Maxim MAX1653ESE High-efficiency, pwm, step-down dc-dc controllers in 16-pin qsop Datasheet

19-1357; Rev 1; 7/98
KIT
ATION
EVALU
E
L
B
AVAILA
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
____________________________Features
♦ 96% Efficiency
The MAX1652–MAX1655 achieve up to 96% efficiency
and deliver up to 10A using a unique Idle Mode™ synchronous-rectified PWM control scheme. These devices
automatically switch between PWM operation at heavy
loads and pulse-frequency-modulated (PFM) operation
at light loads to optimize efficiency over the entire output current range. The MAX1653/MAX1655 also feature
logic-controlled, forced PWM operation for noise-sensitive applications.
All devices operate with a selectable 150kHz/300kHz
switching frequency, which can also be synchronized
to an external clock signal. Both external power switches are inexpensive N-channel MOSFETs, which provide
low resistance while saving space and reducing cost.
The MAX1652 and MAX1654 have an additional feedback pin that permits regulation of a low-cost second
output tapped from a transformer winding. The
MAX1652 provides an additional positive output. The
MAX1654 provides an additional negative output.
The MAX1652–MAX1655 have a 4.5V to 30V input voltage range. The MAX1652/MAX1653/MAX1654’s output
range is 2.5V to 5.5V while the MAX1655’s output range
extends down to 1V. An evaluation kit (MAX1653EVKIT)
is available to speed designs.
♦ 170µA Quiescent Supply Current
♦ Small, 16-Pin QSOP Package
(half the size of a 16-pin narrow SO)
♦ Pin-Compatible with MAX797 (MAX1653/MAX1655)
♦ Output Voltage Down to 1V (MAX1655)
♦ 4.5V to 30V Input Range
♦ 99% Duty Cycle for Lower Dropout
♦ 3µA Logic-Controlled Shutdown
♦ Dual, N-Channel, Synchronous-Rectified Control
♦ Fixed 150kHz/300kHz PWM Switching,
or Synchronized from 190kHz to 340kHz
♦ Programmable Soft Start
♦ Low-Cost Secondary Outputs (MAX1652/MAX1654)
Ordering Information
TEMP. RANGE
PIN-PACKAGE
MAX1652EEE
PART
-40°C to +85°C
16 QSOP
MAX1653ESE
-40°C to +85°C
16 Narrow SO
MAX1653EEE
-40°C to +85°C
16 QSOP
MAX1654EEE
-40°C to +85°C
16 QSOP
MAX1655ESE
-40°C to +85°C
16 Narrow SO
MAX1655EEE
-40°C to +85°C
16 QSOP
Selection Guide
Applications
Notebook Computers
PART
FEEDBACK
VOLTAGE (V)
MAX1652
2.5
Regulates positive Same pin order
secondary voltage as MAX796, but
(such as +12V)
smaller package
MAX1653
2.5
Logic-controlled, Pin-compatible
low-noise mode with MAX797
MAX1654
2.5
Regulates negative Same pin order
secondary voltage as MAX799, but
(such as -5V)
smaller package
PDAs
Cellular Phones
Hand-Held Computers
Handy-Terminals
Mobile Communicators
Distributed Power
Pin Configurations appear at end of data sheet.
Idle Mode is a trademark of Maxim Integrated Products.
MAX1655
1
SPECIAL
FEATURE
Low output voltages (1V to 5.5V);
logic-controlled,
low-noise mode
COMPATIBILITY
Pin compatible
with MAX797
(except for feedback voltage)
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 408-737-7600 ext. 3468.
MAX1652–MAX1655
General Description
The MAX1652–MAX1655 are high-efficiency, pulsewidth-modulated (PWM), step-down DC-DC controllers
in small QSOP packages. The MAX1653/MAX1655 also
come in 16-pin narrow SO packages that are pincompatible upgrades to the popular MAX797. Improvements include higher duty-cycle operation for better
dropout, lower quiescent supply currents for better
light-load efficiency, and an output voltage down to 1V
(MAX1655).
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
ABSOLUTE MAXIMUM RATINGS
V+ to GND ..............................................................-0.3V to +36V
GND to PGND .......................................................-0.3V to +0.3V
VL to GND ................................................................-0.3V to +6V
BST to GND ............................................................-0.3V to +36V
DH to LX .....................................................-0.3V to (BST + 0.3V)
LX to BST..................................................................-6V to +0.3V
SHDN to GND...............................................-0.3V to (V+ + 0.3V)
SYNC, SS, REF, SECFB, SKIP, FB to GND...-0.3V to (VL + 0.3V)
DL to PGND ..................................................-0.3V to (VL + 0.3V)
CSH, CSL to GND ....................................................-0.3V to +6V
VL Short Circuit to GND..............................................Momentary
REF Short Circuit to GND ...........................................Continuous
VL Output Current ...............................................+50mA to -1mA
REF Output Current...............................................+5mA to -1mA
Continuous Power Dissipation (TA = +70°C)
SO (derate 8.70mW/°C above +70°C) .......................696mW
QSOP (derate 8.3mW/°C above +70°C) ....................667mW
Operating Temperature Range
MAX165_E_E ..................................................-40°C to +85°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
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, GND = PGND = 0V, SYNC = REF, IVL = IREF = 0A, TA = 0°C to +85°C, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
30
V
3.3V AND 5V STEP-DOWN CONTROLLERS
Input Supply Range
4.5
5V Output Voltage (CSL)
0 < (CSH - CSL) < 80mV, FB = VL, 6V < V+ < 30V,
includes line and load regulation
4.85
5.06
5.25
V
3.3V Output Voltage (CSL)
0 < (CSH - CSL) < 80mV, FB = 0V, 4.5V < V+ < 30V,
includes line and load regulation
3.20
3.34
3.46
V
Nominal Adjustable Output
Voltage Range
External resistor divider
Feedback Voltage
CSH - CSL = 0V, CSL = FB,
SKIP = 0V, 4.5V < V+ < 30V
MAX1655
Load Regulation
Line Regulation
1
5.5
MAX1652/MAX1653/
MAX1654
2.5
5.5
MAX1655
0.97
1.00
1.03
MAX1652/MAX1653/
MAX1654
2.43
2.50
2.57
0 < (CSH - CSL) < 80mV
2
25mV < (CSH - CSL) < 80mV
0.03
0.06
CSH - CSL, positive
80
100
120
CSH - CSL, negative
-50
-100
-160
SS Source Current
VSS = 0V
2.5
4.0
6.5
SS Fault Sink Current
VSS = 4V
2.0
Falling edge, rising edge, hysteresis = 22mV (MAX1652)
2.45
2.50
2.55
Rising edge, falling edge, hysteresis = 22mV (MAX1654)
-0.05
0
0.05
Current-Limit Voltage
V
%
1.2
6V < V+ < 30V
V
%/V
mV
µA
mA
FLYBACK/PWM CONTROLLER
SECFB Regulation Setpoint
V
INTERNAL REGULATOR AND REFERENCE
VL Output Voltage
SHDN = 2V, 0 < IVL < 25mA, 5.5V < V+ < 30V
4.7
5.0
5.3
V
VL Fault Lockout Voltage
Rising edge, falling edge hysteresis = 50mV
3.8
3.9
4.0
V
VL/CSL Switchover Voltage
Rising edge, falling edge hysteresis = 60mV
4.2
4.5
4.7
V
2
_______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
(V+ = +15V, GND = PGND = 0V, SYNC = REF, IVL = IREF = 0A, TA = 0°C to +85°C, unless otherwise noted.)
MIN
TYP
MAX
UNITS
Reference Output Voltage
PARAMETER
No external load (Note 1)
CONDITIONS
2.46
2.50
2.54
V
Reference Fault Lockout Voltage
Falling edge
2.0
Reference Load Regulation
0 < IREF < 100µA
CSL, CSH Shutdown Leakage
Current
SHDN = 0V, CSL = 5.5V, CSH = 5.5V, V+ = 0 or 30V,
VL = 0V
V+ Shutdown Current
SHDN = 0V, V+ = 30V, CSL = 0 or 5.5V
V+ Off-State Leakage Current
FB = CSH = CSL = 5.5V, VL switched over to CSL
Dropout Power Consumption
Quiescent Power Consumption
2.4
V
5
15
mV
0.1
1
µA
3
7
µA
5
15
µA
V+ = 4.5V, CSH = CSL = 4.0V (Note 2)
1
8
mW
CSH = CSL = 5.5V
1
2
mW
OSCILLATOR AND INPUTS/OUTPUTS
Oscillator Frequency
SYNC = REF
270
300
330
SYNC = 0 or 5V
125
150
175
kHz
SYNC High Pulse Width
200
ns
SYNC Low Pulse Width
200
ns
SYNC Rise/Fall Time
Guaranteed by design, not tested
Oscillator Sync Range
Dropout-Mode Maximum Duty
Cycle
190
SYNC = REF
97
98
SYNC = 0 or 5V
98
99
SYNC
Input High Voltage
SHDN, SKIP
Input Low Voltage
Input Current
200
ns
340
kHz
%
VL - 0.5
V
2.0
SYNC
0.8
SHDN, SKIP
0.5
SHDN, 0 or 30V
3.0
SECFB, 0 or 4V
0.1
SYNC, SKIP
1.0
CSH, CSL, CSH = CSL ≤ 4V
70
FB, FB = REF
V
µA
±0.1
DL Sink/Source Current
DL forced to 2V
1
DH Sink/Source Current
DH forced to 2V, BST - LX = 4.5V
1
A
DL On-Resistance
High or low
1.5
5
Ω
DH On-Resistance
High or low, BST - LX = 4.5V
1.5
5
Ω
A
Note 1: Since the reference uses VL as its supply, V+ line-regulation error is insignificant.
Note 2: At very low input voltages, quiescent supply current may increase due to excessive PNP base current in the VL linear
regulator. This occurs if V+ falls below the preset VL regulation point (5V nominal).
_______________________________________________________________________________________
3
MAX1652–MAX1655
ELECTRICAL CHARACTERISTICS (continued)
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
ELECTRICAL CHARACTERISTICS (continued)
(V+ = +15V, GND = PGND = 0V, SYNC = REF, IVL = IREF = 0A, TA = -40°C to +85°C, unless otherwise noted.) (Note 3)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
4.5
30
V
3.3V and 5V STEP-DOWN CONTROLLERS
Input Supply Range
5V Output Voltage (CSL)
0 < (CSH - CSL) < 70mV, FB = VL, 6V < V+ < 30V,
includes line and load regulation
4.80
5.30
V
3.3V Output Voltage (CSL)
0 < (CSH - CSL) < 70mV, FB = VL, 4.5V < V+ < 30V,
includes line and load regulation
3.16
3.50
V
MAX1655
0.96
1.04
Feedback Voltage
CSH - CSL = 0V, 5V < V+ < 30V,
CSL = FB, SKIP = 0V
MAX1652/MAX1653/
MAX1654
2.40
2.60
V
V
Line Regulation
6V < V+ < 30V
0.06
%/V
Current-Limit Voltage
CSH - CSL, positive
70
130
CSH - CSL, negative
-40
-160
Falling edge, hysteresis = 22mV (MAX1652)
2.40
2.60
Falling edge, hysteresis = 22mV (MAX1654)
-0.08
0.08
mV
FLYBACK/PWM CONTROLLER
SECFB Regulation Setpoint
V
INTERNAL REGULATOR AND REFERENCE
VL Output Voltage
SHDN = 2V, 0 < IVL < 25mA, 5.5V < V+ < 30V
4.7
5.3
V
VL Fault Lockout Voltage
Rising edge, hysteresis = 50mV
3.75
4.05
V
VL/CSL Switchover Voltage
Rising edge, hysteresis = 60mV
4.2
4.7
V
Reference Output Voltage
No external load (Note 1)
2.43
2.57
V
Reference Load Regulation
0 < IREF < 100µA
15
mV
V+ Shutdown Current
SHDN = 0V, V+ = 30V, CSL = 0 or 5.5V
10
µA
V+ Off-State Leakage Current
FB = CSH = CSL = 5.5V, VL switched over to CSL
15
µA
2
mW
Quiescent Power Consumption
OSCILLATOR AND INPUTS/OUTPUTS
Oscillator Frequency
SYNC = REF
250
350
SYNC = 0 or 5V
120
180
SYNC High Pulse Width
250
SYNC Low Pulse Width
250
Oscillator Sync Range
210
Maximum Duty Cycle
SYNC = REF
97
SYNC = 0 or 5V
98
kHz
ns
ns
320
kHz
%
DL On-Resistance
High or low
5
Ω
DH On-Resistance
High or low, BST - LX = 4.5V
5
Ω
Note 3: Specifications from 0°C to -40°C are guaranteed by design, not production tested.
4
_______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
INPUT
4.5V TO 30V
V+
VL
SHDN
MAX1653
MAX1655
DH
BST
SS
LX
REF
DL
+3.3V
OUTPUT
PGND
SYNC
CSH
GND
CSL
SKIP
FB
INPUT
6V TO 30V
V+
SHDN
SECFB
FB
+12V
OUTPUT
VL
MAX1652
DH
BST
+5V
OUTPUT
LX
DL
SS
PGND
REF
CSH
GND
SYNC
CSL
_______________________________________________________________________________________
5
MAX1652–MAX1655
Typical Operating Circuits
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
MAX1652–MAX1655
Typical Operating Circuits (continued)
INPUT
6V TO 30V
FROM
REF
V+
SECFB
SHDN
FB
VL
MAX1654
-5V
OUTPUT
DH
BST
+5V
OUTPUT
LX
DL
SS
PGND
REF
CSH
GND
CSL
SYNC
__________________________________________Typical Operating Characteristics
(Circuit of Figure 1, SKIP = GND, TA = +25°C, unless otherwise noted.)
EFFICIENCY vs.
LOAD CURRENT (3.3V/2A CIRCUIT)
80
V+ = 28V
70
V+ = 12V
V+ = 6V
V+ = 6V
90
EFFICIENCY (%)
EFFICIENCY (%)
90
100
MAX1652 toc02
V+ = 6V
90
EFFICIENCY (%)
100
MAX1652 toc01
100
EFFICIENCY vs.
LOAD CURRENT (3.3V/3A CIRCUIT)
80
V+ = 28V
70
V+ = 12V
MAX1652 toc03
EFFICIENCY vs.
LOAD CURRENT (3.3V/1A CIRCUIT)
80
V+ = 28V
70
V+ = 12V
60
60
60
MAX1653
f = 300kHz
50
0.001
0.01
0.1
LOAD CURRENT (A)
6
1
10
MAX1653
f = 300kHz
50
0.001
0.01
0.1
LOAD CURRENT (A)
1
MAX1653
f = 300kHz
10
50
0.001
0.01
0.1
LOAD CURRENT (A)
_______________________________________________________________________________________
1
10
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
V+ = 28V
70
0.01
V+ = 28V
V+ = 12V
70
0.1
1
V+ = 24V
V+ = 12V
50
0.001
0.01
0.1
1
50
0.001
10
0.01
0.1
1
10
LOAD CURRENT (A)
LOAD CURRENT (A)
IDLE-MODE SUPPLY CURRENT vs.
INPUT VOLTAGE (3.3V/3A CIRCUIT)
PWM-MODE SUPPLY CURRENT vs.
INPUT VOLTAGE (3.3V/3A CIRCUIT)
SHUTDOWN SUPPLY CURRENT
vs. INPUT VOLTAGE
20
15
10
8
5
10
15
20
25
4
0
0
30
6
2
0
0.01
MAX1652 toc08
SHDN = 0V
SUPPLY CURRENT (µA)
SUPPLY CURRENT (mA)
0.1
MAX1653
SKIP = VL
f = 300kHz
NO LOAD
25
10
MAX1652 toc07
MAX1652 toc06
30
5
5
10
15
20
25
0
30
5
10
15
20
25
30
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
REF LOAD-REGULATION ERROR
vs. REF LOAD CURRENT
VL LOAD-REGULATION ERROR
vs. VL LOAD CURRENT
MAX1652 MAXIMUM SECONDARY OUTPUT
CURRENT vs. SUPPLY VOLTAGE
15
10
40
35
30
25
20
15
10
5
5
50
100 150
200 250 300 350
LOAD CURRENT (µA)
400
1200
VSEC > 12.75V,
+5V OUTPUT > 4.75V,
CIRCUIT OF FIGURE 9
900
+5V LOAD = 0A
600
+5V LOAD = 3A
300
0
0
0
1500
MAX1652 toc12
45
MAXIMUM SECONDARY CURRENT (mA)
20
MAX1652 toc011
25
50
LOAD REGULATION ∆V (mV)
MAX1652 toc010
30
LOAD REGULATION ∆V (mV)
70
LOAD CURRENT (A)
1
0
80
MAX1653
f = 300kHz
10
MAX1653
SKIP = 0
NO LOAD
0
V+ = 6V
90
60
MAX1653
f = 300kHz
10
SUPPLY CURRENT (mA)
80
MAX1655
f = 300kHz
60
V+ = 12V
50
0.001
100
EFFICIENCY (%)
80
V+ = 6V
90
EFFICIENCY (%)
EFFICIENCY (%)
90
EFFICIENCY vs.
LOAD CURRENT (1.8V/2.5A CIRCUIT)
MAX1652 toc04a
V+ = 6V
60
100
MAX1652 toc04
100
EFFICIENCY vs.
LOAD CURRENT (5V/3A CIRCUIT)
MAX1652 toc05
EFFICIENCY vs.
LOAD CURRENT (3.3V/5A CIRCUIT)
0
10
20
30
40
50
LOAD CURRENT (mA)
60
70
80
0
5
10
15
20
25
30
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
7
MAX1652–MAX1655
____________________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, SKIP = GND, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(Circuit of Figure 1, SKIP = GND, TA = +25°C, unless otherwise noted.)
DROPOUT VOLTAGE vs.
LOAD CURRENT (3.3V/3A CIRCUIT)
MAX1652 toc09
500
OUTPUT SET FOR 5V (FB = VL)
VOUT > 4.85V
DROPOUT VOLTAGE (mV)
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
400
300
f = 300kHz
200
100
f = 150kHz
0
0.01
0.1
1
10
LOAD CURRENT (A)
PULSE-WIDTH-MODULATION
MODE WAVEFORMS
IDLE-MODE WAVEFORMS
MAX1652-14
MAX1652-13
OUTPUT
VOLTAGE
10mV/div,
AC
LX
VOLTAGE
5V/div
OUTPUT
VOLTAGE
50mV/div,
AC
LX
VOLTAGE
5V/div
TIME (2.5µs)
TIME (1µs)
ILOAD = 300mA, VIN = 10V, 3.3V/3A CIRCUIT
VIN = 6V, 3.3V/3A CIRCUIT
DROPOUT WAVEFORMS
LOAD-TRANSIENT RESPONSE
MAX1652-15
10mV/div,
AC
OUTPUT
VOLTAGE
LX
VOLTAGE
OUTPUT
VOLTAGE
100mV/div,
AC
LOAD
CURRENT
2A/div
5V/div
TIME (5µs)
VIN = 5.1V, NO LOAD, 3.3V/3A CIRCUIT,
SET TO 5V OUTPUT (FB = VL)
8
MAX1652-16
TIME (10µs)
VIN = 15V, 3.3V/3A CIRCUIT
_______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
PIN
NAME
1
SS
FUNCTION
Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF.
SECFB
(MAX1652/
MAX1654)
Secondary Winding Feedback Input. Normally connected to a resistor divider from an auxiliary output.
Don’t leave SECFB unconnected.
• MAX1652: SECFB regulates at VSECFB = 2.50V. Tie to VL if not used.
• MAX1654: SECFB regulates at VSECFB = 0V. Tie to a negative voltage through a high-value currentlimiting resistor (IMAX = 100µA) if not used.
SKIP
(MAX1653/
MAX1655)
Disables pulse-skipping mode when high. Connect to GND for normal use. Don’t leave SKIP unconnected.
With SKIP grounded, the device will automatically change from pulse-skipping operation to full PWM operation when the load current exceeds approximately 30% of maximum (Table 3).
2
3
REF
Reference Voltage Output. Bypass to GND with 0.33µF minimum.
4
GND
Low-Noise Analog Ground and Feedback Reference Point
5
SYNC
Oscillator Synchronization and Frequency Select. Tie to GND or VL for 150kHz operation; tie to REF for
300kHz operation. A high-to-low transition begins a new cycle. Drive SYNC with 0 to 5V logic levels (see the
Electrical Characteristics table for VIH and VIL specifications). SYNC capture range is 190kHz to 340kHz.
6
SHDN
Shutdown Control Input, active low. Logic threshold is set at approximately 1V (VTH of an internal N-channel
MOSFET). Tie SHDN to V+ for automatic start-up.
7
FB
Feedback Input. Regulates at the feedback voltage in adjustable mode. FB is a Dual ModeTM input that also
selects the fixed output voltage settings as follows:
• Connect to GND for 3.3V operation.
• Connect to VL for 5V operation.
• Connect FB to a resistor divider for adjustable mode. FB can be driven with +5V CMOS logic in order to
change the output voltage under system control.
8
CSH
Current-Sense Input, high side. Current-limit level is 100mV referred to CSL.
9
CSL
Current-Sense Input, low side. Also serves as the feedback input in fixed-output modes.
10
V+
Battery Voltage Input (4.5V to 30V). Bypass V+ to PGND close to the IC with a 0.1µF capacitor. Connects to a
linear regulator that powers VL.
11
VL
5V Internal Linear-Regulator Output. VL is also the supply voltage rail for the chip. VL is switched to the output voltage via CSL (VCSL > 4.5V) for automatic bootstrapping. Bypass to GND with 4.7µF. VL can supply up
to 5mA for external loads.
12
PGND
13
DL
Low-Side Gate-Drive Output. Normally drives the synchronous-rectifier MOSFET. Swings from 0V to VL.
14
BST
Boost Capacitor Connection for High-Side Gate Drive (0.1µF)
15
LX
Switching Node (inductor) Connection. Can swing 2V below ground without hazard.
16
DH
High-Side Gate-Drive Output. Normally drives the main buck switch. DH is a floating driver output that swings
from LX to BST, riding on the LX switching-node voltage.
Power Ground
Dual Mode is a trademark of Maxim Integrated Products.
_______________________________________________________________________________________
9
MAX1652–MAX1655
Pin Description
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
Standard Application Circuits
Detailed Description
It’s easy to adapt the basic MAX1653 single-output 3.3V
buck converter (Figure 1) to meet a wide range of applications with inputs up to 30V (limited by choice of external MOSFET). Simply substitute the appropriate
components from Table 1 (candidate suppliers are provided in Table 2). These circuits represent a good set of
trade-offs among cost, size, and efficiency while staying
within the worst-case specification limits for stress-related parameters such as capacitor ripple current.
The MAX1652 family are BiCMOS, switch-mode powersupply controllers designed primarily for buck-topology
regulators in battery-powered applications where high
efficiency and low quiescent supply current are critical.
The parts also work well in other topologies such as
boost, inverting, and Cuk due to the flexibility of their
floating high-speed gate driver. Light-load efficiency is
enhanced by automatic idle-mode operation—a variable-frequency pulse-skipping mode that reduces
losses due to MOSFET gate charge. The step-down
power-switching circuit consists of two N-channel
MOSFETs, a rectifier, and an LC output filter. The output voltage is the average of the AC voltage at the
switching node, which is adjusted and regulated by
changing the duty cycle of the MOSFET switches. The
gate-drive signal to the N-channel high-side MOSFET
must exceed the battery voltage and is provided by a
flying capacitor boost circuit that uses a 100nF capacitor connected to BST.
Don’t change the frequency of these circuits without
first recalculating component values (particularly inductance value at maximum battery voltage).
For a discussion of dual-output circuits using the
MAX1652 and MAX1654, see Figure 9 and the
Secondary Feedback-Regulation Loop section.
INPUT
C1
C7
0.1µF
10
V+
ON/OFF
CONTROL
LOW-NOISE
CONTROL
6
+5V AT
5mA
11
D2
CMPSH-3
VL
DH
SHDN
BST
2
MAX1653
SKIP
LX
DL
PGND
1
CSH
SS
C6
0.01µF
(OPTIONAL)
CSL
GND
FB
7
NOTE: KEEP CURRENT-SENSE
LINES SHORT AND CLOSE
TOGETHER. SEE FIGURE 8.
SYNC
REF
16
C4
4.7µF
Q1
14
15
C3
0.1µF
L1
+3.3V
OUTPUT
R1
C2
13
Q2
D1
GND
OUT
12
8
9
4
3
5
C5
0.33µF
J1
150kHz/300kHz
JUMPER
Figure 1. Standard 3.3V Application Circuit (see Table 1 for Component Values)
10
______________________________________________________________________________________
REF OUTPUT
+2.5V AT 100µA
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
COMPONENT
3.3V at 1A
3.3V at 2A
5V/3.3V at 3A
3.3V at 5A
1.8V at 2.5A
Input Range
4.75V to 28V
4.75V to 28V
4.75V to 28V
4.75V to 28V
4.75V to 22V
Frequency
300kHz
300kHz
300kHz
300kHz
150kHz
Q1 High-Side International Rectifier
MOSFET
1/2 IRF7101
International Rectifier
1/2 IRF7303 or
Fairchild
Semiconductor
1/2 NDS8936
International Rectifier
IRF7403 or
Fairchild
Semiconductor
NDS 8410A
Fairchild
Semiconductor
FDS6680
International Rectifier
1/2 IRF7303 or
Fairchild
Semiconductor
1/2 NDS8936
Q2 Low-Side
MOSFET
International Rectifier
1/2 IRF7101
International Rectifier
1/2 IRF7303 or
Fairchild
Semiconductor
1/2 NDS8936
International Rectifier
IRF7403 or
Fairchild
Semiconductor
NDS 8410A
Fairchild
Semiconductor
FDS6680
International Rectifier
1/2 IRF7303 or
Fairchild
Semiconductor
1/2 NDS8936
C1 Input
Capacitor
10µF, 35V
AVX
TPSD106M035R0300
22µF, 35V
AVX
TPSE226M035R0300
(2) 22µF, 35V
AVX
TPSE226M035R0300
(3) 22µF, 35V
AVX
TPSE226M035R0300
10µF, 25V ceramic
Taiyo Yuden
TMK325F106Z
C2 Output
Capacitor
100µF, 6.3V
AVX TPSC107M006R
220µF, 10V
AVX
TPSE227M010R0100
or Sprague
594D227X001002T
470µF, 6V (for 3.3V)
Kemet
T510X477M006AS
or
(2) 220µF, 10V (for 5V)
AVX
TPSE227M010R011
(3) 330µF, 10V
Sprague
594D337X0010R2T
or
(2) 470µF, 6V
Kemet
T510X477M006AS
470µF, 4V
Sprague
594D477X0004R2T
or
470µF, 6V
Kemet
T510X477M006AS
D1 Rectifier
1N5819 or Motorola
MBR0520L
1N5819 or Motorola
MBRS130LT3
1N5819 or Motorola
MBRS130LT3
1N5821 or Motorola
MBRS340T3
1N5817 or Motorola
MBRS130LT3
R1 Sense
Resistor
70mΩ
33mΩ
25mΩ
12mΩ
Dale WSL-1206-R070F Dale WSL-2010-R033F Dale WSL-2010-R025F
Dale WSL-2512-R012F
or IRC LR2010-01-R070 or IRC LR2010-01-R033 or IRC LR2010-01-R025
L1 Inductor
33µH
Sumida CDR74B-330
15µH
10µH
Sumida CDR105B-150 Sumida CDRH125-100
30mΩ
Dale WSL-2010-R030F
or IRC LR2010-01-R030
4.7µH
15µH
Sumida CDRH127-4R7 Sumida CDRH125-150
Table 2. Component Suppliers
MANUFACTURER
AVX
Central Semiconductor
Coilcraft
Coiltronics
Dale
Fairchild
International Rectifier
IRC
Kemet
Matsuo
Motorola
USA PHONE
803-946-0690
516-435-1110
847-639-6400
561-241-7876
605-668-4131
408-822-2181
310-322-3331
512-992-7900
408-986-0424
714-969-2491
602-303-5454
FACTORY FAX
[Country Code]
[1] 803-626-3123
[1] 516-435-1824
[1] 847-639-1469
[1] 561-241-9339
[1] 605-665-1627
[1] 408-721-1635
[1] 310-322-3332
[1] 512-992-3377
[1] 408-986-1442
[1] 714-960-6492
[1] 602-994-6430
USA PHONE
FACTORY FAX
[Country Code]
Murata
814-237-1431
800-831-9172
[1] 814-238-0490
NIEC
Sanyo
805-867-2555*
619-661-6835
[81] 3-3494-7414
[81] 7-2070-1174
Siliconix
408-988-8000
800-554-5565
[1] 408-970-3950
Sprague
Sumida
Taiyo Yuden
TDK
Transpower Technologies
603-224-1961
847-956-0666
408-573-4150
847-390-4461
702-831-0140
[1] 603-224-1430
[81] 3-3607-5144
[1] 408-573-4159
[1] 847-390-4405
[1] 702-831-3521
MANUFACTURER
* Distributor
______________________________________________________________________________________
11
MAX1652–MAX1655
Table 1. Component Selection for Standard Applications
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
The MAX1652–MAX1655 contain nine major circuit
blocks, which are shown in Figure 2:
PWM Controller Blocks:
• Multi-Input PWM Comparator
• Current-Sense Circuit
• PWM Logic Block
• Dual-Mode Internal Feedback Mux
• Gate-Driver Outputs
• Secondary Feedback Comparator
Bias Generator Blocks:
• +5V Linear Regulator
• Automatic Bootstrap Switchover Circuit
• +2.50V Reference
These internal IC blocks aren’t powered directly from
the battery. Instead, a +5V linear regulator steps down
the battery voltage to supply both the IC internal rail (VL
pin) as well as the gate drivers. The synchronousswitch gate driver is directly powered from +5V VL,
while the high-side-switch gate driver is indirectly powered from VL via an external diode-capacitor boost circuit. An automatic bootstrap circuit turns off the +5V
linear regulator and powers the IC from its output voltage if the output is above 4.5V.
PWM Controller Block
The heart of the current-mode PWM controller is a
multi-input open-loop comparator that sums three signals: output voltage error signal with respect to the reference voltage, current-sense signal, and slope
compensation ramp (Figure 3). The PWM controller is a
direct summing type, lacking a traditional error amplifier and the phase shift associated with it. This directsumming configuration approaches the ideal of
cycle-by-cycle control over the output voltage.
Under heavy loads, the controller operates in full PWM
mode. Each pulse from the oscillator sets the main
PWM latch that turns on the high-side switch for a period determined by the duty factor (approximately
VOUT/VIN). As the high-side switch turns off, the synchronous rectifier latch is set. 60ns later the low-side
switch turns on, and stays on until the beginning of the
next clock cycle (in continuous mode) or until the
inductor current crosses zero (in discontinuous mode).
Under fault conditions where the inductor current
exceeds the 100mV current-limit threshold, the highside latch resets and the high-side switch turns off.
If the load is light in Idle Mode (SKIP = low), the inductor current does not exceed the 25mV threshold set by
the Idle Mode comparator. When this occurs, the controller skips most of the oscillator pulses in order to
reduce the switching frequency and cut back gate12
charge losses. The oscillator is effectively gated off at
light loads because the Idle Mode comparator immediately resets the high-side latch at the beginning of each
cycle, unless the feedback signal falls below the reference voltage level.
When in PWM mode, the controller operates as a fixedfrequency current-mode controller where the duty ratio
is set by the input/output voltage ratio. The currentmode feedback system regulates the peak inductor
current as a function of the output voltage error signal.
Since the average inductor current is nearly the same
as the peak current, the circuit acts as a switch-mode
transconductance amplifier and pushes the second output LC filter pole, normally found in a duty-factorcontrolled (voltage-mode) PWM, to a higher frequency.
To preserve inner-loop stability and eliminate regenerative inductor current “staircasing,” a slope-compensation ramp is summed into the main PWM comparator to
reduce the apparent duty factor to less than 50%.
The relative gains of the voltage- and current-sense
inputs are weighted by the values of current sources
that bias three differential input stages in the main PWM
comparator (Figure 4). The relative gain of the voltage
comparator to the current comparator is internally fixed
at K = 2:1. The resulting loop gain (which is relatively
low) determines the 2% typical load regulation error.
The low loop-gain value helps reduce output filter
capacitor size and cost by shifting the unity-gain
crossover to a lower frequency.
The output filter capacitor C2 sets a dominant pole in
the feedback loop. This pole must roll off the loop gain
to unity before the zero introduced by the output
capacitor’s parasitic resistance (ESR) is encountered
(see Design Procedure section). A 12kHz pole-zero
cancellation filter provides additional rolloff above the
unity-gain crossover. This internal 12kHz lowpass compensation filter cancels the zero due to the filter capacitor’s ESR. The 12kHz filter is included in the loop in
both fixed- and adjustable-output modes.
Synchronous-Rectifier Driver (DL Pin)
Synchronous rectification reduces conduction losses in
the rectifier by shunting the normal Schottky diode with
a low-resistance MOSFET switch. The synchronous rectifier also ensures proper start-up of the boost-gate driver circuit. If you must omit the synchronous power
MOSFET for cost or other reasons, replace it with a
small-signal MOSFET such as a 2N7002.
If the circuit is operating in continuous-conduction mode,
the DL drive waveform is simply the complement of the
DH high-side drive waveform (with controlled dead
time to prevent cross-conduction or “shoot-through”).
______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
MAX1652–MAX1655
BATTERY VOLTAGE
TO
CSL
V+
+5V LINEAR
REGULATOR
4.5V
OUT
VL
+5V AT 5mA
AUXILIARY
OUTPUT
SHDN
BST
SECFB
DH
PWM
LOGIC
LX
MAIN
OUTPUT
DL
+2.50V
REF
+2.50V
AT 100µA
PGND
PWM
COMPARATOR
CSH
REF
CSL
LPF
12kHz
GND
ON/OFF
3.3V FB
5V FB
SHDN
SS
ADJ FB
FB
MAX1652
MAX1653
MAX1654
MAX1655
4V
SYNC
1V
Figure 2. MAX1652–MAX1655 Functional Diagram
______________________________________________________________________________________
13
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
CSH
CSL
2.5V (1V, MAX1655)
FROM
FEEDBACK
DIVIDER
MAIN PWM
COMPARATOR
BST
R
LEVEL
SHIFT
Q
S
DH
LX
SLOPE COMP
OSC
IDLE MODE
COMPARATOR
SKIP
(MAX1653/
MAX1655
ONLY)
25mV
VL
4µA
CURRENT
LIMIT
SHOOTTHROUGH
CONTROL
24R
SS
2.5V
N
SHDN
1R
SYNCHRONOUSRECTIFIER CONTROL
R
-100mV
VL
Q
S
LEVEL
SHIFT
DL
PGND
REF (MAX1652)
GND (MAX1654)
SECFB
COMPARATOR
1µs
SINGLE-SHOT
(NOTE 1)
MAX1652, MAX1654 ONLY
NOTE 1: COMPARATOR INPUT POLARITIES
ARE REVERSED FOR THE MAX1654.
Figure 3. PWM Controller Detailed Block Diagram
14
______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
MAX1652–MAX1655
VL
R1
R2
TO PWM
LOGIC
UNCOMPENSATED
HIGH-SPEED
LEVEL TRANSLATOR
AND BUFFER
OUTPUT DRIVER
FB
I1
I2
I3
REF
CSH
CSL
SLOPE COMPENSATION
Figure 4. Main PWM Comparator Block Diagram
In discontinuous (light-load) mode, the synchronous
switch is turned off as the inductor current falls through
zero. The synchronous rectifier works under all operating conditions, including idle mode. The synchronousswitch timing is further controlled by the secondary
feedback (SECFB) signal in order to improve multipleoutput cross-regulation (see Secondary FeedbackRegulation Loop section).
Internal VL and REF Supplies
An internal regulator produces the 5V supply (VL) that
powers the PWM controller, logic, reference, and other
blocks. This +5V low-dropout linear regulator can supply up to 5mA for external loads, with a reserve of
20mA for gate-drive power. Bypass VL to GND with
4.7µF. Important: VL must not be allowed to exceed
5.5V. Measure VL with the main output fully loaded. If
VL is being pumped up above 5.5V, the probable
cause is either excessive boost-diode capacitance or
excessive ripple at V+. Use only small-signal diodes for
D2 (10mA to 100mA Schottky or 1N4148 are preferred)
and bypass V+ to PGND with 0.1µF directly at the
package pins.
The 2.5V reference (REF) is accurate to ±1.6% over
temperature, making REF useful as a precision system
reference. Bypass REF to GND with 0.33µF minimum.
REF can supply up to 1mA for external loads. However,
if tight-accuracy specs for either V OUT or REF are
essential, avoid loading REF with more than 100µA.
Loading REF reduces the main output voltage slightly,
according to the reference-voltage load regulation
error. In MAX1654 applications, ensure that the SECFB
divider doesn’t load REF heavily.
When the main output voltage is above 4.5V, an internal
P-channel MOSFET switch connects CSL to VL while
simultaneously shutting down the VL linear regulator.
This action bootstraps the IC, powering the internal circuitry from the output voltage, rather than through a linear regulator from the battery. Bootstrapping reduces
power dissipation caused by gate-charge and quiescent losses by providing that power from a 90%-efficient switch-mode source, rather than from a less
efficient linear regulator.
It’s often possible to achieve a bootstrap-like effect,
even for circuits that are set to VOUT < 4.5V, by powering VL from an external-system +5V supply. To achieve
this pseudo-bootstrap, add a Schottky diode between
the external +5V source and VL, with the cathode to the
VL side. This circuit provides a 1% to 2% efficiency
boost and also extends the minimum battery input to
less than 4V. The external source must be in the range
of 4.8V to 5.5V.
Boost High-Side
Gate-Driver Supply (BST Pin)
Gate-drive voltage for the high-side N-channel switch is
generated by a flying-capacitor boost circuit as shown
in Figure 5. The capacitor is alternately charged from
the VL supply and placed in parallel with the high-side
MOSFET’s gate-source terminals.
On start-up, the synchronous rectifier (low-side MOSFET) forces LX to 0V and charges the BST capacitor to
5V. On the second half-cycle, the PWM turns on the
high-side MOSFET by closing an internal switch
between BST and DH. This provides the necessary
enhancement voltage to turn on the high-side switch,
______________________________________________________________________________________
15
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
BATTERY
+5V
VL SUPPLY INPUT
VL
VL
MAX1652
MAX1653
MAX1654
MAX1655
BST
DH
LEVEL
TRANSLATOR
PWM
LX
Oscillator Frequency and
Synchronization (SYNC Pin)
The SYNC input controls the oscillator frequency.
Connecting SYNC to GND or to VL selects 150kHz
operation; connecting SYNC to REF selects 300kHz.
SYNC can also be used to synchronize with an external
5V CMOS clock generator. SYNC has a guaranteed
190kHz to 340kHz capture range.
300kHz operation optimizes the application circuit for
component size and cost. 150kHz operation provides
increased efficiency and improved low-duty factor
operation (see Dropout Operation section).
VL
Dropout Operation
DL
Figure 5. Boost Supply for Gate Drivers
an action that “boosts” the 5V gate-drive signal above
the battery voltage.
Ringing seen at the high-side MOSFET gate (DH) in
discontinuous-conduction mode (light loads) is a natural operating condition caused by the residual energy in
the tank circuit formed by the inductor and stray capacitance at the switching node LX. The gate-driver negative rail is referred to LX, so any ringing there is directly
coupled to the gate-drive output.
Current-Limiting and
Current-Sense Inputs (CSH and CSL)
The current-limit circuit resets the main PWM latch and
turns off the high-side MOSFET switch whenever the
voltage difference between CSH and CSL exceeds
100mV. This limiting is effective for both current flow
directions, putting the threshold limit at ±100mV. The
tolerance on the positive current limit is ±20%, so the
external low-value sense resistor must be sized for
80mV/R1 to guarantee enough load capability, while
components must be designed to withstand continuous
current stresses of 120mV/R1.
For breadboarding purposes or very-high-current applications, it may be useful to wire the current-sense inputs
with a twisted pair rather than PC traces.
16
Dropout (low input-output differential operation) is enhanced by stretching the clock pulse width to increase
the maximum duty factor. The algorithm follows: if the output voltage (VOUT) drops out of regulation without the
current limit having been reached, the controller skips an
off-time period (extending the on-time). At the end of the
cycle, if the output is still out of regulation, another off-time
period is skipped. This action can continue until three offtime periods are skipped, effectively dividing the clock
frequency by as much as four.
The typical PWM minimum off-time is 300ns, regardless
of the operating frequency. Lowering the operating frequency raises the maximum duty factor above 98%.
Low-Noise Mode (SKIP Pin)
The low-noise mode (SKIP = high) is useful for minimizing RF and audio interference in noise-sensitive applications such as audio-equipped systems, cellular
phones, RF communicating computers, and electromagnetic pen-entry systems. See the summary of operating modes in Table 3. SKIP can be driven from an
external logic signal.
The MAX1653 and MAX1655 can reduce interference
due to switching noise by ensuring a constant switching frequency regardless of load and line conditions,
thus concentrating the emissions at a known frequency
outside the system audio or IF bands. Choose an oscillator frequency where harmonics of the switching frequency don’t overlap a sensitive frequency band. If
necessary, synchronize the oscillator to a tight-tolerance external clock generator.
The low-noise mode (SKIP = high) forces two changes
upon the PWM controller. First, it ensures fixed-frequency operation by disabling the minimum-current comparator and ensuring that the PWM latch is set at the
beginning of each cycle, even if the output is in regulation. Second, it ensures continuous inductor current
______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
SHDN
SKIP
LOAD
CURRENT
MODE
NAME
Low
X
X
Shutdown
DESCRIPTION
All circuit blocks
turned off; supply
current = 3µA typ
High
Low
Low,
<10%
Idle
Pulse-skipping;
supply current =
300µA typ at VIN =
10V; discontinuous
inductor current
High
Low
Medium,
<30%
Idle
Pulse-skipping;
continuous inductor
current
High
Low
High,
>30%
PWM
Constant-frequency
PWM; continuous
inductor current
High
High
X
Constant-frequency
PWM regardless of
Low Noise*
load; continuous
(PWM)
inductor current
even at no load
* MAX1652/MAX1654 have no SKIP pin and therefore can’t go
into low-noise mode.
X = Don’t care
put. Adjusting the main output voltage with external
resistors is easy for any of the devices in this family, via
the circuit of Figure 6. The feedback voltage is nominally 2.5 for all family members except the MAX1655,
which has a nominal FB voltage of 1V. The output voltage (given by the formula in Figure 6) should be set
approximately 2% high in order to make up for the
MAX1652’s load-regulation error. For example, if
designing for a 3.0V output, use a resistor ratio that
results in a nominal output voltage of 3.06V. This slight
offsetting gives the best possible accuracy.
Recommended normal values for R5 range from 5kΩ to
100kΩ.
Remote sensing of the output voltage, while not possible in fixed-output mode due to the combined nature of
the voltage- and current-sense input (CSL), is easy to
achieve in adjustable mode by using the top of the
external resistor divider as the remote sense point.
Duty-Factor Limitations for
Low VOUT/VIN Ratios
The MAX1652/MAX1653/MAX1654’s output voltage is
adjustable down to 2.5V and the MAX1655’s output is
adjustable as low as 1V. However, the minimum duty
factor may limit the choice of operating frequency, high
input voltage, and low output voltage.
flow, and thereby suppresses discontinuous-mode
inductor ringing by changing the reverse current-limit
detection threshold from 0 to -100mV, allowing the
inductor current to reverse at very light loads.
In most applications, SKIP should be tied to GND in
order to minimize quiescent supply current. Supply current with SKIP high is typically 10mA to 20mA, depending on external MOSFET gate capacitance and
switching losses.
Forced continuous conduction via SKIP can improve
cross regulation of transformer-coupled multiple-output
supplies. This second function of the SKIP pin produces
a result that is similar to the method of adding secondary regulation via the SECFB feedback pin, but with
much higher quiescent supply current. Still, improving
cross regulation by enabling SKIP instead of building in
SECFB feedback can be useful in noise-sensitive applications, since SECFB and SKIP are mutually exclusive
pins/functions in the MAX1652 family.
Adjustable-Output Feedback
(Dual-Mode FB Pin)
The MAX1652–MAX1655 family has both fixed and
adjustable output voltage modes. For fixed mode, connect FB to GND for a 3.3V output and to VL for a 5V out-
V+
DH
REMOTE
SENSE
LINES
MAX1652
MAX1653
DL
MAX1654
MAX1655
MAIN
OUTPUT
R4
CSH
CSL
FB
GND
R5
R4
VOUT = VREF 1 + –––
R5
WHERE VREF (NOMINAL) = 2.5V (MAX1652–MAX1654)
= 1.0V (MAX1655)
(
)
Figure 6. Adjusting the Main Output Voltage
______________________________________________________________________________________
17
MAX1652–MAX1655
Table 3. Operating-Mode Truth Table
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
With high input voltages, the required duty factor is
approximately (VOUT + VQ2)/ VIN, where VQ2 is the voltage drop across the synchronous rectifier. The
MAX1652’s minimum duty factor is determined by
delays through the feedback network, error comparator,
internal logic gate drivers, and the external MOSFETs,
which typically total 400ns. This delay is about 12% of
the switching period at 300kHz and 6% at 150kHz, limiting the typical minimum duty factor to these values.
Even if the circuit can not attain the required duty factor
dictated by the input and output voltages, the output
voltage will remain in regulation. However, there may be
intermittent or continuous half-frequency operation. This
can cause a factor-of-two increase in output voltage ripple and current ripple, which will increase noise and
reduce efficiency. Choose 150kHz operation for highinput-voltage/low-output-voltage circuits.
Secondary Feedback-Regulation Loop
(SECFB Pin)
A flyback winding control loop regulates a secondary
winding output (MAX1652/MAX1654 only), improving
cross-regulation when the primary is lightly loaded
or when there is a low input-output differential voltage.
If SECFB crosses its regulation threshold, a 1µs oneshot is triggered that extends the low-side switch’s
on-time beyond the point where the inductor current
crosses zero (in discontinuous mode). This causes the
inductor (primary) current to reverse, which in turn pulls
current out of the output filter capacitor and causes the
flyback transformer to operate in the forward mode. The
low impedance presented by the transformer secondary
in the forward mode dumps current into the secondary
output, charging up the secondary capacitor and bringing SECFB back into regulation. The SECFB feedback
loop does not improve secondary output accuracy in
normal flyback mode, where the main (primary) output is
heavily loaded. In this mode, secondary output accuracy is determined (as usual) by the secondary rectifier
drop, turns ratio, and accuracy of the main output voltage. Hence, a linear post-regulator may still be needed
in order to meet tight output accuracy specifications.
The secondary output voltage-regulation point is determined by an external resistor-divider at SECFB. For negative output voltages, the SECFB comparator is
referenced to GND (MAX1654); for positive output voltages, SECFB regulates at the 2.50V reference
(MAX1652). As a result, output resistor-divider connections and design equations for the two device types differ slightly (Figure 7). Ordinarily, the secondary
regulation point is set 5% to 10% below the voltage normally produced by the flyback effect. For example, if the
0.33µF
REF
R3
R3
SECFB
SECFB
1-SHOT
TRIG
1-SHOT
TRIG
R2
2.5V REF
R2
POSITIVE
SECONDARY
OUTPUT
V+
NEGATIVE
SECONDARY
OUTPUT
V+
DH
DH
MAIN
OUTPUT
MAX1652
MAX1654
MAIN
OUTPUT
DL
DL
R2
+VTRIP = VREF 1 + –––
R3
(
)
WHERE VREF (NOMINAL) = 2.5V
R2
-VTRIP = -VREF –––
R3
( )
R3 = 100kΩ (RECOMMENDED)
Figure 7. Secondary-Output Feedback Dividers
18
______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
Shutdown
Shutdown mode (SHDN = 0V) reduces the V+ supply
current to typically 3µA. In this mode, the reference and
VL are inactive. SHDN is a logic-level input, but it can
be safely driven to the full V+ range. Connect SHDN to
V+ for automatic start-up. Do not allow slow transitions
(slower than 0.02V/µs) on SHDN.
Soft-Start Circuit (SS)
Soft-start allows a gradual increase of the internal current-limit level at start-up for the purpose of reducing
22µF, 35V
VIN (6.5V TO 18V)
22µF, 35V
2
7
SECFB
ON/OFF
6
C2
4.7µF
11
VL
FB
V+
DH
SHDN
BST
10
16
+15V AT
250mA
D1
CMPSH
-3A
210k, 1%
C2
4.7µF
Si9410
0.01µF
D2
EC11FS1
14
49.9k, 1%
C3
15µF
2.5V
18V
1/4 W
+5V
AT 3A
0.1µF
MAX1652
1
LX
DL
SS
PGND
0.01µF
(OPTIONAL)
4
GND
CSH
CSL
SYNC
5
15
13
T1
15µH
2.2:1
Si9410
20mΩ
1N5819
220µF
10V
220µF
10V
12
8
22Ω*
9
4700pF*
REF
3
0.33µF
T1 = TRANSPOWER TTI5870
* = OPTIONAL, MAY NOT BE NEEDED
Figure 8. 5V/15V Dual-Output Application Circuit (MAX1652)
______________________________________________________________________________________
19
MAX1652–MAX1655
input surge currents, and perhaps for power-supply
sequencing. In shutdown mode, the soft-start circuit
holds the SS capacitor discharged to ground. When
SHDN goes high, a 4µA current source charges the SS
capacitor up to 3.2V. The resulting linear ramp waveform causes the internal current-limit level to increase
proportionally from 0 to 100mV. The main output capacitor thus charges up relatively slowly, depending on the
SS capacitor value. The exact time of the output rise
depends on output capacitance and load current and is
typically 1ms per nanofarad of soft-start capacitance.
With no SS capacitor connected, maximum current limit
is reached within 10µs.
output voltage as determined by the turns ratio is +15V,
the feedback resistor ratio should be set to produce
about +13.5V; otherwise, the SECFB one-shot might be
triggered unintentionally, causing an unnecessary
increase in supply current and output noise. In negativeoutput (MAX1654) applications, the resistor-divider acts
as a load on the internal reference, which in turn can
cause errors at the main output. Avoid overloading REF
(see the Reference Load-Regulation Error vs. Load
Current graph in the Typical Operating Characteristics).
100kΩ is a good value for R3 in MAX1654 circuits.
Output current on secondary winding applications is
limited at low input voltages. See the MAX1652
Maximum Secondary Output Current vs. Supply Voltage
graph in the Typical Operating Characteristics for data
from the application circuit of Figure 8.
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
__________________Design Procedure
The predesigned standard application circuits (Figure
1 and Table 1) contain ready-to-use solutions for common applications. Use the following design procedure
to optimize the basic schematic for different voltage or
current requirements. Before beginning a design, firmly
establish the following:
VIN(MAX), the maximum input (battery) voltage. This
value should include the worst-case conditions, such
as no-load operation when a battery charger or AC
adapter is connected but no battery is installed.
VIN(MAX) must not exceed 30V. This 30V upper limit is
determined by the breakdown voltage of the BST floating gate driver to GND (36V absolute maximum).
VIN(MIN), the minimum input (battery) voltage. This
should be at full-load under the lowest battery conditions. If VIN(MIN) is less than 4.5V, a special circuit must
be used to externally hold up VL above 4.8V. If the minimum input-output difference is less than 1V, the filter
capacitance required to maintain good AC load regulation increases.
Inductor Value
The exact inductor value isn’t critical and can be
adjusted freely in order to make trade-offs among size,
cost, and efficiency. Although lower inductor values will
minimize size and cost, they will also reduce efficiency
due to higher peak currents. To permit use of the physically smallest inductor, lower the inductance until the
circuit is operating at the border between continuous
and discontinuous modes. Reducing the inductor value
even further, below this crossover point, results in discontinuous-conduction operation even at full load. This
helps reduce output filter capacitance requirements but
causes the core energy storage requirements to
increase again. On the other hand, higher inductor values will increase efficiency, but at some point resistive
losses due to extra turns of wire will exceed the benefit
gained from lower AC current levels. Also, high inductor values affect load-transient response; see the VSAG
equation in the Low-Voltage Operation section.
The following equations are given for continuous-conduction operation since the MAX1652 family is mainly intended for high-efficiency, battery-powered applications. See
Appendix A in Maxim’s Battery Management and DC-DC
Converter Circuit Collection for crossover point and discontinuous-mode equations. Discontinuous conduction
doesn’t affect normal Idle Mode operation.
Three key inductor parameters must be specified:
inductance value (L), peak current (IPEAK), and DC
resistance (RDC). The following equation includes a
constant LIR, which is the ratio of inductor peak-to-peak
20
AC current to DC load current. A higher value of LIR
allows smaller inductance, but results in higher losses
and ripple. A good compromise between size and losses is found at a 30% ripple current to load current ratio
(LIR = 0.3), which corresponds to a peak inductor current 1.15 times higher than the DC load current.
VOUT (VIN(MAX) - VOUT)
L = ———————————
VIN(MAX) x f x IOUT x LIR
where:
f = switching frequency, normally 150kHz or
300kHz
IOUT = maximum DC load current
LIR = ratio of AC to DC inductor current,
typically 0.3
The peak inductor current at full load is 1.15 x IOUT if
the above equation is used; otherwise, the peak current
can be calculated by:
IPEAK = ILOAD +
VOUT (VIN(MAX) - VOUT )
2 x f x L x VIN(MAX)
The inductor’s DC resistance is a key parameter for efficiency performance and must be ruthlessly minimized,
preferably to less than 25mΩ at IOUT = 3A. If a standard off-the-shelf inductor is not available, choose a
core with an LI2 rating greater than L x IPEAK2 and wind
it with the largest diameter wire that fits the winding
area. For 300kHz applications, ferrite core material is
strongly preferred; for 150kHz applications, Kool-mu
(aluminum alloy) and even powdered iron can be
acceptable. If light-load efficiency is unimportant (in
desktop 5V-to-3V applications, for example) then lowpermeability iron-powder cores may be acceptable,
even at 300kHz. For high-current applications, shielded
core geometries (such as toroidal or pot core) help
keep noise, EMI, and switching-waveform jitter low.
Current-Sense Resistor Value
The current-sense resistor value is calculated according to the worst-case, low-current-limit threshold voltage
(from the Electrical Characteristics table) and the peak
inductor current. The continuous-mode peak inductorcurrent calculations that follow are also useful for sizing
the switches and specifying the inductor-current saturation ratings. In order to simplify the calculation, ILOAD
may be used in place of IPEAK if the inductor value has
been set for LIR = 0.3 or less (high inductor values)
and 300kHz operation is selected. Low-inductance
resistors, such as surface-mount metal-film resistors,
are preferred.
80mV
RSENSE = ————
IPEAK
______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
IRMS = ILOAD x
VOUT (VIN − VOUT)
VIN
IRMS = ILOAD / 2 when VIN is 2 x VOUT
Output Filter Capacitor Value
The output filter capacitor values are determined by the
ESR, capacitance, and voltage rating requirements.
Electrolytic and tantalum capacitors are generally chosen by voltage rating and ESR specifications, as they
will generally have more output capacitance than is
required for AC stability. Use only specialized low-ESR
capacitors intended for switching-regulator applications,
such as AVX TPS, Sprague 595D, Sanyo OS-CON, or
Nichicon PL series. To ensure stability, the capacitor
must meet both minimum capacitance and maximum
ESR values as given in the following equations:
VREF (1 + VOUT / VIN(MIN))
COUT > ––––––––––––––––———–––
VOUT x RSENSE x f
RSENSE x VOUT
RESR < ————————
VREF
(can be multiplied by 1.5, see note below)
These equations are “worst-case” with 45 degrees of
phase margin to ensure jitter-free fixed-frequency operation and provide a nicely damped output response for
zero to full-load step changes. Some cost-conscious
designers may wish to bend these rules by using less
expensive (lower quality) capacitors, particularly if the
load lacks large step changes. This practice is tolerable if
some bench testing over temperature is done to verify
acceptable noise and transient response.
There is no well-defined boundary between stable and
unstable operation. As phase margin is reduced, the
first symptom is a bit of timing jitter, which shows up as
blurred edges in the switching waveforms where the
scope won’t quite sync up. Technically speaking, this
(usually) harmless jitter is unstable operation, since the
switching frequency is now nonconstant. As the capacitor quality is reduced, the jitter becomes more pronounced and the load-transient output voltage
waveform starts looking ragged at the edges.
Eventually, the load-transient waveform has enough
ringing on it that the peak noise levels exceed the
allowable output voltage tolerance. Note that even with
zero phase margin and gross instability present, the
output voltage noise never gets much worse than IPEAK
x RESR (under constant loads, at least).
Note: Designers of RF communicators or other noisesensitive analog equipment should be conservative
and stick to the ESR guidelines. Designers of notebook
computers and similar commercial-temperature-range
digital systems can multiply the RESR value by a factor
of 1.5 without hurting stability or transient response.
The output voltage ripple is usually dominated by the
ESR of the filter capacitor and can be approximated as
IRIPPLE x RESR. There is also a capacitive term, so the
full equation for ripple in the continuous mode is
VNOISE(p-p) = IRIPPLE x [RESR + 1 / (8 x f x COUT)]. In
Idle Mode, the inductor current becomes discontinuous
with high peaks and widely spaced pulses, so the noise
can actually be higher at light load compared to full load.
In Idle Mode, the output ripple can be calculated as:
0.025 x RESR
VNOISE(p-p) = —————— +
RSENSE
(0.025)2 x L x [1 / VOUT + 1 / (VIN - VOUT)]
———————————————————
(RSENSE)2 x COUT
Transformer Design
(MAX1652/MAX1654 Only)
Buck-plus-flyback applications, sometimes called “coupled-inductor” topologies, use a transformer to generate
multiple output voltages. The basic electrical design is a
simple task of calculating turns ratios and adding the
power delivered to the secondary in order to calculate the
current-sense resistor and primary inductance. However,
extremes of low input-output differentials, widely different
output loading levels, and high turns ratios can complicate the design due to parasitic transformer parameters
such as interwinding capacitance, secondary resistance,
and leakage inductance. For examples of what is possible with real-world transformers, see the graphs of
Maximum Secondary Current vs. Input Voltage in the
Typical Operating Characteristics.
______________________________________________________________________________________
21
MAX1652–MAX1655
Input Capacitor Value
Place a small ceramic capacitor (0.1µF) between V+ and
GND, close to the device. Also, connect a low-ESR bulk
capacitor directly to the drain of the high-side MOSFET.
Select the bulk input filter capacitor according to input
ripple-current requirements and voltage rating, rather
than capacitor value. Electrolytic capacitors that have
low enough effective series resistance (ESR) to meet the
ripple-current requirement invariably have more than
adequate capacitance values. Ceramic capacitors or
low-ESR aluminum-electrolytic capacitors such as Sanyo
OS-CON or Nichicon PL are preferred. Tantalum types
are also acceptable but may be less tolerant of high
input surge currents. RMS input ripple current is determined by the input voltage and load current, with the
worst possible case occurring at VIN = 2 x VOUT:
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
Power from the main and secondary outputs is lumped
together to obtain an equivalent current referred to the
main output voltage (see Inductor Value section for definitions of parameters). Set the value of the currentsense resistor at 80mV / ITOTAL.
PTOTAL = the sum of the output power from
all outputs
ITOTAL = PTOTAL / VOUT = the equivalent output
current referred to VOUT
VOUT (VIN(MAX) - VOUT)
L(primary) = —————————————
VIN(MAX) x f x ITOTAL x LIR
VSEC + VFWD
Turns Ratio N = ——————————————
VOUT(MIN) + VRECT + VSENSE
where: VSEC is the minimum required rectified
secondary-output voltage
VFWD is the forward drop across the
secondary rectifier
VOUT(MIN) is the minimum value of the main
output voltage (from the Electrical
Characteristics)
VRECT is the on-state voltage drop across the
synchronous-rectifier MOSFET
VSENSE is the voltage drop across the sense
resistor
In positive-output (MAX1652) applications, the transformer secondary return is often referred to the main
output voltage rather than to ground in order to reduce
the needed turns ratio. In this case, the main output
voltage must first be subtracted from the secondary
voltage to obtain VSEC.
______Selecting Other Components
MOSFET Switches
The two high-current N-channel MOSFETs must be
logic-level types with guaranteed on-resistance specifications at VGS = 4.5V. Lower gate threshold specs are
better (i.e., 2V max rather than 3V max). Drain-source
breakdown voltage ratings must at least equal the maximum input voltage, preferably with a 20% derating
factor. The best MOSFETs will have the lowest on-resistance per nanocoulomb of gate charge. Multiplying
RDS(ON) x QG provides a meaningful figure by which to
compare various MOSFETs. Newer MOSFET process
technologies with dense cell structures generally give
the best performance. The internal gate drivers can tolerate more than 100nC total gate charge, but 70nC is a
more practical upper limit to maintain best switching
times.
22
In high-current applications, MOSFET package power
dissipation often becomes a dominant design factor.
I2R losses are distributed between Q1 and Q2 according to duty factor (see the equations below). Switching
losses affect the upper MOSFET only, since the
Schottky rectifier clamps the switching node before the
synchronous rectifier turns on. Gate-charge losses are
dissipated by the driver and don’t heat the MOSFET.
Ensure that both MOSFETs are within their maximum
junction temperature at high ambient temperature by
calculating the temperature rise according to package
thermal-resistance specifications. The worst-case dissipation for the high-side MOSFET occurs at the minimum
battery voltage, and the worst-case for the low-side
MOSFET occurs at the maximum battery voltage.
PD (upper FET) = ILOAD2 x RDS(ON) x DUTY
(
)
VIN x CRSS
+ VIN x ILOAD x f x ––––––––––– +20ns
IGATE
PD (lower FET) = ILOAD2 x RDS(ON) x (1 - DUTY)
DUTY = (VOUT + VQ2) / (VIN - VQ1 + VQ2)
where the on-state voltage drop VQ_ = ILOAD x RDS(ON)
CRSS = MOSFET reverse transfer capacitance
IGATE = DH driver peak output current capability
(1A typically)
20ns = DH driver inherent rise/fall time
Under output short circuit, the synchronous-rectifier
MOSFET suffers extra stress and may need to be oversized if a continuous DC short circuit must be tolerated.
During short circuit, Q2’s duty factor can increase to
greater than 0.9 according to:
Q2 DUTY (short circuit) = 1 - [VQ2 / (VIN(MAX) - VQ1 + VQ2)]
where the on-state voltage drop VQ = (120mV / RSENSE)
x RDS(ON).
Rectifier Diode D1
Rectifier D1 is a clamp that catches the negative inductor swing during the 60ns dead time between turning
off the high-side MOSFET and turning on the low-side.
D1 must be a Schottky type in order to prevent the
lossy parasitic MOSFET body diode from conducting. It
is acceptable to omit D1 and let the body diode clamp
the negative inductor swing, but efficiency will drop one
or two percent as a result. Use an MBR0530 (500mA
rated) type for loads up to 1.5A, a 1N5819 type for
loads up to 3A, or a 1N5822 type for loads up to 10A.
D1’s rated reverse breakdown voltage must be at least
equal to the maximum input voltage, preferably with a
20% derating factor.
______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
Rectifier Diode D3
(Transformer Secondary Diode)
The secondary diode in coupled-inductor applications
must withstand high flyback voltages greater than 60V,
which usually rules out most Schottky rectifiers.
Common silicon rectifiers such as the 1N4001 are also
prohibited, as they are far too slow. This often makes
fast silicon rectifiers such as the MURS120 the only
choice. The flyback voltage across the rectifier is related to the VIN-VOUT difference according to the transformer turns ratio:
VFLYBACK = VSEC + (VIN - VOUT) x N
where: N is the transformer turns ratio SEC/PRI
VSEC is the maximum secondary DC output voltage
VOUT is the primary (main) output voltage
Subtract the main output voltage (VOUT) from VFLYBACK
in this equation if the secondary winding is returned to
VOUT and not to ground. The diode reverse breakdown
rating must also accommodate any ringing due to leakage inductance. D3’s current rating should be at least
twice the DC load current on the secondary output.
_____________Low-Voltage Operation
Low input voltages and low input-output differential voltages each require some extra care in the design. Low
absolute input voltages can cause the VL linear regulator to enter dropout, and eventually shut itself off. Low
input voltages relative to the output (low VIN-VOUT differential) can cause bad load regulation in multi-output flyback applications. See Transformer Design section.
Finally, low VIN-VOUT differentials can also cause the
output voltage to sag when the load current changes
abruptly. The amplitude of the sag is a function of inductor value and maximum duty factor (DMAX an Electrical
Characteristics parameter, 98% guaranteed over temperature at f = 150kHz) as follows:
(ISTEP)2 x L
VSAG = ———————————————
2 x COUT x (VIN(MIN) x DMAX - VOUT)
The cure for low-voltage sag is to increase the value of
the output capacitor. For example, at VIN = 5.5V, VOUT
= 5V, L = 10µH, f = 150kHz, a total capacitance of
660µF will prevent excessive sag. Note that only the
capacitance requirement is increased and the ESR
requirements don’t change. Therefore, the added
capacitance can be supplied by a low-cost bulk
capacitor in parallel with the normal low-ESR capacitor.
Table 4 summarizes low-voltage operational issues.
Table 4. Low-Voltage Troubleshooting
SYMPTOM
CONDITION
ROOT CAUSE
SOLUTION
Sag or droop in VOUT
under step load change
Low VIN-VOUT differential, Limited inductor-current slew
<1V
rate per cycle.
Increase bulk output capacitance per
formula above. Reduce inductor value.
Dropout voltage is too
high (VOUT follows VIN as
VIN decreases)
Low VIN-VOUT differential, Maximum duty-cycle limits
<0.5V
exceeded.
Reduce f to 150kHz. Reduce MOSFET
on-resistance and coil DCR.
Unstable—jitters between
two distinct duty factors
Low VIN-VOUT differential, Normal function of internal low<0.5V
dropout circuitry.
Increase the minimum input voltage or
ignore.
Secondary output won’t
support a load
Not enough duty cycle left to
Reduce f to 150kHz. Reduce secondary
Low VIN-VOUT differential, initiate forward-mode operation.
impedances—use Schottky if possible.
VIN < 1.3 x VOUT(main)
Small AC current in primary can’t
Stack secondary winding on main output.
store energy for flyback operation.
High supply current,
poor efficiency
Low input voltage, <5V
VL linear regulator is going into
dropout and isn’t providing
good gate-drive levels.
Won’t start under load or
quits before battery is
completely dead
Low input voltage, <4.5V
VL output is so low that it hits the Supply VL from an external source other
VL UVLO threshold at 4.2V max. than VBATT, such as the system 5V supply.
Use a small 20mA Schottky diode for
boost diode D2. Supply VL from an
external source.
______________________________________________________________________________________
23
MAX1652–MAX1655
Boost-Supply Diode D2
A 10mA to 100mA Schottky diode or signal diode such
as a 1N4148 works well for D2 in most applications. If
the input voltage can go below 6V, use a Schottky
diode for slightly improved efficiency and dropout characteristics. Don’t use large power diodes such as
1N5817 or 1N4001, since high junction capacitance
can cause VL to be pumped up to excessive voltages.
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
__________Applications Information
Heavy-Load Efficiency Considerations
The major efficiency loss mechanisms under loads (in
the usual order of importance) are:
• P(I2R), I2R losses
• P(gate), gate-charge losses
• P(diode), diode-conduction losses
• P(tran), transition losses
• P(cap), capacitor ESR losses
• P(IC), losses due to the operating supply current
of the IC
Inductor-core losses are fairly low at heavy loads
because the inductor’s AC current component is small.
Therefore, they aren’t accounted for in this analysis.
Ferrite cores are preferred, especially at 300kHz, but
powdered cores such as Kool-mu can work well.
Efficiency = POUT / PIN x 100%
= POUT / (POUT + PTOTAL) x 100%
PTOTAL = P(I2R) + P(gate) + P(diode) + P(tran) +
P(cap) + P(IC)
P(I2R) = (ILOAD)2 x (RDC + RDS(ON) + RSENSE)
where RDC is the DC resistance of the coil, RDS(ON) is
the MOSFET on-resistance, and RSENSE is the currentsense resistor value. The RDS(ON) term assumes identical MOSFETs for the high- and low-side switches
because they time-share the inductor current. If the
MOSFETs aren’t identical, their losses can be estimated by averaging the losses according to duty factor.
P(gate) = gate-driver loss = qG x f x VL
where VL is the MAX1652 internal logic supply voltage
(5V), and qG is the sum of the gate-charge values for
low- and high-side switches. For matched MOSFETs,
qG is twice the data sheet value of an individual
MOSFET. If VOUT is set to less than 4.5V, replace VL in
this equation with VBATT. In this case, efficiency can be
improved by connecting VL to an efficient 5V source,
such as the system +5V supply.
P(diode) = diode conduction losses
= ILOAD x VFWD x tD x f
where tD is the diode conduction time (120ns typ) and
VFWD is the forward voltage of the Schottky.
PD(tran) = transition loss =
VBATT x CRSS
VBATT x ILOAD x f x ——————— + 20ns
IGATE
(
)
where CRSS is the reverse transfer capacitance of the
high-side MOSFET (a data sheet parameter), IGATE is
24
the DH gate-driver peak output current (1A typ), and
20ns is the rise/fall time of the DH driver.
P(cap) = input capacitor ESR loss = (IRMS)2 x RESR
where IRMS is the input ripple current as calculated in the
Input Capacitor Value section of the Design Procedure.
Light-Load Efficiency Considerations
Under light loads, the PWM operates in discontinuous
mode, where the inductor current discharges to zero at
some point during the switching cycle. This causes the
AC component of the inductor current to be high compared to the load current, which increases core losses
and I2R losses in the output filter capacitors. Obtain best
light-load efficiency by using MOSFETs with moderate
gate-charge levels and by using ferrite, MPP, or other
low-loss core material. Avoid powdered iron cores; even
Kool-mu (aluminum alloy) is not as good as ferrite.
__PC Board Layout Considerations
Good PC board layout is required to achieve specified
noise, efficiency, and stability performance. The PC
board layout artist must be provided with explicit
instructions, preferably a pencil sketch of the placement of power switching components and high-current
routing. See the evaluation kit PC board layouts in the
MAX1653, MAX796, and MAX797 EV kit manuals for
examples. A ground plane is essential for optimum performance. In most applications, the circuit will be located on a multilayer board, and full use of the four or
more copper layers is recommended. Use the top layer
for high-current connections, the bottom layer for quiet
connections (REF, SS, GND), and the inner layers for
an uninterrupted ground plane. Use the following stepby-step guide.
1) Place the high-power components (C1, C2, Q1, Q2,
D1, L1, and R1) first, with their grounds adjacent.
Priority 1: Minimize current-sense resistor trace
lengths (see Figure 9).
Priority 2: Minimize ground trace lengths in the
high-current paths (discussed below).
Priority 3: Minimize other trace lengths in the highcurrent paths. Use >5mm wide traces.
C1 to Q1: 10mm max length. D1 anode to
Q2: 5mm max length LX node (Q1
source, Q2 drain, D1 cathode, inductor):
15mm max length
Ideally, surface-mount power components are
butted up to one another with their ground terminals
almost touching. These high-current grounds (C1-,
C2-, source of Q2, anode of D1, and PGND) are
then connected to each other with a wide filled zone
______________________________________________________________________________________
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
2) Place the IC and signal components. Keep the main
switching node (LX node) away from sensitive analog components (current-sense traces and REF and
SS capacitors). Placing the IC and analog components on the opposite side of the board from the
power-switching node is desirable. Important: the
IC must be no farther than 10mm from the currentsense resistor. Keep the gate-drive traces (DH, DL,
and BST) shorter than 20mm and route them away
from CSH, CSL, REF, and SS.
3) Employ a single-point star ground where the input
ground trace, power ground (subground plane),
and normal ground plane all meet at the output
ground terminal of the supply.
MAX1652–MAX1655
of top-layer copper, so that they don’t go through
vias. The resulting top-layer “sub-ground-plane” is
connected to the normal inner-layer ground plane at
the output ground terminals. This ensures that the
analog GND of the IC is sensing at the output terminals of the supply, without interference from IR
drops and ground noise. Other high-current paths
should also be minimized, but focusing ruthlessly
on short ground and current-sense connections
eliminates about 90% of all PC board layout difficulties. See the evaluation kit PC board layouts for
examples.
FAT, HIGH-CURRENT TRACES
MAIN CURRENT PATH
SENSE RESISTOR
MAX1652
MAX1653
MAX1654
MAX1655
Figure 9. Kelvin Connections for the Current-Sense Resistor
Pin Configurations
TOP VIEW
SS 1
16 DH
SS 1
16 DH
(SECFB) SKIP 2
15 LX
SKIP 2
15 LX
14 BST
REF 3
REF 3
GND 4
SYNC 5
SHDN 6
MAX1652
MAX1653
MAX1654
MAX1655
13 DL
SYNC 5
12 PGND
11 VL
SHDN 6
11 VL
FB 7
10 V+
9
QSOP
13 DL
12 PGND
10 V+
FB 7
CSH 8
GND 4
14 BST
MAX1653
MAX1655
CSL
CSH 8
9
CSL
Narrow SO
( ) ARE FOR MAX1652/ MAX1654.
______________________________________________________________________________________
25
___________________Chip Information
TRANSISTOR COUNT: 1990
________________________________________________________Package Information
QSOP.EPS
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
26
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High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
SOICN.EPS
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27
MAX1652–MAX1655
___________________________________________Package Information (continued)
MAX1652–MAX1655
High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
NOTES
28
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