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 varactor tuning. The

MAX629’s unique control scheme provides high efficiency

and a wide range of output voltages with only 80μA qui-

escent supply current, making it ideal for battery-powered

applications. The internal N-channel DMOS switch has

a pin-programmable current limit (250mA and 500mA),

allowing optimization of output current and component

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

(ISET = GND) (see Setting the Current Limit). After the

on-cycle terminates, the switch turns off, charging the out-

put capacitor through the diode. In normal operation, the

minimum off-time is set to 1μs for positive output voltages

and 3.5μs for negative output voltages. When the output

is well below regulation, however, the off-time is increased

to 5μs to provide 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 cur-

rent drops to less than 1μA. SHDN is a logic-level input;

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 configura-

tion (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 external

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 without

PIN

NAME

FUNCTION

1

SHDN

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

current to 1µA.

2

POL

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

voltage.

3

REF

4

FB

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

Output Voltage.

5

ISET

LX current limit. See Setting the Current Limit.

6

GND

Ground

7

LX

Internal N-Channel DMOS Switch Drain

8

Power-Supply Input

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Maxim Integrated │ 5

MAX629

28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

Pin Description