_______________Detailed Description

The MAX685 DC-DC converter accepts an input voltage

between +2.7V and +5.5V and generates both a positive

and negative voltage, using a single inductor (Figure 1).

It alternates between acting as a step-up converter and

as an inverting converter on a cycle-by-cycle basis. Both

output voltages are independently regulated.

Each output is separately controlled by a pulse-width-

modulated (PWM) current mode regulator. This allows

the part to operate at a fixed frequency for use in noise-

sensitive applications. An internal oscillator runs at

220kHz or 400kHz, or can be synchronized to an exter-

nal signal. Since switching alternates between the two

regulators, each operates at half the oscillator frequency

(110kHz, 200kHz, or half the sync frequency). The oscil-

lator can be synchronized to a 200kHz to 480kHz clock.

On the first cycle of operation, the part operates as a

ground, and the inductor current rises. Once the induc-

tor current rises to a level set by the positive-side error

amplifier, LXN releases and the inductor current flows

through D2 to the positive output. When the inductor

current drops to zero (which happens each cycle under

normal, discontinuous operation), LXN returns to the

input voltage.

On the second cycle, LXN is held at ground. LXP is

pulled up to the input voltage until the current reaches

the limit set by the negative error amplifier. Then LXP is

released and the inductor current flows through D1 to

the negative output. Once the inductor current reaches

zero, the voltage at LXP returns to ground. The wave-

forms at LXN and LXP are shown in Figure 2 for a typi-

cal pair of cycles.

The current into the LXN pin is sensed to measure the

inductor current. The MAX685 controls the inductor cur-

rent to regulate both the positive and negative output

voltages.

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

6

_______________________________________________________________________________________

Pin Description

PIN

P-Channel Switching Inductor Node. LXP turns off when the part enters shutdown.

LXP

1

FUNCTION

NAME

Internally Connected. Do not externally connect.

I.C.

2, 15

100k

POK

4

VP

3

SHDN

6

8

Sync Input. This pin synchronizes the oscillator to an external clock frequency between 200kHz and 480kHz.

SYNC

7

Power-Up Sequence Select Input. Connect SEQ to GND to power the negative output voltage first. Connect

SEQ

5

Ground

GND

9

1.25V Reference Voltage Output. Bypass with 0.22µF to GND.

REF

11

Power Ground. Connect PGND to GND.

PGND

13, 14

Feedback for the Positive Output Voltage. Connect a resistor-divider between the positive output and GND

with the center to FBP to set the positive output voltage.

FBP

12

Feedback Input for the Negative Output Voltage. Connect a resistor-divider between the negative output

and REF with the center to FBN to set the negative output voltage.

FBN

10

N-Channel Switching Inductor Node. LXN pulls to GND through the internal transistor when the part is shut

down.

LXN

16