For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim's website at www.maxim-ic.com.
General Description
The MAX8625A PWM step-up/down regulator is intend-
ed to power digital logic, hard disk drives, motors, and
other loads in portable, battery-powered devices such
as PDAs, cell phones, digital still cameras (DSCs), and
MP3 players. The MAX8625A provides either a fixed
3.3V or adjustable output voltage (1.25V to 4V) at up to
0.8A from a 2.5V to 5.5V input. The MAX8625A utilizes
a 2A peak current limit.
Maxim’s proprietary H-bridge topology provides a
seamless transition through all operating modes without
the glitches commonly seen with other devices. Four
internal MOSFETs (two switches and two synchronous
rectifiers) with internal compensation minimize external
components. A SKIP input selects a low-noise, fixed-
frequency PWM mode, or a high-efficiency skip mode
where the converter automatically switches to PFM
mode under light loads for best light-load efficiency.
The internal oscillator operates at 1MHz to allow for a
small external inductor and capacitors.
The MAX8625A features current-limit circuitry that shuts
down the IC in the event of an output overload. In addi-
tion, soft-start circuitry reduces inrush current during
startup. The IC also features True ShutdownTM, which
disconnects the output from the input when the IC is
disabled. The MAX8625A is available in a 3mm x 3mm,
14-pin TDFN package.
Applications
PDAs and Smartphones
DSCs and Camcorders
MP3 Players and Cellular Phones
Battery-Powered Hard Disk Drive (HDD)
Features
Four Internal MOSFET True H-Bridge Buck/Boost
Glitch-Free, Buck-Boost Transitions
Minimal Output Ripple Variation on Transitions
Up to 92% Efficiency
37µA (typ) Quiescent Current in Skip Mode
2.5V to 5.5V Input Range
Fixed 3.3V or Adjustable Output
1µA (max) Logic-Controlled Shutdown
True Shutdown
Output Overload Protection
Internal Compensation
Internal Soft-Start
1MHz Switching Frequency
Thermal-Overload Protection
Small 3mm x 3mm, 14-Pin TDFN Package
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
________________________________________________________________
Maxim Integrated Products
1
Ordering Information
MAX8625A
SKIP
IN
GND
FB
OUT
LX1 LX2
ON
INPUT
2.7V TO 5.5V
OUTPUT
3.3V
OFF
ON
PWM
SKIP
REF
Typical Operating Circuit
19-1006; Rev 4; 4/09
True Shutdown is a trademark of Maxim Integrated Products, Inc.
EVALUATION KIT
AVAILABLE
Note: The device is specified over the -40°C to +85°C extended
temperature range.
+
Denotes a lead(Pb)-free/RoHS-compliant package.
**
EP = Exposed pad.
PART PIN-
PACKAGE TOP MARK
MAX8625AETD+
14 TDFN-EP**
ABQ
MAX8625A
TDFN-EP
TOP VIEW
245
13 11 10
IN
GND
OUT
LX1
LX2
ON
1
+
14
INLX1
3
12
GNDLX2
6
9
OUTSKIP
7
8
REFFB
EP
EP = EXPOSED PAD.
Pin Configuration
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN = 3.6V, ON = SKIP = IN, FB = GND, VOUT = 3.3V, LX_ unconnected, CREF = C5 = 0.1µF to GND, Figure 4. TA= -40°C to +85°C.
Typical values are at TA= +25°C, unless otherwise noted.) (Note 2)
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.
IN, OUT, SKIP, ON to GND ......................................-0.3V to +6V
REF, FB to GND.............................................-0.3V to (IN + 0.3V)
LX2, LX1 (Note 1).........................................................±1.5ARMS
Continuous Power Dissipation (TA= +70°C)
Single-Layer Board (derate 18.5mW/°C
above TA= +70°C) ...................................................1482mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Range VIN 2.5 5.5 V
UVLO Threshold UVLO VIN rising, 60mV hysteresis 2.20 2.49 V
Quiescent Supply Current, FPWM
Mode, Switching IIN No load, VOUT = 3.2V 15 22 mA
Quiescent Supply Current, Skip
Mode, Switching IIN SKIP = GND, no load 37 µA
Quiescent Supply Current, No
Switching, Skip Mode IIN SKIP = GND, FB = 1.3V 35 45 µA
ON = GND, TA = +25°C 0.1 1
Shutdown Supply Current IIN TA = +85°C 0.2 µA
PWM mode, VIN = 2.5V to 5.5V 3.30 V
IOUT = 0 to 0.5A, VIN = 2.5V to 5.5V,
TA = -40°C to +85°C (Note 3) -1 +1 %
SKIP mode, valley regulation value 3.28 V
Average skip voltage 3.285
Output Voltage Accuracy
(Fixed Output)
Load step +0.5A -3 %
Output Voltage Range
(Adjustable Output) 1.25 4.00 V
Maximum Output Current VIN = 3.6V 0.80 A
Soft-Start L = 3.3µH; COUT = C3 + C4 = 44µF 250 mA/ms
Load Regulation IOUT = 0 to 500mA 0.1 %/A
Line Regulation VIN = 2.5V to 5.5V 0.03 %/V
OUT Bias Current IOUT VOUT = 3.3V 3 µA
REF Output Voltage VREF VIN = 2.5V to 5.5V 1.244 1.25 1.256 V
REF Load Regulation IREF = 10µA 1 mV
FB Feedback Threshold VFB IOUT = 0 to full load, PWM mode; VIN = 2.5V
to 5.5V 1.244 1.25 1.258 V
Note 1: LX1 and LX2 have internal clamp diodes to IN, GND and OUT, GND, respectively. Applications that forward bias these
diodes should take care not to exceed the device's power-dissipation limits.
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
_______________________________________________________________________________________ 3
Note 2: The device is production tested at TA= +25°C. Specifications over the operating temperature range are guaranteed by
design and characterization.
Note 3: Limits are guaranteed by design and not production tested.
Note 4: The idle-mode current threshold is the transition point between fixed-frequency PWM operation and idle-mode operation.
The specification is given in terms of output load current for an inductor value of 3.3µH. For the step-up mode, the idle-mode
transition varies with input to the output-voltage ratios.
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, ON = SKIP = IN, FB = GND, VOUT = 3.3V, LX_ unconnected, CREF = C5 = 0.1µF to GND, Figure 4. TA= -40°C to +85°C.
Typical values are at TA= +25°C, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
FB Dual-Mode Threshold VFBDM 75 100 125 mV
VFB = 1.3V, TA = +25°C 0.001 0.1
FB Leakage Current IFB VFB = 1.3V, TA = +85°C 0.01 µA
ON, SKIP Input High Voltage VIH 2.5V < VIN < 5.5V 1.6 V
ON, SKIP Input Low Voltage VIL 2.5V < VIN < 5.5V 0.45 V
2.5V < VIN < 5.5V, TA = +25°C 0.001 1
ON Input Leakage Current IIHL TA = +85°C 0.01 µA
ISKIPH VSKIP = 3.6V 3 12
SKIP Input Leakage Current ISKIPL VSKIP = 0V -2 -0.2 µA
Peak Current Limit ILIMP LX1 PMOS 1700 2000 2300 mA
Fault Latch-Off Delay 100 ms
Each MOSFET, TA = +25°C 0.05 0.1
MOSFET On-Resistance RON Each MOSFET, VIN = 2.5V to 5.5V,
TA = -40°C to +85°C 0.2
Rectifier-Off Current Threshold ILX1OFF SKIP = GND 125 mA
SKIP = GND, load decreasing 100
Idle-Mode Current Threshold
(Note 4) ISKIP Load increasing 300 mA
VIN = VOUT = 5.5V, VLX1 = 0V to VIN,
VLX2 = 0V to VOUT, TA = +25°C 0.01 1
LX1, LX2 Leakage Current ILXLKG
TA = +85°C 0.2
µA
VIN = VLX1 = VLX2 = 0V, VOUT = 5.5V,
measure I (LX2), TA = +25°C 0.01 1
Out Reverse Current ILXLKGR
TA = +85°C 0.5
µA
Minimum TON TONMIN 25 %
OSC Frequency FOSCPWM 850 1000 1150 kHz
Thermal Shutdown 15°C hysteresis +165 °C
Typical Operating Characteristics
(VIN = 3.6V, SKIP = GND, TA= +25°C, Figure 4, unless otherwise noted.)
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
4 _______________________________________________________________________________________
EFFICIENCY vs. LOAD CURRENT
SKIP AND FPWM MODES
MAX8625A toc01
LOAD CURRENT (mA)
EFFICIENCY (%)
100101
10
20
30
40
50
60
70
80
90
100
0
0.1 1000
VOUT = 3.3V
VIN = 2.7V,
3.0V,
3.3V,
3.6V,
4.2V,
5.0V 60
70
65
80
75
85
90
95
100
2.0 3.0 3.52.5 4.0 4.5 5.0 5.5 6.0
SKIP-MODE EFFICIENCY
vs. INPUT VOLTAGE
MAX8625A toc02
INPUT VOLTAGE (V)
EFFICIENCY (%)
100mA
300mA
500mA
VOUT = 3.3V
LOAD CURRENT = 100mA,
300mA, 500mA
EFFICIENCY vs. LOAD CURRENT
FPWM MODE (FIGURE 3)
MAX8625A toc03
LOAD CURRENT (mA)
EFFICIENCY (%)
100101
10
20
30
40
50
60
70
80
90
100
0
0.1 1000
VOUT = 2.8V
VIN = 2.7V,
3.0V,
3.3V,
3.6V,
4.2V,
5.0V
EFFICIENCY vs. LOAD CURRENT
FPWM MODE (FIGURE 3)
MAX8625A toc04
LOAD CURRENT (mA)
EFFICIENCY (%)
100101
10
20
30
40
50
60
70
80
90
100
0
0.1 1000
VOUT = 3.45V
VIN = 2.7V,
3.0V,
3.3V,
3.6V,
4.2V,
5.0V
OUTPUT VOLTAGE (3.3V INTERNAL FB)
vs. LOAD CURRENT
MAX8625A toc05
LOAD CURRENT (mA)
DEVIATION (%)
100101
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
-2.0
0.1 1000
VOUT = 3.3V
TA = +25°C, TA = -40°C, TA = +85°C
OUTPUT VOLTAGE (2.8V EXTERNAL FB)
vs. LOAD CURRENT (FIGURE 3)
MAX8625A toc06
LOAD CURRENT (mA)
DEVIATION (%)
100101
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
-2.0
0.1 1000
VOUT = 2.8V
TA = +25°C, TA = -40°C, TA = +85°C
3.27
3.29
3.28
3.31
3.30
3.32
3.33
3.0 4.0 4.53.5 5.0 5.5 6.0
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH INTERNAL FB RESISTORS
MAX8625A toc07
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
LOAD: 500mA, VOUT = 3.3V
TA = +25°C, TA = -40°C, TA = +85°C
2.75
2.77
2.76
2.79
2.78
2.81
2.80
2.82
3.0 4.0 4.53.5 5.0 5.5 6.0
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH EXTERNAL FB RESISTORS
MAX8625A toc08
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
LOAD: 500mA, VOUT = 2.8V
TA = +25°C, TA = -40°C, TA = +85°C (FIGURE 3)
SUPPLY CURRENT vs. INPUT VOLTAGE
WITH NO LOAD
MAX8625A toc09
INPUT VOLTAGE (V)
SUPPLY CURRENT (mA)
5.55.04.54.03.53.02.5
0.1
1
10
100
0.01
2.0 6.0
NO LOAD VOUT = 3.3V
FPWM MODE
Typical Operating Characteristics (continued)
(VIN = 3.6V, SKIP = GND, TA= +25°C, Figure 4, unless otherwise noted.)
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
_______________________________________________________________________________________
5
0
200
100
400
300
600
500
700
900
800
1000
2.0 3.0 3.52.5 4.0 4.5 5.0 5.5 6.0
MAXIMUM LOAD CURRENT
vs. INPUT VOLTAGE
MAX8625A toc10
INPUT VOLTAGE (V)
MAXIMUM LOAD CURRENT (mA)
VOUT = 3.3V
1µs/div
SWITCHING WAVEFORMS
VIN = 3V, LOAD = 500mA, VOUT = 3.3V
MAX8625A toc11
VLX1
2V/div
VOUT
50mV/div
(AC-COUPLED)
VLX2
2V/div
ILX
500mA/div
1µs/div
SWITCHING WAVEFORMS
VIN = 3.3V, LOAD = 500mA, VOUT = 3.3V
MAX8625A toc12
VLX1
2V/div
VOUT
50mV/div
(AC-COUPLED)
VLX2
2V/div
ILX
500mA/div
1µs/div
SWITCHING WAVEFORMS
VIN = 3.6V, LOAD = 500mA, VOUT = 3.3V
MAX8625A toc13
VLX1
2V/div
VOUT
50mV/div
(AC-COUPLED)
VLX2
2V/div
ILX
500mA/div
10µs/div
SKIP MODE
VIN = 3V, LOAD = 20mA,
VOUT = 3.288V
MAX8625A toc14
CH1 = VLX1
2V/div
VOUT
20mV/div
(AC-COUPLED)
CH2 = VLX2
2V/div
ILX
500mA/div
1µs/div
FPWM MODE
VIN = 3V, LOAD = 20mA,
VOUT = 3.308V
MAX8625A toc15
VLX1
2V/div
VOUT
20mV/div
(AC-COUPLED)
VLX2
2V/div
ILX
500mA/div
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
6 _______________________________________________________________________________________
2ms/div
STARTUP WAVEFORMS
VIN = 3.6V, LOAD = 5, VOUT = 3.288V
MAX8625A toc16
ON
2V/div
VOUT
20mV/div
IBATT
500mA/div
ILX
500mA/div
2ms/div
STARTUP WAVEFORMS (FIGURE 3)
VIN = 3.6V, LOAD = 30, VOUT = 1.5V
MAX8625A toc17
ON
2V/div
IBATT
100mA/div
VOUT
1V/div
ILX
500mA/div
400µs/div
LOAD TRANSIENT
VOUT = 3.3V
MAX8625A toc18
VOUT
100mV/div
(DC OFFSET = 3.3V)
ILX
500mA/div
IBATT
250mA/div
Typical Operating Characteristics (continued)
(VIN = 3.6V, SKIP = GND, TA= +25°C, Figure 4, unless otherwise noted.)
1ms/div
LINE TRANSIENT
VOUT = 3.3V, LOAD = 5.5,
VIN RAMP 3V TO 4V
MAX8625A toc19
CH1 = VIN
500mV/div
3V OFFSET
CH2 = VOUT
50mV/div
(AC-COUPLED)
BODE PLOT
GAIN AND PHASE vs. FREQUENCY
MAX8625A toc20
FREQUENCY (kHz)
GAIN (dB)
10010
-50
-40
-30
-20
-10
0
10
20
30
40
-60
1 1000
VIN = 3.6V
VOUT = 3.3V
LOAD = 200mA
-180
-144
-108
-72
-36
0
36
72
108
144
180
PHASE (DEG)
GAIN
PHASE
0.90
0.94
0.92
0.98
0.96
1.04
1.02
1.00
1.06
-40 0-20 20406080100
OSCILLATOR FREQUENCY
vs. TEMPERATURE
MAX8625A toc21
TEMPERATURE (°C)
OSCILLATOR FREQUENCY (MHz)
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
_______________________________________________________________________________________
7
Typical Operating Characteristics (continued)
(VIN = 3.6V, SKIP = GND, TA= +25°C, Figure 4, unless otherwise noted.)
2.28
2.34
2.32
2.30
2.36
2.38
2.40
2.42
2.44
2.46
2.48
-50 0-25 255075100
MINIMUM STARTUP VOLTAGE
vs. TEMPERATURE
MAX8625A toc22
TEMPERATURE (°C)
MINIMUM STARTUP VOLTAGE (V)
VOUT = 3.3V, NO LOAD
1.22
1.24
1.23
1.26
1.25
1.27
1.28
-40 20 40-20 0 60 80 100
REFERENCE vs. TEMPERATURE
NO LOAD
MAX8625A toc23
TEMPERATURE (°C)
REFERENCE (V)
VOUT = 3.3V
VIN = 3.0V,
3.6V,
4.2V,
5.0V
100µs/div
SHUTDOWN DUE TO OVERLOAD
VIN = 3.6V, VOUT = 3.288V
MAX8625A toc25
VLX1
2V/div
VLX2
2V/div
VOUT
500mV/div
ILX
500mA/div
2µs/div
BOOST-TO-BUCK TRANSITION
FPWM MODE VIN = 3.6V, VOUT = 3.288V
MAX8625A toc26
VIN
1V/div
DC OFFSET = 3V
VOUT
100mV/div
AC-COUPLED
ILX
200mA/div
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
8 _______________________________________________________________________________________
Detailed Description
The MAX8625A step-up/down architecture employs a
true H-bridge topology that combines a boost converter
and a buck converter topology using a single inductor
and output capacitor (Figure 1). The MAX8625A utilizes
a pulse-width modulated (PWM), current-mode control
scheme and operates at a 1MHz fixed frequency to
minimize external component size. A proprietary
H-bridge design eliminates mode changes when transi-
tioning from buck to boost operation. This control
scheme provides very low output ripple using a much
smaller inductor than a conventional H-bridge, while
avoiding glitches that are commonly seen during mode
transitions with competing devices.
The MAX8625A switches at an internally set frequency
of 1MHz, allowing for tiny external components. Internal
compensation further reduces the external component
count in cost- and space-sensitive applications. The
MAX8625A is optimized for use in HDDs, DSCs, and
other devices requiring low-quiescent current for opti-
mal light-load efficiency and maximum battery life.
Control Scheme
The MAX8625A basic noninverting step-up/down con-
verter operates with four internal switches. The control
logic determines which two internal MOSFETs operate
to maintain the regulated output voltage. Unlike a tradi-
tional H-bridge, the MAX8625A utilizes smaller peak-
inductor currents, thus improving efficiency and
lowering input/output ripple.
The MAX8625A uses three operating phases during
each switching cycle. In phase 1 (fast-charge), the
inductor current ramps up with a di/dt of VIN/L. In phase
2 (slow charge/discharge), the current either ramps up
or down depending on the difference between the input
voltage and the output voltage (VIN - VOUT)/L. In phase 3
(discharge), the inductor current discharges at a rate of
VOUT/L through MOSFETs P2 and N1 (see Figure 1). An
additional fourth phase (phase 4: hold) is entered when
the inductor current falls to zero during phase 3. This
fourth phase is only used during skip operation.
The state machine (Figure 2) decides which phase to
use and when to switch phases. The converter goes
through the first three phases in the same order at all
Pin Description
PIN NAME FUNCTION
1, 2 LX1 Inductor Connection 1. Connect the inductor between LX1 and LX2. Both LX1 pins must be connected
together externally. LX1 is internally connected to GND during shutdown.
3, 4 LX2 Inductor Connection 2. Connect the inductor between LX1 and LX2. Both LX2 pins must be connected
together externally. LX2 is internally connected to GND during shutdown.
5 ON Enable Input. Connect ON to the input or drive high to enable the IC. Drive ON low to disable the IC.
6SKIP
Mode Select Input. Connect SKIP to GND to enable skip mode. This mode provides the best overall
efficiency curve.
Connect SKIP to IN to enable forced-PWM mode. This mode provides the lowest noise, but reduces light-
load efficiency compared to skip mode.
7FB
Feedback Input. Connect to ground to set the fixed 3.3V output. Connect FB to the center tap of an
external resistor-divider from the output to GND to set the output voltage to a different value. VFB regulates
to 1.25V.
8 REF Reference Output. Bypass REF to GND with a 0.1µF ceramic capacitor. VREF is 1.25V and is internally
pulled to GND during shutdown.
9, 10 OUT Power Output. Bypass OUT to GND with two 22µF ceramic capacitors. Both OUT pins must be connected
together externally.
11, 12 GND Ground. Connect the exposed pad and GND directly under the IC.
13, 14 IN Power-Supply Input. Bypass IN to GND with two 22µF ceramic capacitors. Connect IN to a 2.5V to 5.5V
supply. Both IN pins must be connected together externally.
—EP
Exposed Pad. Connect to GND directly under the IC. Connect to a large ground plane for increased
thermal performance.
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
_______________________________________________________________________________________ 9
times. This reduces the ripple and removes any mode
transitions from boost-only or buck-only to hybrid modes
as seen in competing H-bridge converters.
The time spent in each phase is set by a PWM con-
troller, using timers and/or peak-current regulation on a
cycle-by-cycle basis. The heart of the PWM control
block is a comparator that compares the output volt-
age-error feedback signal and the sum of the current-
sense and slope compensation signals. The current-
mode control logic regulates the inductor current as a
function of the output error voltage signal. The current-
sense signal is monitored across the MOSFETs (P1, N1,
and N2). A fixed time delay of approximately 30ns
occurs between turning the P1 and N2 MOSFETs off,
and turning the N1 and P2 MOSFETs on. This dead
time prevents efficiency loss by preventing “shoot-
through” current.
Step-Down Operation (V
IN
> V
OUT
)
During medium and heavy loads and VIN > VOUT,
MOSFETs P1 and N2 turn on to begin phase 1 at the
clock edge and ramp up the inductor current. The
duration of phase 1 is set by an internal timer. During
phase 2, N2 turns off, and P2 turns on to further ramp
up inductor current and also transfer charge to the out-
put. This slow charge phase is terminated on a clock
edge and P1 is turned off. The converter now enters the
fast discharge phase (phase 3). In phase 3, N1 turns
on and the inductor current ramps down to the valley
current-regulation point set by the error signal. At the
end of phase 3, both P2 and N1 turn off and another
phase 1 is initiated and the cycle repeats.
With SKIP asserted low, during light loads when induc-
tor current falls to zero in phase 3, the converter switch-
es to phase 4 to reduce power consumption and avoid
Figure 1. Simplified Block Diagram
MAX8625A
Gm
FB
UVLO
P1
CURRENT SENSE
REFERENCE
PWM/PFM
CONTROL
P1 P2
N2
OUT
IN
REF
LX1 LX2
N1
ON
SKIP OSCILLATOR
GND
1.25V
125mV
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
10 ______________________________________________________________________________________
shuttling current in and out of the output capacitor. If
SKIP is asserted high for forced-PWM mode, phase 4 is
not entered and current shuttling is allowed (and is
necessary to maintain the PWM operation frequency
when no load is present).
Step-Up Operation (V
IN
< V
OUT
)
During medium and heavy loads when VIN < VOUT,
MOSFETs P1 and N2 turn on at the clock edge to ramp
up the inductor current. Phase 1 terminates when the
inductor current reaches the peak target current set by
the PWM comparator and N2 turns off. This is followed
by a slow-discharge phase (phase 2) instead of a
charge phase (since VIN is less than VOUT) when P2
turns on. The slow-discharge phase terminates on a
clock edge. The converter now enters the fast-dis-
charge phase (phase 3). During phase 3, P1 turns off
and N1 turns on. At the end of the minimum time, both
P2 and N1 turn off and the cycle repeats.
If SKIP is asserted low, during light loads when inductor
current falls to zero in phase 3, the converter switches to
phase 4 (hold) to reduce power consumption and avoid
shuttling current in and out of the output. If SKIP is high
to assert forced-PWM mode, the converter never enters
phase 4 and allows negative inductor current.
Step-Up/Down Transition-Zone Operation
(V
IN
= V
OUT
)
When VIN = VOUT, the converter still goes through the
three phases for moderate to heavy loads. However,
the maximum time is now spent in phase 2 where
inductor current di/dt is almost zero, since it is propor-
tional to (VIN - VOUT). This eliminates transition glitches
Figure 2. State Diagram
FAULT
TIMEOUT
(ASYNCHRONOUS
FROM ANYWHERE)
ERROR
ON = 1
P1, P2 = OFF
N1, N2 = ON
OFF
ON = 0
P1, P2 = OFF
N1, N2 = ON
IQ = 0µA
ON = 0
(ASYNCHRONOUS
FROM
ANYWHERE)
REFOK = 0 OR
UVLO = 0
(ASYNCHRONOUS
FROM ANYWHERE)
PHASE 2
SLOW CHARGE/
DISCHARGE
OSC = ON
P1, P2 = ON
N1, N2 = OFF
PHASE 3
FAST DISCHARGE
OSC = ON
P2, N1 = ON
P1, N2 = OFF
PHASE 1
FAST-CHARGE
OSC = ON
P1, N2 = ON
P2, N1 = OFF
PHASE 4
HOLD
OSC = OFF
N1, N2 = ON
P1, P2 = OFF
POWER-UP
ON = 1, P1, P2 = OFF, N1, N2 = ON,
OSC = ON AND REF = ON IF UVLO OK
T2-3
T3-4
T1-2
T3-1
T1-3
T4-1
TRUN
TPUP
(SKIP)
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
______________________________________________________________________________________ 11
or oscillation between the boost and buck modes as
seen in other step-up/down converters. See the switch-
ing waveforms for each of the three modes and transi-
tion waveforms in the
Typical Operating Characteristics
section.
Forced-PWM Mode
Drive SKIP high to operate the MAX8625A in forced-
PWM mode. In this mode, the IC operates at a constant
1MHz switching frequency with no pulse skipping. This
scheme is desirable in noise-sensitive applications
because the output ripple is minimized and has a pre-
dictable noise spectrum. Forced PWM consumes higher
supply current at light loads due to constant switching.
Skip Mode
Drive SKIP low to operate the MAX8625A in skip mode
to improve light-load efficiency. In skip mode, the IC
switches only as necessary to maintain the output at
light loads, but still operates with fixed-frequency PWM
at medium and heavy loads. This maximizes light-load
efficiency and reduces the input quiescent current to
37µA (typ).
Do not dynamically transition between skip and FPWM.
The MAX8625A is not designed for dynamic transitions
between FPWM and skip modes. Spikes of negative
inductor current are possible when making these types
of dynamic transitions. The magnitude of the spike
depends on the load and output capacitance. The
MAX8625A has no protection against these types of
negative current spikes.
Load Regulation and Transient Response
During a load transient, the output voltage instantly
changes due to the ESR of the output capacitors by an
amount equal to their ESR times the change in load
current (VOUT = RESR x ILOAD). The output voltage
then deviates further based on the speed at which the
loop compensates for the load step. Increasing the out-
put capacitance reduces the output-voltage droop. See
the
Capacitor Selection
section. The typical application
circuit limits the output transient droop to less than 3%.
See the
Typical Operating Characteristics
section.
Soft-Start
Soft-start prevents input inrush current during startup.
Internal soft-start circuitry ramps the peak inductor cur-
rent with an internal DAC in 8ms. Once the output
reaches regulation, the current limit immediately jumps
to the maximum threshold. This allows full load capabil-
ity as soon as regulation is reached, even if it occurs
before the 8ms soft-start time is complete.
When using the MAX8625A at low input voltages (close
to UVLO and < 3V), it is recommended that the ON pin
should not be tied to the BATT or supply voltage node
directly. The ON pin should be held low for > 1ms after
power to the MAX8625A is applied before it is driven
high for normal operation.
Shutdown
Drive ON low to place the MAX8625A in shutdown
mode and reduce supply current to less than 1µA.
During shutdown, OUT is disconnected from IN, and
LX1 and LX2 are connected to GND. Drive ON high for
normal operation.
Fault and Thermal Shutdown
The MAX8625A contains current-limit and thermal shut-
down circuitry to protect the IC from fault conditions.
When the inductor current exceeds the current limit (2A
for the MAX8625A), the converter immediately enters
phase 3 and an internal 100ms timer starts. The con-
verter continues to commutate through the three phas-
es, spending most of its time in phase 1 and phase 3. If
the overcurrent event continues and the output is out of
regulation for the duration of the 100ms timer, the IC
enters shutdown mode and the output latches off. ON
must then be toggled to clear the fault. If the overload
is removed before the 100ms timer expires, the timer is
cleared and the converter resumes normal operation.
The thermal-shutdown circuitry disables the IC switching
if the die temperature exceeds +165°C. The IC begins
soft-start once the die temperature cools by 15°C.
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
12 ______________________________________________________________________________________
Applications Information
Selecting the Output Voltage
The MAX8625A output is nominally fixed at 3.3V.
Connect FB to GND to select the internally fixed-output
voltage. For an adjustable output voltage, connect FB
to the center tap of an external resistor-divider connect-
ed from the output to GND (R1 and R2 in Figure 3).
Select 100kfor R2 and calculate R1 using the follow-
ing equation:
where VFB = 1.25V and VOUT is the desired output reg-
ulation voltage. VOUT must be between 1.25V and 4V.
Note that the minimum output voltage is limited by the
minimum duty cycle. VOUT cannot be below 1.25V.
Calculating Maximum Output Current
The maximum output current provided by the MAX8625A
circuit depends on the inductor value, switching frequen-
cy, efficiency, and input/output voltage.
See the
Typical Operating Characteristics
section for
the Maximum Load Current vs. Input Voltage graph.
Capacitor Selection
The input and output ripple currents are both discontin-
uous in this topology. Therefore, select at least two
22µF ceramic capacitors at the input. Select two 22µF
ceramic output capacitors. For best stability over a
wide temperature range, use X5R or better dielectric.
Inductor Selection
The recommended inductance range for the
MAX8625A is 3.3µH to 4.7µH. Larger values of L give a
smaller ripple, while smaller L values provide a better
transient response. This is because, for boost and step-
up/down topologies, the crossover frequency is
inversely proportional to the value of L for a given load
and input voltage. The MAX8625A is internally compen-
sated, and therefore, the choice of power components
for stable operation is bounded. A 3.3µH inductor with
2A rating is recommended for the 3.3V fixed output with
0.8A load.
PCB Layout and Routing
Good PCB layout is important to achieve optimal per-
formance from the MAX8625A. Poor design can cause
excessive conducted and/or radiated noise.
Conductors carrying discontinuous currents and any
high-current path should be made as short and wide as
possible. Keep the feedback network (R1 and R2) very
close to the IC, preferably within 0.2 inches of the FB
and GND pins. Nodes with high dv/dt (switching
nodes) should be kept as small as possible and routed
away from FB. Connect the input and output capacitors
as close as possible to the IC. Refer to the MAX8625A
evaluation kit for a PCB layout example.
Rk
V
V
OUT
FB
1 100 1
Figure 3. Typical Application Circuit (Adjustable Output)
U1
MAX8625A
R2
100k
R1
140k
IN
SKIP
IN
OUT
FB
OUT
LX1LX1 LX2 LX2
12
13
14
6
ON
5
REF
8
34
L
3.3µH
C1, C2
22µF
C5
0.1µF
9
7
10 C3, C4
22µF
INPUT
2.7V TO 5.5V
MODE
SELECTION
INPUT
OUTPUT
3V
OFF
ON
GND
GND
11
12
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
______________________________________________________________________________________ 13
Chip Information
PROCESS: BiCMOS
Figure 4. Typical Application Circuit (Fixed 3.3V Output)
U1
MAX8625A
IN
SKIP
IN
OUT
FB
OUT
LX1LX1 LX2 LX2
12
13
14
6
ON
5
REF
8
34
L
3.3µH
C1, C2
22µF
C5
0.1µF
9
7
10 C3, C4
22µF
INPUT
2.7V TO 5.5V
MODE
SELECTION
INPUT
OUTPUT
3.3V
OFF
ON
GND
GND
11
12
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
14 TDFN-EP T1433-2 21-0137
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages.
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
14 ______________________________________________________________________________________
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
______________________________________________________________________________________ 15
COMMON DIMENSIONS
SYMBOL MIN. MAX.
A 0.70 0.80
D2.90 3.10
E 2.90 3.10
A1 0.00 0.05
L0.20
0.40
PKG. CODE ND2 E2 eJEDEC SPEC b[(N/2)-1] x e
PACKAGE VARIATIONS
0.25 MIN.
k
A2 0.20 REF.
2.00 REF
0.25±0.05
0.50 BSC
2.30±0.10
10
T1033-1
2.40 REF
0.20±0.05- - - -
0.40 BSC
1.70±0.10 2.30±0.10
14
T1433-1
1.50±0.10 MO229 / WEED-3
0.40 BSC - - - - 0.20±0.05 2.40 REF
T1433-2 14 2.30±0.10
1.70±0.10
T633-2 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF
T833-2 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF
T833-3 81.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF
2.30±0.10 MO229 / WEED-3 2.00 REF
0.25±0.050.50 BSC1.50±0.10
10
T1033-2
0.25±0.05 2.00 REF
10 0.50 BSC MO229 / WEED-3
2.30±0.10
1.50±0.10
T1033MK-1
0.40 BSC - - - - 0.20±0.05 2.40 REFT1433-3F 14 2.30±0.101.70±0.10
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 3/08 Initial release
1 5/08 Added PCB Layout and Routing section 12
2 10/08 Updated Electrical Characteristics, Skip Mode and Soft-Start sections 2, 11
3 12/08 Corrected P1 and P2 symbols in Figure 1 9