down.
Disabled.
Enabled.
Discharge sequence activated;
DL forced high when shut down.
GND
No.
DL forced low when
shut down.
Disabled.
Disabled.
Enabled.
Discharge sequence activated;
DL forced high when shut down.
VCC
UVP PROTECTION
*Discharge-mode state latched on power-up.
Undervoltage Protection (UVP)
When the output voltage drops below 70%, the nominal
regulation voltage and the UVP are enabled (OVP/UVP =
VCC or REF), and the controller sets the fault latch and
begins the discharge mode (see the Shutdown and
Output Discharge section). When the output voltage
drops to 0.3V, the synchronous rectifiers turn on, clamping the outputs to GND. UVP is ignored for at least 10ms
(min) after startup (SHDN rising edge) and when transitions are detected on GATE (MAX1993 only, FBLANK
enabled). Toggle SHDN or cycle VCC power below 1V to
clear the fault latch and restart the controller.
UVP is disabled when OVP/UVP is left open or connected to GND (Table 6).
Thermal Fault Protection
The MAX1992/MAX1993 feature a thermal fault protection circuit. When the junction temperature rises above
+160°C, a thermal sensor activates the fault latch, pulls
PGOOD low, and shuts down using discharge mode
regardless of the OVP/UVP setting. Toggle SHDN or
cycle VCC power below 1V to reactivate the controller
after the junction temperature cools by 15°C.
Output Voltage
Preset Output Voltages (MAX1992 Only)
The MAX1992’s Dual Mode operation allows the selection of common voltages without requiring external
components (Figure 7). Connect FB to AGND for a
fixed 2.5V output, to VCC for a fixed 1.8V output, or connect FB directly to OUT for a fixed 0.7V output.
Setting VOUT with a Resistive Voltage-Divider at FB
The output voltage can be adjusted from 0.7V to 5.5V
using a resistive voltage-divider (Figure 8). The MAX1992
regulates FB to a fixed reference voltage (0.7V).
Alternatively, the MAX1993 regulates FB to the voltage
set at REFIN, making the MAX1993 ideal for memory
applications in which the termination supply must track
the supply voltage. The adjusted output voltage is:
⎛ R ⎞
VOUT = VFB ⎜1 + C ⎟
⎝ RD ⎠
where VFB is 0.7V for the MAX1992 and VFB = VREFIN
for the MAX1993.
MAX1992
OUT
TO ERROR
AMPLIFIER
FB
1.8V (FIXED)
REF
(2.0V)
2.5V (FIXED)
0.1 x REF
(0.2V)
Figure 7. Dual-Mode Feedback Decoder (MAX1992)
______________________________________________________________________________________
23
MAX1992/MAX1993
Table 6. Fault Protection and Shutdown Setting Truth Table
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
L
RSENSE
LX
MAX1992
MAX1993
COUT
DL
NL
PGND
AGND
CSP
CSN
OUT
RC
FB
RD
Figure 8. Setting VOUT with a Resistive Voltage-Divider
Dynamic Output Voltages (MAX1993 Only)
The MAX1993 regulates FB to the voltage set at REFIN.
By changing the voltage at REFIN, the MAX1993 can
be used in applications that require dynamic output
voltage changes between two set points. Figure 9
shows a dynamically adjustable resistive voltagedivider network at REFIN. Using the GATE signal and
open-drain output (OD), a resistor can be switched in
and out of the REFIN resistor-divider, changing the voltage at REFIN. A logic high on GATE turns on the internal N-channel MOSFET, forcing OD to a lowimpedance state. A logic low on GATE disables the Nchannel MOSFET, so OD is high impedance. The two
output voltages (FB = OUT) are determined by the following equations:
⎛ R6 ⎞
VOUT(LOW) = VREF ⎜
⎟
⎝ R5 + R6 ⎠
⎡ (R6 + R7) ⎤
VOUT(HIGH) = VREF ⎢
⎥
⎢⎣ R5 + (R6 + R7) ⎥⎦
24
The MAX1993 automatically enters forced-PWM operation on the rising and falling edges of GATE and
remains in forced-PWM mode for a minimum time
selected by FBLANK (Table 5). Forced-PWM operation
is required to ensure fast, accurate negative voltage
transitions when REFIN is lowered. Because forcedPWM operation disables the zero-crossing comparator,
the inductor current can reverse under light loads,
quickly discharging the output capacitors. If fault blanking is enabled, the MAX1993 also disables the overvoltage and undervoltage fault protection and forces
PGOOD to a high-impedance state for the period
selected by FBLANK (Table 5).
For a step voltage change at REFIN, the rate of change
of the output voltage is limited by the inductor current
ramp, the total output capacitance, the current limit, and
the load during the transition. The inductor current ramp
is limited by the voltage across the inductor and the
inductance. The total output capacitance determines
how much current is needed to change the output voltage. Additional load current slows the output voltage
change during a positive REFIN voltage change, and
speeds the output voltage change during a negative
REFIN voltage change. Increasing the current-limit setting speeds a positive output voltage change.
Adding a capacitor across REFIN and GND filters noise
and controls the rate-of-change of the REFIN voltage
during dynamic transitions. With the additional capacitance, the REFIN voltage slews between the two set
points with a time constant determined by the equivalent parallel resistance seen by the slew capacitor
(CREFIN). Referring to Figure 9, the time constant for a
positive REFIN voltage transition is:
⎡ R5 × (R6 + R7) ⎤
τ POS = ⎢
⎥CREFIN
⎢⎣ R5 + (R6 + R7) ⎥⎦
and the time constant for a negative REFIN voltage
transition is:
⎛ R5 × R6 ⎞
τ NEG = ⎜
⎟C
⎝ R5 + R6 ⎠ REFIN
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
R1
20Ω
+5V BIAS
SUPPLY
C2
1μF
VDD
VCC
R2
100kΩ
LSAT
OVP/UVP
POWER GOOD
DBST
CMPSH-3
V+
PGOOD
BST
SHDN
DH
CIN
10μF
INPUT (VIN)*
4.5V TO 5.5V
NH
ON OFF
OUTPUT
VOUT(HIGH) = 1.5V
VOUT(LOW) = 1.0V
RSENSE
15mΩ
LX
REF
R3
100kΩ
CILIM
470pF
L1
1.4μH
CBST
0.1μF
CREF
0.22μF
MAX1992/MAX1993
C1
1μF
MAX1993
ILIM
R4
49.9kΩ
NL
DL
COUT
220μF
DL
GND
CSP
CSN
CREFIN
470pF
R5
75kΩ
OUT
REFIN
R6
75kΩ
FB
SKIP
OD
TON
R7
150kΩ
FBLANK
GATE
GND (600kHz)
FLOAT (90μs MIN, FAULT BLANKING)
VOUT (LOW) = VREF
(R5R6+ R6)
VOUT (HIGH) = VREF
[R5 +R6(R6+ R7+ R7)]
VOUT (LOW)
VOUT (HIGH)
POWER GROUND
ANALOG GROUND
*LOWER INPUT VOLTAGES REQUIRE
ADDITIONAL INPUT CAPACITANCE.
SEE TABLE 1 FOR COMPONENT SPECIFICATIONS.
BOLD LINES INDICATE HIGH CURRENT TRACES.
Figure 9. MAX1993 Standard Application Circuit
______________________________________________________________________________________
25
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
Design Procedure
Firmly establish the input voltage range and maximum
load current before choosing a switching frequency
and inductor operating point (ripple-current ratio). The
primary design trade-off lies in choosing a good switching frequency and inductor operating point, and the following four factors dictate the rest of the design:
• Input Voltage Range. The maximum value (VIN(MAX))
must accommodate the worst-case, high AC-adapter
voltage. The minimum value (VIN(MIN)) must account
for the lowest battery voltage after drops due to connectors, fuses, and battery selector switches. If there
is a choice, lower input voltages result in better efficiency.
• Maximum Load Current. There are two values to
consider. The peak load current (ILOAD(MAX)) determines the instantaneous component stresses and filtering requirements and thus drives output capacitor
selection, inductor saturation rating, and the design
of the current-limit circuit. The continuous load current (ILOAD) determines the thermal stresses and
thus drives the selection of input capacitors,
MOSFETs, and other critical heat-contributing components.
• Switching Frequency. This choice determines the
basic trade-off between size and efficiency. The
optimal frequency is largely a function of maximum
input voltage, due to MOSFET switching losses proportional to frequency and VIN2. The optimum frequency is also a moving target, due to rapid
improvements in MOSFET technology that are making higher frequencies more practical.
• Inductor Operating Point. This choice provides
trade-offs: size vs. efficiency and transient response
vs. output ripple. Low inductor values provide better
transient response and smaller physical size but also
result in lower efficiency and higher output ripple
due to increased ripple currents. The minimum practical inductor value is one that causes the circuit to
operate at the edge of critical conduction (where the
inductor current just touches zero with every cycle at
maximum load). Inductor values lower than this grant
no further size-reduction benefit. The optimum operating point is usually found between 20% and 50%
ripple current. When pulse skipping (SKIP low and
light loads), the inductor value also determines the
load-current value at which PFM/PWM switchover
occurs.
26
Inductor Selection
The switching frequency and inductor operating point
determine the inductor value as follows:
L=
VOUT ( VIN − VOUT )
VINfSW ILOAD(MAX)LIR
For example: I LOAD(MAX) = 5A, V IN = 12V, V OUT =
2.5V, fSW = 300kHz, 30% ripple current or LIR = 0.3
L=
2.5V(12V − 2.5)
12V × 300kHz × 5A × 0.3
= 4.40μH
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered
iron is inexpensive and can work well at 200kHz. The
core must be large enough not to saturate at the peak
inductor current (IPEAK):
⎛ LIR ⎞
IPEAK = ILOAD (MAX) ⎜1 +
⎟
⎝
2 ⎠
Most inductor manufacturers provide inductors in standard values, such as 1.0µH, 1.5µH, 2.2µH, 3.3µH, etc.
Also look for nonstandard values, which can provide a
better compromise in LIR across the input voltage range.
If using a swinging inductor (where the no-load inductance decreases linearly with increasing current), evaluate the LIR with properly scaled inductance values.
Transient Response
The inductor ripple current also affects transientresponse performance, especially at low VIN - VOUT differentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The output sag is also a function of the maximum duty
factor, which can be calculated from the on-time and
minimum off-time:
(
L ΔILOAD (MAX)
VSAG =
)
2 ⎡⎛ VOUTK ⎞
⎢⎜
⎢⎣⎝
⎤
+ t OFF(MIN) ⎥
VIN ⎟⎠
⎥⎦
⎡⎛ ( VIN − VOUT )K ⎞
⎤
2COUT VOUT ⎢⎜
⎟ + t OFF(MIN) ⎥
VIN
⎢⎝
⎥
⎠
⎣
⎦
where t OFF(MIN) is the minimum off-time (see the
Electrical Characteristics) and K is from Table 3.
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
CURRENT-SENSE
ACCURACY
INDUCTOR SATURATION
PROTECTION
CURRENT-SENSE POWER LOSS
(EFFICIENCY)
A) Output Current-Sense Resistor
High
Allowed
(highest accuracy)
RSENSE x IOUT2
B) Low-Side Current-Sense Resistor
High
Not allowed
(LSAT = GND)
C) Low-Side MOSFET On-Resistance
Low
Not allowed
(LSAT = GND)
No additional loss
D) Equivalent Inductor DC Resistance
Low
Allowed
No additional loss
METHOD
The overshoot during a full-load to no-load transient due
to stored inductor energy can be calculated as:
VSOAR
(ΔILOAD(MAX) )
≈
2
L
2COUT VOUT
Setting the Current Limit
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The valley of the inductor current occurs at ILOAD(MAX) minus
half the ripple current; therefore:
⎛ ILOAD (MAX)LIR ⎞
ILIM (VAL) > ILOAD (MAX) − ⎜
⎟
2
⎝
⎠
where ILIM(VAL) equals the minimum valley current-limit
threshold voltage divided by the current-sense resistance (RSENSE). For the 50mV default setting, the minimum valley current-limit threshold is 40mV.
Connect ILIM to VCC for a default 50mV valley currentlimit threshold. In adjustable mode, the valley currentlimit threshold is precisely 1/10th the voltage seen at
ILIM. For an adjustable threshold, connect a resistive
divider from REF to analog ground (GND) with ILIM
connected to the center tap. The external 250mV to 2V
adjustment range corresponds to a 25mV to 200mV
valley current-limit threshold. When adjusting the current limit, use 1% tolerance resistors and a divider current of approximately 10µA to prevent significant
inaccuracy in the valley current-limit tolerance.
The current-sense method (Figure 10) and magnitude
determine the achievable current-limit accuracy and
power loss (Table 7). Typically, higher current-sense
voltage limits provide tighter accuracy but also dissipate more power.
⎛ VOUT ⎞
2
⎜1 − V ⎟ × RSENSE × IOUT
⎝
⎠
IN
Most applications employ a valley current-sense voltage (VLIM(VAL)) of 50mV to 100mV, so the sense resistor can be determined by:
RSENSE = VLIM(VAL) / ILIM(VAL)
For the best current-sense accuracy and overcurrent
protection, use a 1% tolerance current-sense resistor
between the inductor and output as shown in Figure
10a. This configuration constantly monitors the inductor
current, allowing accurate valley current-limiting and
inductor saturation protection.
For low output voltage applications that require higher
efficiency, the current-sense resistor can be connected
between the source of the low-side MOSFET (NL) and
power ground (Figure 10b) with CSN connected to the
drain of NL and CSP connected to power ground. In
this configuration, the additional current-sense resistance only dissipates power when NL is conducting
current. Inductor saturation protection must be disabled with this configuration (LSAT = GND) because
the inductor current is only properly sensed when the
low-side MOSFET is turned on.
For high-power applications that do not require highaccuracy current sensing or inductor saturation protection, the MAX1992/MAX1993 can use the low-side
MOSFET’s on-resistance as the current-sense element
(RSENSE = RDS(ON)) by connecting CSN to the drain of
NL and CSP to the source of NL (Figure 10c). Use the
worst-case maximum value for R DS(ON) from the
MOSFET data sheet, and add some margin for the rise
in RDS(ON) with temperature. A good general rule is to
allow 0.5% additional resistance for each °C of temperature rise. Inductor saturation protection must be disabled with this configuration (LSAT = GND) because
the inductor current is properly sensed only when the
low-side MOSFET is turned on.
______________________________________________________________________________________
27
MAX1992/MAX1993
Table 7. Current-Sense Configurations
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
VIN
DH
CIN
RSENSE
L
VOUT
LX
MAX1992
MAX1993
COUT
DL
GND
CONNECT TO
PREFERRED
LSAT SETTING
CSP
LSAT
CSN
A) OUTPUT SERIES RESISTOR SENSING
VIN
CIN
DH
L
VOUT
LX
MAX1992
MAX1993
COUT
DL
CSN
RSENSE
DISABLE
LSAT
CSP
GND
LSAT
B) LOW-SIDE SERIES RESISTOR SENSING
VIN
DH
CIN
L
VOUT
LX
COUT
CSN
MAX1992
MAX1993
DISABLE
LSAT
DL
VIN
CSP
LSAT
DH
GND
CIN
INDUCTOR
RL
L
VOUT
LX
C) LOW-SIDE MOSFET SENSING
MAX1992
MAX1993
DL
GND
CONNECT TO
PREFERRED
LSAT SETTING
COUT
REQ
CEQ
CSP
LSAT
CSN
D) LOSSLESS INDUCTOR SENSING
RBIAS = REQ
Figure 10. Current-Sense Configurations
28
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
L
= CEQ × REQ
RL
where RL is the inductor’s series DC resistance. In this
configuration, the current-sense resistance is equivalent to the inductor’s DC resistance (RSENSE = RL). Use
the worst-case inductance and RL values provided by
the inductor manufacturer, adding some margin for the
inductance drop over temperature and load.
In all cases, ensure an acceptable valley current-limit
threshold voltage and inductor saturation configurations despite inaccuracies in sense resistance values.
When using low-capacity filter capacitors, such as
ceramic capacitors, size is usually determined by the
capacity needed to prevent V SAG and V SOAR from
causing problems during load transients. Generally,
once enough capacitance is added to meet the overshoot requirement, undershoot at the rising load edge
is no longer a problem (see the VSAG and VSOAR equations in the Transient Response section). However, lowcapacity filter capacitors typically have high-ESR zeros
that can affect the overall stability (see the Output
Capacitor Stability Considerations section).
Output Capacitor Stability Considerations
For Quick-PWM controllers, stability is determined by
the value of the ESR zero relative to the switching frequency. The boundary of instability is given by the following equation:
f
fESR ≤ SW
π
Output Capacitor Selection
The output filter capacitor must have low enough equivalent series resistance (ESR) to meet output ripple and
load-transient requirements, yet have high enough ESR
to satisfy stability requirements.
For processor core voltage converters and other applications in which the output is subject to violent load
transients, the output capacitor’s size depends on how
much ESR is needed to prevent the output from dipping too low under a load transient. Ignoring the sag
due to finite capacitance:
RESR ≤
VSTEP
ΔILOAD(MAX)
In applications without large and fast load transients,
the output capacitor’s size often depends on how much
ESR is needed to maintain an acceptable level of output voltage ripple. The output ripple voltage of a stepdown controller equals the total inductor ripple current
multiplied by the output capacitor’s ESR. Therefore, the
maximum ESR required to meet ripple specifications is:
RESR ≤
VRIPPLE
ΔILOAD(MAX)LIR
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as to
the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value (this is true of tantalums, OSCONs, polymers, and other electrolytics).
where
fESR =
1
2πRESR COUT
For a typical 300kHz application, the ESR zero frequency must be well below 95kHz, preferably below 50kHz.
Tantalum and OSCON capacitors in widespread use at
the time of publication have typical ESR zero frequencies of 25kHz. In the design example used for inductor
selection, the ESR needed to support 25mVP-P ripple is
25mV/1.5A = 16.7mΩ. One 220µF/4V Sanyo polymer
(TPE) capacitor provides 15mΩ (max) ESR. This results
in a zero at 48kHz, well within the bounds of stability.
Do not put high-value ceramic capacitors directly
across the feedback sense point without taking precautions to ensure stability. Large ceramic capacitors can
have a high-ESR zero frequency and cause erratic,
unstable operation. However, it is easy to add enough
series resistance by placing the capacitors a couple of
inches downstream from the feedback sense point,
which should be as close as possible to the inductor.
Unstable operation manifests itself in two related but
distinctly different ways: double pulsing and fast-feedback loop instability. Double pulsing occurs due to
noise on the output or because the ESR is so low that
there is not enough voltage ramp in the output voltage
signal. This “fools” the error comparator into triggering
a new cycle immediately after the 400ns minimum offtime period has expired.
______________________________________________________________________________________
29
MAX1992/MAX1993
Alternatively, high-power applications that require
inductor saturation protection can constantly detect the
inductor current by connecting a series RC circuit
across the inductor (Figure 10d) with an equivalent time
constant:
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
Double pulsing is more annoying than harmful, resulting in nothing worse than increased output ripple.
However, it can indicate the possible presence of loop
instability due to insufficient ESR. Loop instability can
result in oscillations at the output after line or load
steps. Such perturbations are usually damped but can
cause the output voltage to rise above or fall below the
tolerance limits.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully
observe the output voltage ripple envelope for overshoot and ringing. It can help to simultaneously monitor
the inductor current with an AC current probe. Do not
allow more than one cycle of ringing after the initial
step-response under/overshoot.
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (IRMS) imposed by the switching currents:
⎛ V
⎞
OUT ( VIN − VOUT )
⎟
IRMS = ILOAD ⎜
⎜
⎟
VIN
⎝
⎠
For most applications, nontantalum chemistries (ceramic, aluminum, or OSCON) are preferred due to their
resistance to power-up surge currents typical of systems with a mechanical switch or connector in series
with the input. If the MAX1992/MAX1993 are operated
as the second stage of a two-stage power conversion
system, tantalum input capacitors are acceptable. In
either configuration, choose a capacitor that has less
than 10°C temperature rise at the RMS input current for
optimal reliability and lifetime.
Power MOSFET Selection
Most of the following MOSFET guidelines focus on the
challenge of obtaining high load-current capability
when using high-voltage (>20V) AC adapters. Low-current applications usually require less attention.
The high-side MOSFET (NH) must be able to dissipate
the resistive losses plus the switching losses at both
VIN(MIN) and VIN(MAX). Ideally, the losses at VIN(MIN)
should be roughly equal to the losses at VIN(MAX), with
lower losses in between. If the losses at VIN(MIN) are
significantly higher, consider increasing the size of NH.
Conversely, if the losses at VIN(MAX) are significantly
higher, consider reducing the size of NH. If VIN does
not vary over a wide range, maximum efficiency is
achieved by selecting a high-side MOSFET (N H) that
has conduction losses equal to the switching losses.
30
Choose a low-side MOSFET (NL) that has the lowest
possible on-resistance (RDS(ON)), comes in a moderatesized package (i.e., 8-pin SO, DPAK, or D2PAK), and is
reasonably priced. Ensure that the MAX1992/MAX1993
DL gate driver can supply sufficient current to support the
gate charge and the current injected into the parasitic
drain-to-gate capacitor caused by the high-side MOSFET
turning on; otherwise, cross-conduction problems can
occur. Switching losses are not an issue for the low-side
MOSFET since it is a zero-voltage switched device when
used in the step-down topology.
Power MOSFET Dissipation
Worst-case conduction losses occur at the duty factor
extremes. For the high-side MOSFET (NH), the worstcase power dissipation due to resistance occurs at
minimum input voltage:
⎛V
⎞
2
PD(NH Re sistive) = ⎜ OUT ⎟ (ILOAD ) RDS(ON)
⎝ VIN ⎠
Generally, use a small high-side MOSFET to reduce
switching losses at high-input voltages. However, the
RDS(ON) required to stay within package power-dissipation limits often limits how small the MOSFET can be.
The optimum efficiency occurs when the switching
losses equal the conduction (RDS(ON)) losses. Highside switching losses do not become an issue until the
input is greater than approximately 15V.
Calculating the power dissipation in high-side
MOSFETs (NH) due to switching losses is difficult, since
it must allow for difficult-to-quantify factors that influence the turn-on and turn-off times. These factors
include the internal gate resistance, gate charge,
threshold voltage, source inductance, and PC board
layout characteristics. The following switching loss calculation provides only a very rough estimate and is no
substitute for breadboard evaluation, preferably including verification using a thermocouple mounted on NH:
PD(NH
(VIN(MAX) )
Switching) =
2
CRSSfSW ILOAD
IGATE
where CRSS is the reverse transfer capacitance of NH,
and IGATE is the peak gate-drive source/sink current
(1A typ).
Switching losses in the high-side MOSFET can become
a heat problem when maximum AC adapter voltages
are applied, due to the squared term in the switchingloss equation (C x VIN2 x fSW).
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
⎡ ⎛V
⎞⎤
2
PD(NL Re sistive) = ⎢1 − ⎜ OUT ⎟ ⎥(ILOAD ) RDS(ON)
V
⎢⎣ ⎝ IN ⎠ ⎥⎦
The absolute worst case for MOSFET power dissipation
occurs under heavy overload conditions that are
greater than ILOAD(MAX) but are not high enough to
exceed the current limit and cause the fault latch to trip.
To protect against this possibility, “overdesign” the circuit to tolerate:
⎛ ILOAD(MAX)LIR ⎞
ILOAD = IVALLEY(MAX) + ⎜
⎟
2
⎝
⎠
where I VALLEY(MAX) is the maximum valley current
allowed by the current-limit circuit, including threshold
tolerance and sense-resistance variation. The
MOSFETs must have a relatively large heatsink to handle the overload power dissipation.
Choose a Schottky diode (DL) with a forward voltage
drop low enough to prevent the low-side MOSFET’s
body diode from turning on during the dead time. As a
general rule, select a diode with a DC current rating
equal to 1/3 the load current. This diode is optional and
can be removed if efficiency is not critical.
Applications Information
Dropout Performance
The output voltage adjustable range for continuous-conduction operation is restricted by the nonadjustable minimum off-time one-shot. For best dropout performance,
use the slower (200kHz) on-time setting. When working
with low input voltages, the duty-factor limit must be calculated using worst-case values for on- and off-times.
Manufacturing tolerances and internal propagation
delays introduce an error to the TON K-factor. This error
is greater at higher frequencies (Table 3). Also, keep in
mind that transient response performance of buck regulators operated too close to dropout is poor, and bulk output capacitance must often be added (see the VSAG
equation in the Design Procedure section).
The absolute point of dropout is when the inductor current ramps down during the minimum off-time (ΔIDOWN)
as much as it ramps up during the on-time (ΔIUP). The
ratio h = ΔIUP/ΔIDOWN indicates the controller’s ability
to slew the inductor current higher in response to
increased load, and must always be greater than 1. As
h approaches 1, the absolute minimum dropout point,
the inductor current cannot increase as much during
each switching cycle, and V SAG greatly increases,
unless additional output capacitance is used.
A reasonable minimum value for h is 1.5, but adjusting
this up or down allows trade-offs between VSAG, output
capacitance, and minimum operating voltage. For a
given value of h, the minimum operating voltage can be
calculated as:
⎡
⎤
⎢
⎥
⎢ VOUT + VDROP1 ⎥
VIN(MIN) = ⎢
⎥ + VDROP2 − VDROP1
⎢ ⎛ h × t OFF(MIN) ⎞ ⎥
⎟⎥
⎢1 − ⎜
K
⎠⎦
⎣ ⎝
where VDROP1 and VDROP2 are the parasitic voltage
drops in the discharge and charge paths (see the OnTime One-Shot (TON) section), tOFF(MIN) is from the
Electrical Characteristics, and K is taken from Table 3.
The absolute minimum input voltage is calculated with h
= 1.
If the calculated VIN(MIN) is greater than the required
minimum input voltage, then operating frequency must
be reduced or output capacitance added to obtain an
acceptable VSAG. If operation near dropout is anticipated, calculate VSAG to be sure of adequate transient
response.
A dropout design example follows:
VOUT = 2.5V
fSW = 300kHz
K = 3.3µs, worst-case KMIN = 3.0µs
tOFF(MIN) = 500ns
VDROP1 = VDROP2 = 100mV
h = 1.5
⎡
⎤
⎢
⎥
2.5V + 0.1V ⎥
⎢
VIN(MIN) = ⎢
+ 0.1V − 0.1V = 3.47V
⎛
⎞⎥
⎢ 1 − ⎜ 1.5 × 500ns ⎟ ⎥
⎢⎣ ⎝ 3.0μs ⎠ ⎥⎦
______________________________________________________________________________________
31
MAX1992/MAX1993
If the high-side MOSFET chosen for adequate RDS(ON)
at low battery voltages becomes extraordinarily hot
when subjected to VIN(MAX), consider choosing another
MOSFET with lower parasitic capacitance.
For the low-side MOSFET (NL) the worst-case power
dissipation always occurs at maximum battery voltage:
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
REF
R4
R1
REFIN
B
R3
C1
R2
MAX1993
A
GND
1kΩ
1000pF
GATE
1kΩ
1000pF
Figure 11. Multiple Output Voltage Settings
Calculating again with h = 1 and the typical K-factor
value (K = 3.3µs) gives the absolute limit of dropout:
⎡
⎤
⎢
⎥
2.5V + 0.1V ⎥
⎢
VIN(MIN) = ⎢
+ 0.1V − 0.1V = 3.06V
⎛
⎞⎥
⎢ 1 − ⎜ 1.5 × 500ns ⎟ ⎥
⎢⎣ ⎝ 3.3μs ⎠ ⎥⎦
Therefore, VIN must be greater than 3.06V, even with very
large output capacitance, and a practical input voltage
with reasonable output capacitance would be 3.47V.
Multiple Output Voltage Settings
(MAX1993 Only)
While the MAX1993 is optimized to work with applications that require two dynamic output voltages, it can
produce three or more output voltages if required by
using discrete logic or a DAC.
Figure 11 shows an application circuit providing four
voltage levels using discrete logic. Switching resistors
in and out of the resistor network changes the voltage
at REFIN. An edge detection circuit is added to generate a 1µs pulse on GATE to trigger the fault-blanking
and forced-PWM operation. When using PWM mode
(SKIP = V CC ), the edge detection circuit is only
required if fault blanking is enabled. Otherwise, leave
OD unconnected.
32
Active Bus Termination (MAX1993 Only)
Active bus termination power supplies generate a voltage rail that tracks a set reference. They are required to
source and sink current. DDR memory architecture
requires active bus termination. In DDR memory architecture, the termination voltage is set at exactly half the
memory supply voltage. Configure the MAX1993 to
generate the termination voltage using a resistordivider at REFIN. In such an application, the MAX1993
must be kept in PWM mode (SKIP = VCC) in order for it
to source and sink current. Figure 12 shows the
MAX1993 configured as a DDR termination regulator.
Connect GATE and FBLANK to GND when unused.
Voltage Positioning
In applications where fast-load transients occur, the
output voltage changes instantly by ESR COUT x
ΔILOAD. Voltage positioning allows the use of fewer output capacitors for such applications, and maximizes
the output voltage AC and DC tolerance window in tight
tolerance applications.
Figure 13 shows the connection of OUT and FB in a
voltage-positioned circuit. In nonvoltage-positioned circuits, the MAX1992/MAX1993 regulate at the output
capacitor. In voltage-positioned circuits, the MAX1992/
MAX1993 regulate on the inductor side of the currentsense resistor. VOUT is reduced to:
VOUT(VPS) = VOUT(NO LOAD) - RSENSEILOAD
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
VCC
VIN
SKIP
DH
CIN
L
1000pF
REFIN
1000pF
RSENSE
V
VTT = DDQ
2
LX
10kΩ
COUT
DL
MAX1993 GND
10kΩ
CSP
OD
CSN
GATE
OUT
FBLANK
FB
VDDQ = DDR MEMORY SUPPLY VOLTAGE
VTT = TERMINATION SUPPLY VOLTAGE
Figure 12. Active Bus Termination
PC Board Layout Guidelines
Layout Procedure
Careful PC board layout is critical to achieve low
switching losses and clean, stable operation. The
switching power stage requires particular attention
(Figure 15). If possible, mount all of the power components on the topside of the board, with their ground terminals flush against one another. Follow these
guidelines for good PC board layout:
• Keep the high-current paths short, especially at the
ground terminals. This practice is essential for stable, jitter-free operation.
• Keep the power traces and load connections short.
This practice is essential for high efficiency. Using
thick copper PC boards (2oz vs. 1oz) can enhance
full-load efficiency by 1% or more. Correctly routing
PC board traces is a difficult task that must be
approached in terms of fractions of centimeters,
where a single milliohm of excess trace resistance
causes a measurable efficiency penalty.
• Minimize current-sensing errors by connecting CSP
and CSN directly across the current-sense resistor
(RSENSE).
• When trade-offs in trace lengths must be made, it is
preferable to allow the inductor-charging path to be
made longer than the discharge path. For example,
it is better to allow some extra distance between the
input capacitors and the high-side MOSFET than to
allow distance between the inductor and the lowside MOSFET or between the inductor and the output filter capacitor.
• Route high-speed switching nodes (BST, LX, DH,
and DL) away from sensitive analog areas (REF, FB,
CSP, and CSN).
1) Place the power components first, with ground terminals adjacent (N L source, C IN , C OUT , and D L
anode). If possible, make all these connections on
the top layer with wide, copper-filled areas.
2) Mount the controller IC adjacent to the low-side
MOSFET, preferably on the backside opposite NL
and NH in order to keep LX, GND, DH, and the DL
gate-drive lines short and wide. The DL and DH gate
traces must be short and wide (50 mils to 100 mils
wide if the MOSFET is 1in from the controller IC) to
keep the driver impedance low and for proper adaptive dead-time sensing.
3) Group the gate-drive components (BST diode and
capacitor, VDD bypass capacitor) together near the
controller IC.
4) Make the DC-DC controller ground connections as
shown in Figures 1 and 9. This diagram can be
viewed as having two separate ground planes:
power ground, where all the high-power components go; and an analog ground plane for sensitive
analog components. The analog ground plane and
power ground plane must meet only at a single point
directly at the IC.
5) Connect the output power planes directly to the output filter capacitor positive and negative terminals
with multiple vias. Place the entire DC-to-DC converter circuit as close to the load as is practical.
______________________________________________________________________________________
33
MAX1992/MAX1993
VDDQ
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
R1
+5V BIAS
SUPPLY
C2
VDD
VCC
DBST
C1
V+
INPUT (VIN)
CIN
BST
NH
DH
CBST
MAX1992
L1
RSENSE
LX
NL
COUT
DL
DL
VOLTAGE-POSITIONED
OUTPUT (VOUT(VPS))
PGND
AGND
CSP
OUT
CSN
FB
VOUT(VPS) = VOUT(NO LOAD) - RSENSEIOUT
Figure 13. Voltage-Positioning Output
CAPACITIVE SOAR
(dV/dt = IOUT/COUT)
VOLTAGE POSITIONING THE OUTPUT
ESR VOLTAGE STEP
(ISTEP x RESR)
A
B
A. CONVENTIONAL CONVERTER
B. VOLTAGE-POSITIONED OUTPUT
VOUT
CAPACITIVE SAG
(dV/dt = IOUT/COUT)
RECOVERY
ILOAD
Figure 14. Voltage-Positioning Transient Response
34
______________________________________________________________________________________
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
MAX1992/MAX1993
KELVIN SENSE VIAS UNDER
THE SENSE RESISTOR
(SEE EVALUATION KIT)
INDUCTOR
VIA TO POWER
GROUND
VIA TO ANALOG
GROUND
COUT
CONNECT GND
AND PGND TO THE
CONTROLLER AT
ONE POINT ONLY,
AS SHOWN
COUT
CIN
OUTPUT
INPUT
GROUND
CONNECT THE
EXPOSED PAD TO
ANALOG GND
MAX1992
Figure 15. PC Board Layout
LX
DH
V+
SKIP
14
13
BST
17
15
DL
18
TOP VIEW
16
Pin Configurations (continued)
VDD
19
12 CSN
GND
20
11 CSP
GATE
21
VCC
22
4
5
6
ILIM
REF
REFIN
PGOOD
7
3
OD
24
LSAT
23
2
FB
8
1
9
TON
OVP/UVP
10 OUT
MAX1993
FBLANK
SHDN
Chip Information
TRANSISTOR COUNT: 2616
PROCESS: BiCMOS
THIN QFN
4mm x 4mm
A "+" SIGN WILL REPLACE THE FIRST PIN INDICATOR ON LEAD-FREE PACKAGES.
______________________________________________________________________________________
35
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
24L QFN THIN.EPS
MAX1992/MAX1993
Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
PACKAGE OUTLINE,
12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm
21-0139
E
1
2
PACKAGE OUTLINE,
12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm
21-0139
E
2
2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
36 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2005 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products, Inc.