voltages that are either greater or less than the output.

DC-DC converters can also be classified by the control

method. The two most common are pulse-width modu-

lation (PWM) and pulse-frequency modulation (PFM).

PWM switch-mode power-supply ICs (of which current-

mode control is one variant) are well-established in

high-power off-line switchers. Both PWM and PFM cir-

cuits control the output voltage by varying duty cycle.

In the PWM circuit, the frequency is held constant and

the width of each pulse is varied. In the PFM circuit, the

pulse width is held constant and duty cycle is con-

trolled by changing the pulse repetition rate.

The MAX630 refines the basic PFM by employing a con-

stant-frequency oscillator. Its output MOSFET is switched

on when the oscillator is high and the output voltages is

lower than desired. If the output voltage is higher than

desired, the MOSFET output is disabled for that oscillator

cycle. This pulse skipping varies the average duty cycle,

and thereby controls the output voltage.

Note that, unlike the PWM ICs, which use an op amp as

the control element, the MAX630 uses a comparator to

compare the output voltage to an onboard reference.

This reduces the number of external components and

operating current.

Typical Applications

+5V to +15V DC-DC Converter

Figure 1 shows a simple circuit that generates +15V at

approximately 20mA from a +5V input. The MAX630

has a ±1.5% reference accuracy, so the output voltage

has an untrimmed accuracy of ±3.5% if R1 and R2 are

1% resistors. Other output voltages can also be select-

sets the oscillator frequency (47pF = 40kHz), while C1

limits output ripple to about 50mV.

With a low-cost molded inductor, the circuit’s efficiency

is about 75%, but an inductor with lower series resis-

tance such as the Dale TE3Q4TA increases efficiency

to around 85%. A key to high efficiency is that the

MAX630 itself is powered from the +15V output. This

provides the onboard N-channel output device with 15V

gate drive, lowering its on-resistance to about 4

Ω.

When +5V power is first applied, current flows through

L1 and D1, supplying the MAX630 with 4.4V for startup.

+5V to ±15V DC-DC Converter

The circuit in Figure 3 is similar to that of Figure 1

except that two more windings are added to the induc-

tor. The 1408 (14mm x 8mm) pot core specified is an

IEC standard size available from many manufacturers

(see Table 1). The -15V output is semiregulated, typi-

cally varying from -13.6V to -14.4V as the +15V load

current changes from no load to 20mA.

2.5W, 3V to 5V DC-DC Converter

Some systems, although battery powered, need high

currents for short periods, and then shut down to a low-

power state. The extra circuitry of Figure 4 is designed to

meet these high-current needs. Operating in the buck-

boost or flyback mode, the circuit converts -3V to +5V.

The left side of Figure 4 is similar to Figure 1 and sup-

plies 15V for the gate drive of the external power MOS-

FET. This 15V gate drive ensures that the external device

is completely turned on and has low on-resistance.

The right side of Figure 4 is a -3V to +5V buck-boost

converter. This circuit has the advantage that when the

MAX630 is turned off, the output voltage falls to 0V,

unlike the standard boost circuit, where the output volt-

When shut down, this circuit uses less than 10µA, with

most of the current being the leakage current of the

power MOSFET.

The inductor and output-filter capacitor values have

been selected to accommodate the increased power

levels. With the values indicated, this circuit can supply

up to 500mA at 5V, with 85% efficiency. Since the left

side of the circuit powers only the right-hand MAX630,

the circuit starts up with battery voltages as low as

1.5V, independent of the loading on the +5V output.

CMOS Micropower Step-Up

Switching Regulator

_______________________________________________________________________________________

9

MAX630

1M

Ω

95.3k

Ω

47pF

7

2

GND

LBD

N.C.

GND

1

3

65

48

+5V

330

μF

25V

330

μF

25V

1:3

:3

220

μH PRIMARY

14 x 8mm POT CORE

ALL DIODES IN4148

Figure 3. +5V to ±15V Converter