Detailed Description

The operation of the MAX630 can best be understood

by examining the voltage regulating loop of Figure 1.

R1 and R2 divide the output voltage, which is com-

pared with the 1.3V internal reference by comparator

COMP1. When the output voltage is lower than desired,

the comparator output goes high and the oscillator out-

put pulses are passed through the NOR gate latch,

As long as the output voltage is less than the desired

voltage, pin 3 drives the inductor with a series of pulses

at the oscillator frequency.

Each time the output N-channel MOSFET is turned on,

the current through the external coil, L1, increases,

storing energy in the coil. Each time the output turns off,

the voltage across the coil reverses sign and the volt-

biased, delivering power to the output.

When the output voltage reaches the desired level,

1.31V x (1 + R1 / R2), the comparator output goes low

and the inductor is no longer pulsed. Current is then

supplied by the filter capacitor, C1, until the output volt-

switched on, repeating the cycle. The average duty

The MAX630/MAX4193 output device is a large

N-channel MOSFET with an on-resistance of 4

Ω and a

peak current rating of 525mA. One well-known advan-

tage that MOSFETs have over bipolar transistors in

switching applications is higher speed, which reduces

switching losses and allows the use of smaller, lighter,

less costly magnetic components. Also important is that

MOSFETs, unlike bipolar transistors, do not require

base current that, in low-power DC-DC converters,

often accounts for a major portion of input power.

The operating current of the MAX630 and MAX4193

increases by approximately 1µA/kHz at maximum

power output due to the charging current required by

increase at a 40kHz operating frequency). In compari-

son, equivalent bipolar circuits typically drive their NPN

bipolar circuit’s operating current to increase by a fac-

tor of 10 between no load and full load.

Oscillator

The oscillator frequency is set by a single external, low-

sets the oscillator to 40kHz, a reasonable compromise

between lower switching losses at low frequencies and

reduced inductor size at higher frequencies.

CMOS Micropower Step-Up

Switching Regulator

4

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Pin Description

PIN

NAME

FUNCTION

1

LBR

Low-Battery Detection Comparator Input. The LBD output, pin 8, sinks current whenever this pin is

below the low-battery detector threshold, typically 1.31V.

An external capacitor connected between this terminal and ground sets the oscillator frequency.

47pF = 40 kHz.

resistance of 4

Ω and a peak current rating of 525mA.

4

GND

Ground

The positive supply voltage, from 2.0V to 16.5V (MAX630).

The MAX630/MAX4193 shut down when this pin is left floating or is driven below 0.2V. For normal

than 1.31V.

8

LBD

The Low-Battery Detector output is an open-drain N-channel MOSFET that sinks up to 600

μA (typ)

whenever the LBR input, pin 1, is below 1.31V.