MAXIM MAX668

19-4778; Rev 0a; 8/98
L
MANUA
ION KIT HEET
T
A
U
L
EVA
TA S
WS DA
FOLLO
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
The MAX668/MAX669 constant-frequency, pulse-widthmodulating (PWM), current-mode DC-DC controllers are
designed for a wide range of DC-DC conversion applications including step-up, SEPIC, flyback, and isolatedoutput configurations. Power levels of 20W or more can
be controlled with conversion efficiencies of over 90%.
The 1.8V to 28V input voltage range supports a wide
range of battery and AC-powered inputs. An advanced
BiCMOS design features low operating current (220µA),
adjustable operating frequency (100kHz to 500kHz),
soft-start, and a SYNC input allowing the MAX668/
MAX669 oscillator to be locked to an external clock.
DC-DC conversion efficiency is optimized with a low
100mV current-sense voltage as well as with Maxim’s
proprietary Idle Mode™ control scheme. The controller
operates in PWM mode at medium and heavy loads for
lowest noise and optimum efficiency, then pulses only as
needed (with reduced inductor current) to reduce operating current and maximize efficiency under light loads.
A logic-level shutdown input is also included, reducing
supply current to 3.5µA.
The MAX669, optimized for low input voltages with a
guaranteed start-up voltage of 1.8V, requires bootstrapped operation (IC powered from boosted output). It
supports output voltages up to 28V. The MAX668 operates with inputs as low as 3V and can be connected in
either a bootstrapped or non-bootstrapped (IC powered
from input supply or other source) configuration. When
not bootstrapped, it has no restriction on output voltage.
Both ICs are available in an extremely compact 10-pin
µMAX package.
Features
♦ 1.8V Minimum Start-Up Voltage (MAX669)
♦ Wide Input Voltage Range (1.8V to 28V)
♦ Tiny 10-Pin µMAX Package
♦ Current-Mode PWM and Idle Mode™ Operation
♦ Efficiency over 90%
♦ Adjustable 100kHz to 500kHz Oscillator or
SYNC Input
♦ 220µA Quiescent Current
♦ Logic-Level Shutdown
♦ Soft-Start
Applications
Cellular Telephones
Telecom Hardware
LANs and Network Systems
POS Systems
Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
MAX668EUB
-40°C to +85°C
10 µMAX
MAX669EUB
-40°C to +85°C
10 µMAX
Idle Mode is a trademark of Maxim Integrated Products.
Pin Configuration
Typical Operating Circuit
VIN = 1.8V to 28V
TOP VIEW
VOUT = 28V
SYNC/
SHDN
VCC
FREQ
EXT
CS+
MAX669
PGND
LDO
FB
REF
LDO 1
FREQ
10 SYNC/SHDN
2
MAX668
MAX669
9
VCC
GND
3
8
EXT
REF
4
7
PGND
FB
5
6
CS+
µMAX
GND
________________________________________________________________ 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 1-800-835-8769.
MAX668/MAX669
General Description
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
ABSOLUTE MAXIMUM RATINGS
VCC to GND ..........................................................-0.3V to +30V
PGND to GND....................................................................±0.3V
SYNC/SHDN to GND .............................................-0.3V to +30V
EXT, REF to GND.....................................-0.3V to (VLDO + 0.3V)
LDO, FREQ, FB, CS+ to GND ................................ -0.3V to +6V
LDO Output Current...........................................-1mA to +20mA
REF Output Current..............................................-1mA to +1mA
LDO Short Circuit to GND .........................................Momentary
REF Short Circuit to GND ..........................................Continuous
Continuous Power Dissipation (TA = +70°C)
10-Pin µMAX (derate 5.6mW/°C above +70°C) ..........444mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering,10sec) ..............................+300°C
ELECTRICAL CHARACTERISTICS
(VCC = LDO = +5V, ROSC = 200kΩ, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
PWM
PWM Controller
CONTROLLER
Input Voltage Range, VCC
MAX668
3
28
MAX669
1.8
28
2.7
5.5
V
1.275
V
Input Voltage Range with VCC Tied to LDO
FB Threshold
1.225
1.250
V
FB Threshold Load Regulation
Typically 0.013% per mV on CS+;
VCS+ range is 0 to 100mV for 0 to full
load current.
0.013
%/mV
FB Threshold Line Regulation
Typically 0.012% per % duty factor on
EXT; EXT duty factor for a step-up is:
100% (1 – VIN/VOUT)
0.012
%/%
FB Input Current
VFB = 1.30V
1
20
nA
Current Limit Threshold
85
100
115
mV
Idle Mode Current-Sense Threshold
5
15
25
mV
CS+ Input Current
CS+ forced to GND
0.2
1
µA
VCC Supply Current (Note 1)
VFB = 1.30V, VCC = 3V to 28V
220
350
µA
Shutdown Supply Current (VCC)
SYNC/SHDN = GND, VCC = 28V
3.5
6
µA
5.00
5.50
REFERENCE
Reference and
AND
LDOLDO
Regulators
REGULATORS
LDO Output Voltage
LDO load =
∞ to 400Ω
5V ≤ VCC ≤ 28V
(includes LDO dropout)
4.50
3V ≤ VCC ≤ 28V
(includes LDO dropout)
2.65
V
5.50
Undervoltage Lockout Threshold
Sensed at LDO, falling edge,
hysteresis = 1%, MAX668 only
2.40
2.50
2.60
REF Output Voltage
No load, CREF = 0.22µF
1.225
1.250
1.275
V
REF Load Regulation
REF load = 0 to 50µA
-2
-10
mV
REF Undervoltage Lockout Threshold
Rising edge, 1% hysteresis
1.0
1.1
1.2
V
ROSC = 200kΩ ±1%
225
250
275
ROSC = 100kΩ ±1%
425
500
575
ROSC = 500kΩ ±1%
85
100
115
V
OSCILLATOR
Oscillator
Oscillator Frequency
2
_______________________________________________________________________________________
kHz
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
MAX668/MAX669
ELECTRICAL CHARACTERISTICS (continued)
(VCC = LDO = +5V, ROSC = 200kΩ, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
Maximum Duty Cycle
CONDITIONS
MIN
TYP
MAX
ROSC = 200kΩ ±1%
87
90
93
ROSC = 100kΩ ±1%
86
90
94
ROSC = 500kΩ ±1%
86
90
94
UNITS
%
Minimum EXT Pulse Width
290
Minimum SYNC Input-Pulse Duty Cycle
20
45
%
Minimum SYNC Input Low Pulse Width
50
200
ns
SYNC Input Rise/Fall Time
Not tested
SYNC Input Frequency Range
100
SYNC/SHDN Input Low Voltage
SYNC/SHDN Input Current
200
ns
500
kHz
70
SYNC/SHDN Falling Edge to Shutdown Delay
SYNC/SHDN Input High Voltage
ns
3.0V < VCC < 28V
2.0
1.8V < VCC < 3.0V (MAX669)
1.5
µs
V
3.0V < VCC < 28V
0.45
1.8V < VCC < 3.0V (MAX669)
0.30
SYNC/SHDN = 5V
0.5
3.0
SYNC/SHDN = 28V
1.5
6.5
EXT Sink/Source Current
EXT forced to 2V
1
EXT On-Resistance
EXT high or low
2
V
µA
A
5
Ω
ELECTRICAL CHARACTERISTICS
(VCC = LDO = +5V, ROSC = 200kΩ, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
CONDITIONS
MIN
MAX
MAX668
3
28
MAX669
1.8
28
UNITS
PWM
PWM Controller
CONTROLLER
Input Voltage Range, VCC
V
Input Voltage Range with VCC Tied to LDO
2.7
5.5
FB Threshold
1.22
1.28
V
20
nA
FB Input Current
VFB = 1.30V
V
Current-Limit Threshold
85
115
mV
Idle Mode Current-Sense Threshold
3
27
mV
CS+ Input Current
CS+ forced to GND
VCC Supply Current (Note 1)
VFB = 1.30V, VCC = 3V to 28V
Shutdown Supply Current (VCC)
SYNC/SHDN = GND, VCC = 28V
1
µA
350
µA
6
µA
Reference
REFERENCE
andAND
LDOLDO
Regulators
REGULATORS
LDO Output Voltage
LDO Undervoltage Lockout Threshold
LDO load =
∞ to 400Ω
5V ≤ VCC ≤ 28V
(includes LDO dropout)
4.50
5.50
3V ≤ VCC ≤ 28V
(includes LDO dropout)
2.65
5.50
V
2.40
2.60
V
Sensed at LDO, falling edge,
hysteresis = 1%, MAX669 only
V
_______________________________________________________________________________________
3
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
ELECTRICAL CHARACTERISTICS (continued)
(VCC = LDO = +5V, ROSC = 200kΩ, TA = -40°C to +85°C, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
MAX
1.22
1.28
V
-10
mV
1.0
1.2
V
ROSC = 200kΩ ±1%
222
278
ROSC =100kΩ ±1%
425
575
ROSC = 500kΩ ±1%
85
115
ROSC = 200kΩ ±1%
87
93
ROSC = 100kΩ ±1%
86
94
ROSC = 500kΩ ±1%
86
94
REF Output Voltage
No load, CREF = 0.22µF
REF Load Regulation
REF load = 0 to 50µA
REF Undervoltage Lockout Threshold
Rising edge, 1% hysteresis
UNITS
OSCILLATOR
Oscillator Frequency
Maximum Duty Cycle
kHz
%
Minimum SYNC Input-Pulse Duty Cycle
45
%
Minimum SYNC Input Low Pulse Width
200
ns
200
ns
500
kHz
SYNC Input Rise/Fall Time
Not tested
SYNC Input Frequency Range
SYNC/SHDN Input High Voltage
SYNC/SHDN Input Low Voltage
SYNC/SHDN Input Current
EXT On-Resistance
100
3.0V < VCC < 28V
2.0
1.8V < VCC < 3.0V (MAX669)
1.5
3.0V < VCC < 28V
0.45
1.8V < VCC < 3.0V (MAX669)
0.30
SYNC/SHDN = 5V
3.0
SYNC/SHDN = 28V
6.5
EXT high or low
Note 1: This is the VCC current consumed when active but not switching. Does not include gate-drive current.
Note 2: Limits at TA = -40°C are guaranteed by design.
4
V
_______________________________________________________________________________________
5
V
µA
Ω
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
MAX668 EFFICIENCY vs.
LOAD CURRENT (VOUT = 12V)
90
VIN = 2.7V
75
VIN = 2V
70
VIN = 5V
NON-BOOTSTRAPPED
FIGURE 4
R4 = 200kΩ
75
BOOTSTRAPPED
FIGURE 3
R4 = 200kΩ
100
1000
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX669 MINIMUM START-UP VOLTAGE
vs. LOAD CURRENT
SUPPLY CURRENT vs.
SUPPLY VOLTAGE
2.0
VOUT = 12V
1.5
1.0
0.5
800
MAX669
600
400
200
BOOTSTRAPPED
FIGURE 2
1
3000
2500
2000
1500
1000
500
5
10
15
20
25
30
0
2
4
6
8
10
LOAD CURRENT (mA)
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
SHUTDOWN CURRENT vs.
SUPPLY VOLTAGE
SUPPLY CURRENT vs.
TEMPERATURE
LDO DROPOUT VOLTAGE vs.
LDO CURRENT
2.0
1.5
1.0
ROSC = 100kΩ
250
230
ROSC = 200kΩ
210
ROSC = 500kΩ
190
12
250
VIN = 3V
200
150
VIN = 4.5V
100
50
170
0.5
300
LDO DROPOUT VOLTAGE (mV)
MAX668
MAX668 toc08
270
SUPPLY CURRENT (µA)
MAX669
2.5
290
MAX668 toc07
3.5
3.0
0
0
100 200 300 400 500 600 700 800 900 1000
10,000
VOUT = 12V
BOOTSTRAPPED
FIGURE 2
R4 = 200kΩ
3500
0
0
100
1000
LOAD CURRENT (mA)
4000
MAX668
0
10
NO-LOAD SUPPLY CURRENT vs.
SUPPLY VOLTAGE
CURRENT INTO VCC PIN
ROSC = 500kΩ
1000
SUPPLY CURRENT (µA)
VOUT = 5V
2.5
NON-BOOTSTRAPPED
FIGURE 4
R4 = 200kΩ
10,000
1200
MAX668 toc04
3.0
80
70
1
10,000
NO-LOAD SUPPLY CURRENT (µA)
10
MAX668 toc05
1
VIN = 5V
75
70
50
85
MAX668 toc09
55
MINIMUM START-UP VOLTAGE (V)
80
65
60
SHUTDOWN CURRENT (µA)
85
VIN = 8V
MAX668 toc06
EFFICIENCY (%)
EFFICIENCY (%)
80
VIN = 12V
90
EFFICIENCY (%)
VIN = 3.3V
85
95
MAX668 toc02
VIN = 3.6V
90
95
MAX668 toc01
95
MAX668 EFFICIENCY vs.
LOAD CURRENT (VOUT = 24V)
MAX668 toc03
EFFICIENCY vs. LOAD CURRENT
(VOUT = 5V)
CURRENT INTO VCC PIN
150
0
0
5
10
15
20
SUPPLY VOLTAGE (V)
25
30
0
-40
-20
0
20
40
60
TEMPERATURE (°C)
80
100
0.1
1
10
20
LDO CURRENT (mA)
_______________________________________________________________________________________
5
MAX668/MAX669
Typical Operating Characteristics
(Circuits of Figures 2, 3, 4, and 5; TA = +25°C; unless otherwise noted.)
Typical Operating Characteristics (continued)
(Circuits of Figures 2, 3, 4, and 5; TA = +25°C; unless otherwise noted.)
REFERENCE VOLTAGE vs.
TEMPERATURE
SWITCHING FREQUENCY vs. ROSC
1.247
1.246
1.245
1.244
1.243
1.242
1.241
MAX668 toc11
1.248
450
400
350
300
250
200
150
100
50
VCC = 5V
1.240
VCC = 5V
0
-40
-20
0
20
40
60
80
100
0
100
200
300
ROSC (kΩ)
SWITCHING FREQUENCY vs.
TEMPERATURE
EXT RISE/FALL TIME vs.
CAPACITANCE
100kΩ
50
EXT RISE/FALL TIME (ns)
500
400
165kΩ
300
200
tR, VCC = 3.3V
40
tF, VCC = 3.3V
30
20
499kΩ
tR, VCC = 5V
10
100
500
60
MAX668 toc12
600
VIN = 5V
tF, VCC = 5V
0
0
-40
-20
0
20
40
60
TEMPERATURE (°C)
6
400
TEMPERATURE (°C)
MAX668 toc13
REFERENCE VOLTAGE (V)
1.249
500
SWITCHING FREQUENCY (kHz)
MAX668 toc10
1.250
SWITCHING FREQUENCY (kHz)
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
80
100
100
1000
CAPACITANCE (pF)
_______________________________________________________________________________________
10,000
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
ENTERING SHUTDOWN
MAX668 toc14
MAX668 toc15
EXITING SHUTDOWN
0V
SHUTDOWN
VOLTAGE
5V/div
OUTPUT
VOLTAGE
5V/div
INDUCTOR
CURRENT
2A/div
0V
0A
0V
OUTPUT
VOLTAGE
5V/div
200µs/div
500µs/div
MAX668, VIN = 5V, VOUT = 12V, LOAD = 1.0A,
LOW VOLTAGE, NON-BOOTSTRAPPED
MAX668, VIN = 5V, VOUT = 12V, LOAD = 1.0A, ROSC = 100kΩ,
LOW VOLTAGE, NON-BOOTSTRAPPED
LIGHT-LOAD SWITCHING WAVEFORM
MAX668 toc16
HEAVY-LOAD SWITCHING WAVEFORM
VOUT
200mV/div
AC-COUPLED
0V
MAX668 toc17
SHUTDOWN
VOLTAGE
5V/div
VOUT
100mV/div
AC-COUPLED
Q1, DRAIN
5V/div
Q1, DRAIN
5V/div
0V
0V
IL
1A/div
0A
IL
1A/div
0A
1µs/div
1µs/div
MAX668, VIN = 5V, VOUT = 12V, ILOAD = 1.0A,
LOW VOLTAGE, NON-BOOTSTRAPPED
MAX668, VIN = 5V, VOUT = 12V, ILOAD = 0.1A,
LOW VOLTAGE, NON-BOOTSTRAPPED
LOAD-TRANSIENT RESPONSE
OUTPUT
VOLTAGE
AC-COUPLED
100mV/div
LOAD
CURRENT
1A/div
MAX668 toc19
MAX668 toc18
LINE-TRANSIENT RESPONSE
OUTPUT
VOLTAGE
100mV/div
AC-COUPLED
INPUT
VOLTAGE
5V/div
1ms/div
MAX668, VIN = 5V, VOUT = 12V, ILOAD = 0.1A TO 1.0A,
LOW VOLTAGE, NON-BOOTSTRAPPED
0V
20ms/div
MAX668, VIN = 5V TO 8V, VOUT = 12V, LOAD = 1.0A,
HIGH VOLTAGE, NON-BOOTSTRAPPED
_______________________________________________________________________________________
7
MAX668/MAX669
Typical Operating Characteristics (continued)
(Circuits of Figures 2, 3, 4, and 5; TA = +25°C; unless otherwise noted.)
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
MAX668/MAX669
Pin Description
PIN
NAME
FUNCTION
1
LDO
5V On-Chip Regulator Output. This regulator powers all internal circuitry including the EXT gate driver.
Bypass LDO to GND with a 1µF or greater ceramic capacitor.
2
FREQ
Oscillator Frequency Set Input. A resistor from FREQ to GND sets the oscillator from 100kHz (ROSC =
500kΩ) to 500kHz (ROSC = 100kΩ). fOSC = 5 x 1010 / ROSC. ROSC is still required if an external clock is used
at SYNC/SHDN. (See SYNC/SHDN and FREQ Inputs section.)
3
GND
Analog Ground
4
REF
1.25V Reference Output. REF can source 50µA. Bypass to GND with a 0.22µF ceramic capacitor.
5
FB
6
CS+
7
PGND
Power Ground for EXT Gate Driver and Negative Current-Sense Input
8
EXT
External MOSFET Gate-Driver Output. EXT swings from LDO to PGND.
9
VCC
Input Supply to On-Chip LDO Regulator. VCC accepts inputs up to 28V. Bypass to GND with a 0.1µF ceramic
capacitor.
10
SYNC/
SHDN
Feedback Input. The FB threshold is 1.25V.
Positive Current-Sense Input. Connect a current-sense resistor, RCS, between CS+ and PGND.
Shutdown control and Synchronization Input. There are three operating modes:
• SYNC/SHDN low: DC-DC off.
• SYNC/SHDN high: DC-DC on with oscillator frequency set at FREQ by ROSC.
• SYNC/SHDN clocked: DC-DC on with operating frequency set by SYNC clock input. DC-DC conversion
cycles initiate on rising edge of input clock.
Detailed Description
The MAX668/MAX669 current-mode PWM controllers
operate in a wide range of DC-DC conversion applications, including boost, SEPIC, flyback, and isolated output configurations. Optimum conversion efficiency is
maintained over a wide range of loads by employing
both PWM operation and Maxim’s proprietary Idle
Mode control to minimize operating current at light
loads. Other features include shutdown, adjustable
internal operating frequency or synchronization to an
external clock, soft start, adjustable current limit, and a
wide (1.8V to 28V) input range.
MAX668 vs. MAX669 Differences
Differences between the MAX668 and MAX669 relate
to their use in bootstrapped or non-bootstrapped circuits (Table 1). The MAX668 operates with inputs as
low as 3V and can be connected in either a bootstrapped or non-bootstrapped (IC powered from input
supply or other source) configuration. When not bootstrapped, the MAX668 has no restriction on output voltage. When bootstrapped, the output cannot exceed
28V.
The MAX669 is optimized for low input voltages (down
to 1.8V) and requires bootstrapped operation (IC powered from VOUT) with output voltages no greater than
8
28V. Bootstrapping is required because the MAX669
does not have undervoltage lockout, but instead drives
EXT with an open-loop, 50% duty-cycle start-up oscillator when LDO is below 2.5V. It switches to closed-loop
operation only when LDO exceeds 2.5V. If a non-bootstrapped connection is used with the MAX669 and if
VCC (the input voltage) remains below 2.7V, the output
voltage will soar above the regulation point. Table 2
recommends the appropriate device for each biasing
option.
Table 1. MAX668/MAX669 Comparison
FEATURE
MAX668
MAX669
VCC Input
Range
3V to 28V
1.8V to 28V
Operation
Bootstrapped or nonbootstrapped. VCC can be connected to input, output, or
other voltage source such as
a logic supply.
Must be bootstrapped (VCC
must be connected to boosted output voltage, VOUT).
Undervoltage
Lockout
IC stops switching for LDO
below 2.5V.
No
Soft-Start
Yes
When LDO is
above 2.5V
_______________________________________________________________________________________
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
Bootstrapped/Non-Bootstrapped Operation
Low-Dropout Regulator (LDO)
Several IC biasing options, including bootstrapped and
non-bootstrapped operation, are made possible by an
on-chip, low-dropout 5V regulator. The regulator input is
at VCC, while its output is at LDO. All MAX668/MAX669
functions, including EXT, are internally powered from
LDO. The V CC -to-LDO dropout voltage is typically
200mV (300mV max at 12mA), so that when VCC is less
than 5.2V, LDO is typically VCC - 200mV. When LDO is
in dropout, the MAX668/MAX669 still operate with VCC
as low as 3V (as long as LDO exceeds 2.7V), but with
reduced amplitude FET drive at EXT. The maximum
VCC input voltage is 28V.
LDO can supply up to 12mA to power the IC, supply
gate charge through EXT to the external FET, and supply small external loads. When driving particularly large
FETs at high switching rates, little or no LDO current
may be available for external loads. For example, when
switched at 500kHz, a large FET with 20nC gate charge
requires 20nC x 500kHz, or 10mA.
VCC and LDO allow a variety of biasing connections to
optimize efficiency, circuit quiescent current, and fullload start-up behavior for different input and output
voltage ranges. Connections are shown in Figures 2, 3,
4, and 5. The characteristics of each are outlined in
Table 1.
In PWM mode, the controller uses fixed-frequency, current-mode operation where the duty ratio is set by the
input/output voltage ratio (duty ratio = (VOUT - VIN) / VIN
in the boost configuration). The current-mode feedback
loop regulates peak inductor current as a function of
the output error signal.
At light loads the controller enters Idle Mode. During
Idle Mode, switching pulses are provided only as needed to service the load, and operating current is minimized to provide best light-load efficiency. The
minimum-current comparator threshold is 15mV, or 15%
of the full-load value (IMAX) of 100mV. When the controller is synchronized to an external clock, Idle Mode
occurs only at very light loads.
VCC
LDO
MAX669 ONLY
1.25V
ANTISAT
EXT
LDO
R1
552k
PGND
(MAX669 ONLY)
UVLO
MAX668
MAX669
R2
276k
R3
276k
FB
CURRENT SENSE
CS+
SLOPE COMPENSATION
100mV
IMAX
15mV
IMIN
LOW-VOLTAGE
START-UP
OSCILLATOR
MUX 0
1
REF
1.25V
MAIN PWM
COMPARATOR
+A
X6
-A
+C
-C X1
+S X1
-S
SYNC/SHDN
FREQ
BIAS
OSC
OSC
S Q
R
Figure 1. MAX668/MAX669 Functional Diagram
_______________________________________________________________________________________
9
MAX668/MAX669
PWM Controller
The heart of the MAX668/MAX669 current-mode PWM
controller is a BiCMOS multi-input comparator that
simultaneously processes the output-error signal, the
current-sense signal, and a slope-compensation ramp
(Figure 1). The main PWM comparator is direct summing, lacking a traditional error amplifier and its associated phase shift. The direct summing configuration
approaches ideal cycle-by-cycle control over the output voltage since there is no conventional error amp in
the feedback path.
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
VIN = 1.8V to 12V
1
C1
68µF
20V
LDO
C4
1µF
EXT
MAX669
9
C2
0.1µF
2
VOUT = 12V @ 0.5A
8
N1
D1
MBRS340T3
IRF7401
6
PGND
FB
GND
FREQ
C5
68µF
20V
C6
68µF
20V
R1
0.02Ω
VCC
10 SYNC/
SHDN
4
REF
C3
0.22µF
CS+
L1
4.7µH
7
C8
0.1µF
R2
218k
1%
5
R3
24.9k
1%
C7
220pF
3
R4
100k
1%
Figure 2. MAX669 High-Voltage Bootstrapped Configuration
VIN = 1.8V to 5V
1
LDO
C2
1µF
EXT
MAX669
9
2
CS+
L1
4.7µH
FREQ
VOUT = 5V @ 1A
8
N1
6
R1
0.02Ω
VCC
10 SYNC/
SHDN
4
REF
C3
0.22µF
C1
68µF
10V
PGND
FB
GND
D1
MBRS340T3
FDS6680
IRF7401
C4
68µF
10V
7
C5
68µF
10V
C6
0.1µF
R2
75k
1%
5
3
C7
220pF
R3
24.9k
1%
R4
100k
1%
Figure 3. MAX669 Low-Voltage Bootstrapped Configuration
Bootstrapped Operation
With bootstrapped operation, the IC is powered from
the circuit output (V OUT ). This improves efficiency
when the input voltage is low, since EXT drives the FET
with a higher gate voltage than would be available from
the low-voltage input. Higher gate voltage reduces the
FET on-resistance, increasing efficiency. Other (undesirable) characteristics of bootstrapped operation are
increased IC operating power (since it has a higher
operating voltage) and reduced ability to start up with
high load current at low input voltages. If the input volt10
age range extends below 2.7V, then bootstrapped
operation with the MAX669 is the only option.
With VCC connected to VOUT, as in Figure 2, EXT voltage swing is 5V when VCC is 5.2V or more, and VCC 0.2V when VCC is less than 5.2V. If the output voltage
does not exceed 5.5V, the on-chip regulator can be
disabled by connecting VCC to LDO (Figure 3). This
eliminates the LDO forward drop and supplies maximum gate drive to the external FET.
______________________________________________________________________________________
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
MAX668/MAX669
VIN = 3V to 12V
C1
68µF
20V
1
LDO
C4
1µF
EXT
MAX668
9
C2
0.1µF
2
VOUT = 12V @ 1A
8
N1
D1
MBRS340T3
FDS6680
6
R1
0.02Ω
VCC
PGND
10 SYNC/
SHDN
4
REF
C3
0.22µF
CS+
L1
4.7µH
FB
GND
FREQ
C5
68µF
20V
C6
68µF
20V
7
C8
0.1µF
R2
218k
1%
5
R3
24.9k
1%
C7
220pF
3
R4
100k
1%
Figure 4. MAX668 High-Voltage Non-Bootstrapped Configuration
VIN = 2.7V to 5.5V
C1
68µF
10V
1
LDO
C2
1µF
MAX668
9
2
CS+
FREQ
VOUT = 12V @ 1A
8
N1
D1
MBRS340T3
FDS6680
6
C4
68µF
20V
R1
0.02Ω
VCC
10 SYNC/
SHDN
4
REF
C3
0.22µF
EXT
L1
4.7µH
PGND
FB
GND
7
C5
68µF
20V
C6
0.1µF
R2
218k
1%
5
3
C7
220pF
R3
24.9k
1%
R4
100k
1%
Figure 5. MAX668 Low-Voltage Non-Bootstrapped Configuration
Non-Bootstrapped Operation
With non-bootstrapped operation, the IC is powered
from the input voltage (VIN) or another source, such as
a logic supply. Non-bootstrapped operation (Figure 4)
is recommended (but not required) for input voltages
above 5V, since the EXT amplitude (limited to 5V by
LDO) at this voltage range is no higher than it would be
with bootstrapped operation. Note that non-bootstrapped operation is required if the output voltage
exceeds 28V, since this level is too high to safely con-
nect to VCC. Also note that only the MAX668 can be
used with non-bootstrapped operation.
If the input voltage does not exceed 5.5V, the on-chip
regulator can be disabled by connecting VCC to LDO
(Figure 5). This eliminates the regulator forward drop
and supplies the maximum gate drive to the external
FET for lowest on-resistance. Disabling the regulator
also reduces the non-bootstrapped minimum input voltage from 3V to 2.7V.
______________________________________________________________________________________
11
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
Table 2. Bootstrapped and Non-Bootstrapped Configurations
CONFIGURATION
FIGURE
USE
WITH:
INPUT
VOLTAGE
RANGE* (V)
OUTPUT
VOLTAGE
RANGE (V)
COMMENTS
High-Voltage,
Bootstrapped
Figure
2
MAX669
1.8 to 28
3V to 28
Connect VCC to VOUT. Provides maximum external
FET gate drive for low-voltage (Input <3V) to highvoltage (output >5.5V) boost circuits. VOUT cannot
exceed 28V.
Low-Voltage,
Bootstrapped
Figure
3
MAX669
1.8 to 5.5
2.7 to 5.5
Connect VOUT to VCC and LDO. Provides maximum possible external FET gate drive for low-voltage designs, but limits VOUT to 5.5V or less.
High-Voltage,
Non-Bootstrapped
Figure
4
MAX668
3 to 28
VIN to ∞
Connect VIN to VCC. Provides widest input and output range, but external FET gate drive is reduced for
VIN below 5V.
VIN to ∞
Connect VIN to VCC and LDO. FET gate-drive
amplitude = VIN for logic-supply (input 3V to 5.5V) to
high-voltage (output >5.5V) boost circuits. IC operating power is less than in Figure 4, since IC current
does not pass through the LDO regulator.
VIN to ∞
Connect VCC and LDO to a separate supply
(VBIAS) that powers only the IC. FET gate-drive
amplitude = VBIAS. Input power source (VIN) and
output voltage range (VOUT) are not restricted,
except that VOUT must exceed VIN.
Low-Voltage,
Non-Bootstrapped
Extra IC supply,
Non-Bootstrapped
Figure
5
None
MAX668
MAX668
2.7 to 5.5
Not
Restricted
* For standard step-up DC-DC circuits (as in Figures 2, 3, 4, and 5), regulation cannot be maintained if VIN exceeds VOUT. SEPIC
and transformer-based circuits do not have this limitation.
In addition to the configurations shown in Table 2, the
following guidelines may help when selecting a configuration:
1) If V IN is ever below 2.7V, V CC must be bootstrapped to VOUT and the MAX669 must be used. If
VOUT never exceeds 5.5V, LDO may be shorted to
VCC and VOUT to eliminate the dropout voltage of
the LDO regulator.
2) If VIN is greater than 3.0V, VCC can be powered
from VIN, rather than from VOUT (non-bootstrapped).
This can save quiescent power consumption, especially when V OUT is large. If V IN never exceeds
5.5V, LDO may be shorted to VCC and VIN to eliminate the dropout voltage of the LDO regulator.
12
3) If VIN is in the 3V to 4.5V range (i.e., 1-cell Li-Ion or
3-cell NiMH battery range), bootstrapping VCC from
VOUT, although not required, may increase overall
efficiency by increasing gate drive (and reducing
FET resistance) at the expense of quiescent power
consumption.
4) If VIN always exceeds 4.5V, VCC should be tied to
V IN , since bootstrapping from V OUT does not
increase gate drive from EXT but does increase
quiescent power dissipation.
______________________________________________________________________________________
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
So a 500kHz operating frequency, for example, is set
with ROSC = 100kΩ.
Rising clock edges on SYNC/SHDN are interpreted as
synchronization inputs. If the sync signal is lost while
SYNC/SHDN is high, the internal oscillator takes over at
the end of the last cycle and the frequency is returned
to the rate set by ROSC. If sync is lost with SYNC/SHDN
low, the IC waits for 70µs before shutting down. This
maintains output regulation even with intermittent sync
signals. When an external sync signal is used, Idle
Mode switchover at the 15mV current-sense threshold
is disabled so that Idle Mode only occurs at very light
loads. Also, ROSC should be set for a frequency 15%
below the SYNC clock rate:
ROSC(SYNC) = 5 x 1010 / (0.85 x fSYNC)
Soft-Start
The MAX668/MAX669 feature a “digital” soft start which
is preset and requires no external capacitor. Upon
start-up, the peak inductor increments from 1/5 of the
value set by RCS, to the full current-limit value, in five
steps over 1024 cycles of fOSC or fSYNC. For example,
with an f OSC of 200kHz, the complete soft-start
sequence takes 5ms. See the Typical Operating
Characteristics for a photo of soft-start operation. Softstart is implemented: 1) when power is first applied to
the IC, 2) when exiting shutdown with power already
applied, and 3) when exiting undervoltage lockout. The
MAX669’s soft-start sequence does not start until LDO
reaches 2.5V.
Design Procedure
The MAX668/MAX669 can operate in a number of DCDC converter configurations including step-up, SEPIC
(single-ended primary inductance converter), and flyback. The following design discussions are limited to
step-up, although SEPIC and flyback examples are
shown in the Application Circuits section.
Setting the Operating Frequency
The MAX668/MAX669 can be set to operate from
100kHz to 500kHz. Choice of operating frequency will
depend on number of factors:
1) Noise considerations may dictate setting (or synchronizing) fOSC above or below a certain frequency
or band of frequencies, particularly in RF applications.
2) Higher frequencies allow the use of smaller value
(hence smaller size) inductors and capacitors.
3) Higher frequencies consume more operating power
both to operate the IC and to charge and discharge
the gate of the external FET. This tends to reduce
efficiency at light loads; however, the MAX668/
MAX669’s Idle Mode feature substantially increases
light-load efficiency.
4) Higher frequencies may exhibit poorer overall efficiency due to more transition losses in the FET;
however, this shortcoming can often be nullified by
trading some of the inductor and capacitor size
benefits for lower-resistance components.
The oscillator frequency is set by a resistor, ROSC, connected from FREQ to GND. ROSC must be connected
whether or not the part is externally synchronized ROSC
is in each case:
ROSC = 5 x 1010 / fOSC
when not using an external clock.
ROSC(SYNC) = 5 x 1010 / (0.85 x fSYNC)
when using an external clock, fSYNC.
Setting the Output Voltage
The output voltage is set by two external resistors (R2
and R3, Figures 2, 3, 4, and 5). First select a value for
R3 in the 10kΩ to 1MΩ range. R2 is then given by:
R2 = R3 [(VOUT / VREF) – 1]
where VREF is 1.25V.
Determining Inductance Value
For most MAX668/MAX669 boost designs, the inductor
value (LIDEAL) can be derived from the following equation, which picks the optimum value for stability based
on the MAX668/MAX669’s internally set slope compensation:
LIDEAL = VOUT / (4 x IOUT x fOSC)
The MAX668/MAX669 allow significant latitude in inductor selection if LIDEAL is not a convenient value. This
may happen if LIDEAL is a not a standard inductance
(such as 10µH, 22µH, etc.), or if LIDEAL is too large to
be obtained with suitable resistance and saturation-current rating in the desired size. Inductance values smaller than LIDEAL may be used with no adverse stability
effects; however, the peak-to-peak inductor current
(ILPP) will rise as L is reduced. This has the effect of
raising the required ILPK for a given output power and
also requiring larger output capacitance to maintain a
______________________________________________________________________________________
13
MAX668/MAX669
SYNC/SHDN and FREQ Inputs
The SYNC/SHDN pin provides both external-clock synchronization (if desired) and shutdown control. When
SYNC/SHDN is low, all IC functions are shut down. A
logic high at SYNC/SHDN selects operation at a frequency set by ROSC, connected from FREQ to GND.
The relationship between fOSC and ROSC is:
ROSC = 5 x 1010 / fOSC
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
given output ripple. An inductance value larger than
LIDEAL may also be used, but output-filter capacitance
must be increased by the same proportion that L has to
LIDEAL. See the Capacitor Selection section for more
information on determining output filter values.
Due the MAX668/MAX669’s high switching frequencies,
inductors with a ferrite core or equivalent are recommended. Powdered iron cores are not recommended
due to their high losses at frequencies over 50kHz.
old NFETs that specify on-resistance with a gatesource voltage (VGS) of 2.7V or less. When selecting an
NFET, key parameters can include:
1) Total gate charge (Qg)
Determining Peak Inductor Current
At high switching rates, dynamic characteristics (parameters 1 and 2 above) that predict switching losses
may have more impact on efficiency than RDS(ON),
which predicts DC losses. Qg includes all capacitances
associated with charging the gate. In addition, this
parameter helps predict the current needed to drive the
gate at the selected operating frequency. The continuous LDO current for the FET gate is:
IGATE = Qg x fOSC
For example, the MMFT3055L has a typical Qg of 7nC
(at VGS = 5V); therefore, the IGATE current at 500kHz is
3.5mA. Use the FET manufacturer’s typical value for Qg
in the above equation, since a maximum value (if supplied) is usually too conservative to be of use in estimating IGATE.
The peak inductor current required for a particular output is:
ILPEAK = ILDC + (ILPP / 2)
where ILDC is the average DC input current and ILPP is
the inductor peak-to-peak ripple current. The ILDC and
ILPP terms are determined as follows:
I
(V
+ VD )
ILDC = OUT OUT
(VIN – VSW )
where V D is the forward voltage drop across the
Schottky rectifier diode (D1), and V SW is the drop
across the external FET, when on.
(VIN – VSW ) (VOUT + VD – VIN )
L x fOSC (VOUT + VD )
where L is the inductor value. The saturation rating of
the selected inductor should meet or exceed the calculated value for ILPEAK, although most coil types can be
operated up to 20% over their saturation rating without
difficulty. In addition to the saturation criteria, the inductor should have as low a series resistance as possible.
For continuous inductor current, the power loss in the
inductor resistance, PLR, is approximated by:
PLR ≅ (IOUT x VOUT / VIN)2 x RL
where RL is the inductor series resistance.
Once the peak inductor current is selected, the currentsense resistor (RCS) is determined by:
ILPP =
RCS = 85mV / ILPEAK
For high peak inductor currents (>1A), Kelvin sensing
connections should be used to connect CS+ and
PGND to RCS. PGND and GND should be tied together
at the ground side of RCS.
Power MOSFET Selection
The MAX668/MAX669 drive a wide variety of N-channel
power MOSFETs (NFETs). Since LDO limits the EXT
output gate drive to no more than 5V, a logic-level
NFET is required. Best performance, especially at low
input voltages (below 5V), is achieved with low-thresh14
2) Reverse transfer capacitance or charge (CRSS)
3) On-resistance (RDS(ON))
4) Maximum drain-to-source voltage (VDS(MAX))
5) Minimum threshold voltage (VTH(MIN))
Diode Selection
The MAX668/MAX669’s high switching frequency
demands a high-speed rectifier. Schottky diodes are
recommended for most applications because of their
fast recovery time and low forward voltage. Ensure that
the diode’s average current rating is adequate using
the diode manufacturer’s data, or approximate it with
the following formula:
I
- I
IDIODE = IOUT + LPEAK OUT
3
Also, the diode reverse breakdown voltage must
exceed VOUT. For high output voltages (50V or above),
Schottky diodes may not be practical because of this
voltage requirement. In these cases, use a high-speed
silicon rectifier with adequate reverse voltage.
Capacitor Selection
Output Filter Capacitor
The minimum output filter capacitance that ensures stability is:
(7.5V x L / L IDEAL )
COUT(MIN) =
(2πRCS x VIN(MIN) x fOSC )
where VIN(MIN) is the minimum expected input voltage.
Typically COUT(MIN), though sufficient for stability, will
______________________________________________________________________________________
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
Input Capacitor
The input capacitor (CIN) in boost designs reduces the
current peaks drawn from the input supply and reduces
noise injection. The value of CIN is largely determined
by the source impedance of the input supply. High
source impedance requires high input capacitance,
particularly as the input voltage falls. Since step-up DCDC converters act as “constant-power” loads to their
input supply, input current rises as input voltage falls.
Consequently, in low-input-voltage designs, increasing
CIN and/or lowering its ESR can add as many as five
percentage points to conversion efficiency. A good
starting point is to use the same capacitance value for
CIN as for COUT.
Bypass Capacitors
In addition to CIN and COUT, three ceramic bypass
capacitors are also required with the MAX668/MAX669.
Bypass REF to GND with 0.22µF or more. Bypass LDO
to GND with 1µF or more. And bypass VCC to GND with
0.1µF or more. All bypass capacitors should be located
as close to their respective pins as possible.
Compensation Capacitor
Output ripple voltage due to COUT ESR affects loop
stability by introducing a left half-plane zero. A small
capacitor connected from FB to GND forms a pole with
the feedback resistance that cancels the ESR zero. The
optimum compensation value is:
ESRCOUT
CFB = COUT x
(R2 x R3) / (R2 + R3)
where R2 and R3 are the feedback resistors (Figures 2,
3, 4, and 5). If the calculated value for CFB results in a
non-standard capacitance value, values from 0.5CFB to
1.5CFB will also provide sufficient compensation.
Applications Information
Starting Under Load
In non-bootstrapped configurations (Figures 4 and 5),
the MAX668 can start up with any combination of output load and input voltage at which it can operate when
already started. In other words, there are no special
limitations to start-up in non-bootstrapped circuits.
In bootstrapped configurations with the MAX668 or
MAX669, there may be circumstances where full load
current can only be applied after the circuit has started
and the output is near its set value. As the input voltage
drops, this limitation becomes more severe. This characteristic of all bootstrapped designs occurs when the
MOSFET gate is not fully driven until the output voltage
rises. This is problematic because a heavily loaded output cannot rise until the MOSFET has low on-resistance. In such situations, low-threshold FETs (VTH <
VIN(MIN)) are the most effective solution. The Typical
Operating Characteristics section shows plots of startup voltage versus load current for a typical bootstrapped design.
Layout Considerations
Due to high current levels and fast switching waveforms
that radiate noise, proper PC board layout is essential.
Protect sensitive analog grounds by using a star ground
configuration. Minimize ground noise by connecting
GND, PGND, the input bypass-capacitor ground lead,
and the output-filter ground lead to a single point (star
ground configuration). Also, minimize trace lengths to
reduce stray capacitance, trace resistance, and radiated noise. The trace between the external gain-setting
resistors and the FB pin must be extremely short, as
must the trace between GND and PGND.
Application Circuits
Low-Voltage Boost Circuit
Figure 3 shows the MAX669 operating in a low-voltage
boost application. The MAX669 is configured in the
bootstrapped mode to improve low input voltage performance. The IRF7401 N-channel MOSFET was selected for Q1 in this application because of its very low
0.7V gate threshold voltage (VGS). This circuit provides
a 5V output at greater than 2A of output current and
operates with input voltages as low as 1.8V. Efficiency
is typically in the 85% to 90% range.
+12V Boost Application
Figure 5 shows the MAX668 operating in a 5V to 12V
boost application. This circuit provides output currents
of greater than 1A at a typical efficiency of 92%. The
MAX668 is operated in non-bootstrapped mode to minimize the input supply current. This achieves maximum
light-load efficiency. If input voltages below 5V are
used, the IC should be operated in bootstrapped mode
to achieve best low-voltage performance.
4-Cell to +5V SEPIC Power Supply
Figure 6 shows the MAX668 in a SEPIC (single-ended
primary inductance converter) configuration. This configuration is useful when the input voltage can be either
______________________________________________________________________________________
15
MAX668/MAX669
not be adequate for low output voltage ripple. Since
output ripple in boost DC-DC designs is dominated by
capacitor equivalent series resistance (ESR), a capacitance value 2 or 3 times larger than COUT(MIN) is typically needed. Low-ESR types must be used. Output
ripple due to ESR is:
VRIPPLE(ESR) = ILPEAK x ESRCOUT
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
larger or smaller than the output voltage, such as when
converting four NiMH, NiCd, or Alkaline cells to a 5V
output. The SEPIC configuration is often a good choice
for combined step-up/step-down applications.
The N-channel MOSFET (Q1) must be selected to withstand a drain-to-source voltage (VDS) greater than the
sum of the input and output voltages. The coupling
capacitor (C2) must be a low-ESR type to achieve maximum efficiency. C2 must also be able to handle high
ripple currents; ordinary tantalum capacitors should not
be used for high-current designs.
The circuit in Figure 6 provides greater than 1A output
current at 5V when operating with an input voltage from
3V to 25V. Efficiency will typically be between 70% and
85%, depending upon the input voltage and output current.
Isolated +5V to +5V Power Supply
The circuit of Figure 7 provides a 5V isolated output at
400mA from a 5V input power supply. Transformer T1
provides electrical isolation for the forward path of the
converter, while the TLV431 shunt regulator and
MOC211 opto-isolator provide an isolated feedback
error voltage for the converter. The output voltage is set
by resistors R2 and R3 such that the mid-point of the
divider is 1.24V (threshold of TLV431). Output voltage
can be adjusted from 1.24V to 6V by selecting the
proper ratio for R2 and R3. For output voltages greater
than 6V, substitute the TL431 for the TLV431, and use
2.5V as the voltage at the midpoint of the voltagedivider.
VIN
3V to 25V
22µF x 3
@ 35V
9
D1
40V
10
VCC
1
LDO
2
VOUT
5V @ 1A
SHDN
C2
10µF @ 35V
MAX668
FREQ
EXT
1µF
4
R3
100k
0.22µF
8
C3
68µF x 3
Q1
30V
FDS6680
REF
CS+
5
4.9µH L1
CTX5-4
FB
GND
3
D1: MBR5340T3, 3A, 40V SCHOTTKY DIODE
R4: WSL-2512-R020F, 0.02Ω
C3: AVX TPSZ686M020R0150, 68µF, 150mΩ ESR
PGND
6
R4
0.02Ω
R1
75k
7
C4
520pF
R2
25k
Figure 6. MAX668 in SEPIC Configuration
16
______________________________________________________________________________________
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
MAX668/MAX669
MBR0540L
47µH
VIN = +5V
+5V @ 400mA
220µF
10V
T1
2:1
1µF
MBR0540L
220µF
10V
+5V RETURN
VCC
LDO
IRF7603
EXT
SHDN
CS+
MAX668
FB
0.1Ω
PGND
REF
FREQ
GND
0.22µF
100k
R2
301k
1%
510Ω
MOC211
10k
0.1µF
0.068µF
610Ω
TLV431
R3
100k
1%
T1: COILTRONICS CTX03-14232
Figure 7. Isolated +5V to +5V at 400mA Power Supply
Chip Information
TRANSISTOR COUNT: 1861
______________________________________________________________________________________
17
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
10LUMAXB.EPS
MAX668/MAX669
Package Information
18
______________________________________________________________________________________