28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

_______________________________________________________________________________________

5

______________________________________________________________Pin Description

current limit. See

Setting the Current Limit.

ISET

5

Ground

GND

6

Feedback Input for setting output voltage. Connect to an external voltage divider. See

Setting the Output

Voltage.

FB

4

drive external loads. For 10µA

REF

3

PIN

Polarity Input. Changes polarity and threshold of FB to allow regulation of either positive or negative output

POL

2

Active-Low Shutdown Input. A logic low puts the MAX629 in shutdown mode and reduces supply current to

1µA.

SHDN

1

FUNCTION

NAME

Internal N-Channel DMOS Switch Drain

LX

7

Power-Supply Input

8

_______________Detailed Description

The MAX629 low-power, boost DC-DC converter pro-

vides either positive or negative output voltages up to

±28V from a wide range of input voltages. It is

designed primarily for use in low-power, high-voltage

applications such as LCD biasing and set-top box var-

actor tuning. The MAX629’s unique control scheme

provides high efficiency and a wide range of output

voltages with only 80µA quiescent supply current, mak-

ing it ideal for battery-powered applications. The inter-

nal N-channel DMOS switch has a pin-programmable

current limit (250mA and 500mA), allowing optimization

of output current and component size. Figure 1 shows

the MAX629 functional diagram.

Control Scheme

A combination of peak-current limiting and a pair of

one-shots controls the MAX629 switching, determining

the maximum on-time and constant off-time. During the

on-cycle, the internal switch closes, and current

through the inductor ramps up until either the fixed

10µs maximum on-time expires (at low input voltages)

or the switch’s peak current limit is reached. The peak

switch current limit is selectable to either 500mA (ISET

Current Limit). After the on-cycle terminates, the switch

turns off, charging the output capacitor through the

diode. In normal operation, the minimum off-time is set

to 1µs for positive output voltages and 3.5µs for nega-

tive output voltages. When the output is well below reg-

ulation, however, the off-time is increased to 5µs to pro-

vide soft-start during start-up. The switching frequency,

which depends upon the load, can be as high as

300kHz.

Shutdown Mode

When SHDN is low, the MAX629 enters shutdown

mode. In this mode, the feedback and control circuit,

reference, and internal biasing circuitry turn off. The

shutdown current drops to less than 1µA. SHDN is a

The output voltage behavior in shutdown mode

depends on the output voltage polarity. In the positive

output voltage configuration (Figure 2), the output is

directly connected to the input through the diode (D1)

and the inductor (L1). When the device is in shutdown

mode, the output voltage falls to one diode drop below

the input voltage, and any load connected to the output

may still conduct current. In the negative output voltage

configuration (Figure 3), there is no DC connection

between the input and the output, and in shutdown

mode the output is pulled to GND.

__________________Design Procedure

Setting the Output Voltage

For either positive or negative output voltage applica-

tions, set the MAX629’s output voltage using two exter-

nal resistors, R1 and R2, as shown in Figures 2 and 3.

Since the input bias current at FB has a 50nA maximum

value, large resistors can be used in the feedback loop