Step-Down Controllers with

Synchronous Rectifier for CPU Power

_______________________________________________________________________________________

9

______________________________________________________________Pin Description

Current-Sense input, High side. Current-limit level is 100mV referred to CSL.

CSH

8

Current-Sense input, Low side. Also serves as the feedback input in fixed-output modes.

CSL

9

Battery voltage input (4.5V to 30V). Bypass V+ to PGND close to the IC with a 0.1µF capacitor. Connects to a

linear regulator that powers VL.

V+

10

5V Internal linear-regulator output. VL is also the supply voltage rail for the chip. VL is switched to the output

supply up to 5mA for external loads.

VL

11

Power Ground.

PGND

12

Low-noise analog Ground and feedback reference point.

GND

4

Oscillator Synchronization and frequency select. Tie to GND or VL for 150kHz operation; tie to REF for

300kHz operation. A high-to-low transition begins a new cycle. Drive SYNC with 0V to 5V logic levels (see the

guaranteed.

SYNC

5

MOSFET). Tie SHDN to V+ for automatic start-up.

SHDN

6

input that also selects the fixed output voltage settings as follows:

• Connect to GND for 3.3V operation.

• Connect to VL for 5V operation.

• Connect FB to a resistor divider for adjustable mode. FB can be driven with +5V rail-to-rail logic in order to

change the output voltage under system control.

FB

7

Reference voltage output. Bypass to GND with 0.33µF minimum.

REF

3

PIN

Secondary winding Feedback input. Normally connected to a resistor divider from an auxiliary output.

Don’t leave SECFB unconnected.

• MAX796: SECFB regulates at VSECFB = 2.505V. Tie to VL if not used.

• MAX799: SECFB regulates at VSECFB = 0V. Tie to a negative voltage through a high-value current-limit-

SECFB

(MAX796/

MAX799)

2

Soft-Start timing capacitor connection. Ramp time to full current limit is approximately 1ms/nF.

SS

1

FUNCTION

NAME

Low-side gate-drive output. Normally drives the synchronous-rectifier MOSFET. Swings 0V to VL.

DL

13

Boost capacitor connection for high-side gate drive (0.1µF).

BST

14

Switching node (inductor) connection. Can swing 2V below ground without hazard.

LX

15

High-side gate-drive output. Normally drives the main buck switch. DH is a floating driver output that swings

from LX to BST, riding on the LX switching-node voltage.

DH

16

Dual Mode is a trademark of Maxim Integrated Products.

Disables pulse-skipping mode when high. Connect to GND for normal use. Don’t leave SKIP unconnected.

With SKIP grounded, the device will automatically change from pulse-skipping operation to full PWM opera-

tion when the load current exceeds approximately 30% of maximum. (See Table 3.)

SKIP

(MAX797)