Datasheet MAX8625, MAX8625A Datasheet (Maxim)

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 intend­ed 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, fixed­frequency 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 addi­tion, soft-start circuitry reduces inrush current during
startup. The IC also features True Shutdown

TM

, 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 1; 5/08

*

US Patent #7,289,119.

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-free package.

**

EP = Exposed pad.

PART

PIN­PACKAGE

TOP MARK

MAX8625AETD+

14 TDFN-EP**
(3mm x 3mm)

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, V

OUT

= 3.3V, LX_ unconnected, C

REF

= C5 = 0.1µF to GND, Figure 4. TA= -40°C to +85°C.

Typical values are at T

A

= +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, (IN + 0.3V)

LX2, LX1 (Note 1).........................................................±1.5A

RMS

Continuous Power Dissipation (TA= +70°C)

Single-Layer Board (derate 18.5mW/°C

above T

A

= +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 V

IN

2.5 5.5 V

UVLO Threshold UVLO VIN rising, 60mV hysteresis 2.20 2.49 V

Quiescent Supply Current, FPWM
Mode, Switching

I

IN

No load, V

OUT

= 3.2V 15 22 mA

Quiescent Supply Current, Skip
Mode, Switching

I

IN

SKIP = GND, no load 37 µA

Quiescent Supply Current, No
Switching, Skip Mode

I

IN

SKIP = GND, FB = 1.3V 35 45 µA

ON = GND, TA = +25°C 0.1 1

Shutdown Supply Current I

IN

TA = +85°C 0.2

µA

PWM mode, VIN = 2.5V to 5.5V 3.30 V

I

OUT

= 0 to 0.5A, VIN = 2.5V to 5.5V,

T

A

= -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; C

OUT

= C3 + C4 = 44µF 250 mA/ms

Load Regulation I

OUT

= 0 to 500mA 0.1 %/mA

Line Regulation VIN = 2.5V to 5.5V 0.03 %/V

OUT Bias Current I

OUT

V

OUT

= 3.3V 3 µA

REF Output Voltage V

REF

VIN = 2.5V to 5.5V 1.244 1.25 1.256 V

REF Load Regulation I

REF

= 10µA 1 mV

FB Feedback Threshold V

FB

I

OUT

= 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, PGND and OUT, PGND, 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: Devices are 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, V

OUT

= 3.3V, LX_ unconnected, C

REF

= C5 = 0.1µF to GND, Figure 4. TA= -40°C to +85°C.

Typical values are at T

A

= +25°C, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

FB Dual-Mode Threshold V

FBDM

75 100 125 mV

VFB = 1.3V, TA = +25°C 0.001 0.1

FB Leakage Current I

FB

VFB = 1.3V, TA = +85°C 0.01

µA

ON, SKIP Input High Voltage V

IH

2.5V < VIN < 5.5V 1.6 V

ON, SKIP Input Low Voltage V

IL

2.5V < VIN < 5.5V 0.45 V

2.5V < VIN < 5.5V, TA = +25°C 0.001 1

ON Input Leakage Current I

IHL

TA = +85°C 0.01

µA

I

SKIPH

V

SKIP

= 3.6V 3 12

SKIP Input Leakage Current

I

SKIPL

V

SKIP

= 0V -2 -0.2

µA

Peak Current Limit I

LIMP

LX1 PMOS 1700 2000 2300 mA

Fault Latch-Off Delay 100 ms

Each MOSFET, TA = +25°C 0.05 0.1

MOSFET On-Resistance R

ON

Each MOSFET, VIN = 2.5V to 5.5V,
T

A

= -40°C to +85°C

0.2

Ω

Rectifier-Off Current Threshold I

LX1OFF

SKIP = GND 125 mA
SKIP = GND, load decreasing 100

Idle-Mode Current Threshold
(Note 4)

I

SKIP

Load increasing 300

mA

VIN = V

OUT

= 5.5V, V

LX1

= 0V to VIN,

V

LX2

= 0V to V

OUT

, TA = +25°C

0.01 1

LX1, LX2 Leakage Current I

LXLKG

TA = +85°C 0.2

µA

VIN = V

LX1

= V

LX2

= 0V, V

OUT

= 5.5V,

measure I (LX2), T

A

= +25°C

0.01 1

Out Reverse Current I

LXLKGR

TA = +85°C 0.5

µA

Minimum T

ON

T

ONMIN

25 %

OSC Frequency F

OSCPWM

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

V

OUT

= 3.3V

V

IN

= 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

V

OUT

= 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

V

OUT

= 2.8V

V

IN

= 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

V

OUT

= 3.45V

V

IN

= 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

V

OUT

= 3.3V

T

A

= +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

V

OUT

= 2.8V

T

A

= +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, V

OUT

= 3.3V

T

A

= +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, V

OUT

= 2.8V

T

A

= +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 V

OUT

= 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)

V

OUT

= 3.3V

1

μs/div

SWITCHING WAVEFORMS

V

IN

= 3V, LOAD = 500mA, V

OUT

= 3.3V

MAX8625A toc11

V

LX1

2V/div

V

OUT

50mV/div
(AC-COUPLED)

V

LX2

2V/div

I

LX

500mA/div

1

μs/div

SWITCHING WAVEFORMS

V

IN

= 3.3V, LOAD = 500mA, V

OUT

= 3.3V

MAX8625A toc12

V

LX1

2V/div

V

OUT

50mV/div
(AC-COUPLED)

V

LX2

2V/div

I

LX

500mA/div

1

μs/div

SWITCHING WAVEFORMS

V

IN

= 3.6V, LOAD = 500mA, V

OUT

= 3.3V

MAX8625A toc13

V

LX1

2V/div

V

OUT

50mV/div
(AC-COUPLED)

V

LX2

2V/div

I

LX

500mA/div

10μs/div

SKIP MODE

V

IN

= 3V, LOAD = 20mA,

V

OUT

= 3.288V

MAX8625A toc14

CH1 = V

LX1

2V/div

V

OUT

20mV/div
(AC-COUPLED)

CH2 = V

LX2

2V/div

I

LX

500mA/div

1μs/div

FPWM MODE

V

IN

= 3V, LOAD = 20mA,

V

OUT

= 3.308V

MAX8625A toc15

V

LX1

2V/div

OUT

20mV/div
(AC-COUPLED)

V

LX2

2V/div

I

LX

500mA/div

MAX8625A

High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter

6 _______________________________________________________________________________________

2ms/div

STARTUP WAVEFORMS

V

IN

= 3.6V, LOAD = 5Ω, V

OUT

= 3.288V

MAX8625A toc16

SHDN

2V/div

V

OUT

20mV/div

I

BATT

500mA/div

I

LX

500mA/div

2ms/div

STARTUP WAVEFORMS (FIGURE 3)

V

IN

= 3.6V, LOAD = 30Ω, V

OUT

= 1.5V

MAX8625A toc17

SHDN

2V/div

I

BATT

100mA/div

V

OUT

500mA/div

I

LX

500mA/div

400

μs/div

LOAD TRANSIENT

V

OUT

= 3.3V

MAX8625A toc18

V

OUT

100mV/div
(DC OFFSET = 3.3V)

I

LX

500mA/div

I

BATT

250mA/div

Typical Operating Characteristics (continued)

(VIN= 3.6V, SKIP = GND, TA= +25°C, Figure 4, unless otherwise noted.)

1ms/div

LINE TRANSIENT

V

OUT

= 3.3V, LOAD = 5.5Ω,

V

IN

RAMP 3V TO 4V

MAX8625A toc19

CH1 = V

IN

500mV/div

3V OFFSET

CH2 = V

OUT

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 = 3.6
V

OUT

= 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 20 40 60 80 100

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 25 50 75 100

MINIMUM STARTUP VOLTAGE

vs. TEMPERATURE

MAX8625A toc22

TEMPERATURE (°C)

MINIMUM STARTUP VOLTAGE (V)

V

OUT

= 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)

V

OUT

= 3.3V

V

IN

= 3.0V,

3.6V,

4.2V,

5.0V

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

WITH 300mA LOAD

MAX8625A toc24

TEMPERATURE (°C)

REFERENCE (V)

V

OUT

= 3.3V

V

IN

= 3.0V,

3.6V,

4.2V,

5.0V

100

μs/div

SHUTDOWN DUE TO OVERLOAD

V

IN

= 3.6V, V

OUT

= 3.288V

MAX8625A toc25

V

LX2

2V/div

V

LX2

2V/div

V

OUT

500mV/div

I

LX

500mA/div

2μs/div

BOOST-TO-BUCK TRANSITION

FPWM MODE V

IN

= 3.6V, V

OUT

= 3.288V

MAX8625A toc26

V

IN

1V/div
DC OFFSET = 3V

V

OUT

100mV/div
AC-COUPLED

I

LX

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 transi­tioning 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 opti­mal light-load efficiency and maximum battery life.

Control Scheme

The MAX8625A basic noninverting step-up/down con­verter operates with four internal switches. The control
logic determines which two internal MOSFETs operate
to maintain the regulated output voltage. Unlike a tradi­tional H-bridge, the MAX8625A utilizes smaller peak­inductor 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- V

OUT

)/L. In phase 3
(discharge), the inductor current discharges at a rate of
V

OUT

/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.

6 SKIP

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 light­load 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. V

REF

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 con­troller, 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 volt­age-error feedback signal and the sum of the current­sense and slope compensation signals. The current­mode control logic regulates the inductor current as a
function of the output error voltage signal. The current­sense 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 “shoot­through” current.

Step-Down Operation (V

IN

> V

OUT

)

During medium and heavy loads and V

IN

> V

OUT

,
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 out­put. 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 induc­tor current falls to zero in phase 3, the converter switch­es to phase 4 to reduce power consumption and avoid

Figure 1. Simplified Block Diagram

MAX8625

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 V

IN

< V

OUT,

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 VINis less than V

OUT

) when P2
turns on. The slow-discharge phase terminates on a
clock edge. The converter now enters the fast-dis­charge 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 V

IN

= V

OUT

, 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 propor­tional to (VIN- V

OUT

). 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

I

Q

= 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 switch­ing waveforms for each of the three modes and transi­tion waveforms in the

Typical Operating Characteristics

section.

Forced-PWM Mode

Drive SKIP high to operate the MAX8625A in forced­PWM 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 pre­dictable 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).

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 (ΔV

OUT

= R

ESR

x ΔI

LOAD

). The output voltage
then deviates further based on the speed at which the
loop compensates for the load step. Increasing the out­put 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 cur­rent with an internal DAC in 8ms. Once the output
reaches regulation, the current limit immediately jumps
to the maximum threshold. This allows full load capabil­ity as soon as regulation is reached, even if it occurs
before the 8ms soft-start time is complete.

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 shut­down 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 con­verter continues to commutate through the three phas­es, 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 connect­ed from the output to GND (R1 and R2 in Figure 3).
Select 100kΩ for R2 and calculate R1 using the follow­ing equation:

where VFB= 1.25V and V

OUT

is the desired output reg-

ulation voltage. V

OUT

must be between 1.25V and 4V.
Note that the minimum output voltage is limited by the
minimum duty cycle. V

OUT

cannot be below 1.25V.

Calculating Maximum Output Current

The maximum output current provided by the MAX8625A
circuit depends on the inductor value, switching frequen­cy, 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 discontin­uous 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 step­up/down topologies, the crossover frequency is
inversely proportional to the value of L for a given load
and input voltage. The MAX8625A is internally compen­sated, 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 per­formance 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 =× −

⎛
⎝

⎜

⎞
⎠

⎟

Ω

Figure 3. Typical Application Circuit (Adjustable Output)

U1

MAX8625A

R2
100kΩ

R1
140kΩ

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 Information

For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages

.

PACKAGE TYPE PACKAGE CODE DOCUMENT NO.

14 TDFN-EP T1433-2

21-0137

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.

14

____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2008 Maxim Integrated Products Printed USA 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