High-Efficiency, Seamless Transition,

Step-Up/Down DC-DC Converter

8

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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

2 (slow charge/discharge), the current either ramps up

or down depending on the difference between the input

(discharge), the inductor current discharges at a rate of

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 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

to 1.25V.

8

REF

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.