External Components

Resistors

as 10nA (max), the current in the dividers R1/R2 and

R3/R4 (Figure 1) may be as low as 1µA without signifi-

cantly affecting accuracy. Normally R2 and R4 are

between 10k

Ω and 1MΩ, which sets the current in the

voltage-dividers in the 1.3µA to 130µA range. R1 and

R3 can then be calculated as follows:

the desired low-battery warning threshold.

through the pullup resistor should be a minimum of 4µA

Inductor Value

The available output current from a DC-DC voltage

boost converter is a function of the input voltage, exter-

nal inductor value, output voltage, and the operating

frequency.

The inductor must 1) have the correct inductance, 2) be

able to handle the required peak currents, and 3) have

acceptable series resistance and core losses. If the

inductance is too high, the MAX630 will not be able to

put on for every oscillator cycle. The available output

power can be increased by either decreasing the

inductance or the frequency. Reducing the frequency

increasing the peak inductor current. The available out-

put power is increased since it is proportional to the

diode (D1) as well as that in the load. If the inductance

rating. The minimum allowed inductor value is

expressed by:

The most common MAX630 circuit is a boost-mode

converter (Figure 1). When the N-channel output device

is on, the current linearly rises since:

At the end of the on-time (14µs for 40kHz, 55% duty-

cycle oscillator) the current is:

The energy in the coil is:

At maximum load, this cycle is repeated 40,000 times

per second, and the power transferred through the coil

is 40,000 x 5.25 = 210mW. Since the coil only supplies

the voltage above the input voltage, at 15V, the DC-DC

converter can supply 210mW / (15V - 5V) = 21mA. The

coil provides 210mW and the battery directly supplies

another 105mW, for a total of 315mW of output power. If

the load draws less than 21mA, the MAX630 turns on its

output only often enough to keep the output voltage at

a constant 15V.

Reducing the inductor value increases the available

output current: lower L increases the peak current,

thereby increasing the available power. The external

inductor required by the MAX630 is readily obtained

from a variety of suppliers (Table 1). Standard coils are

suitable for most applications.

Types of Inductors

Molded Inductors

These are cylindrically wound coils that look similar to

1W resistors. They have the advantages of low cost and

ease of handling, but have higher resistance, higher

losses, and lower power handling capability than other

types.

ImA

pk

VT

L

Vx

s

H

ON

==

μ

μ

=

514

470

150

di

dt

V

L

=

I

IN ON

MAX

=

L

VT

f

P

ce P

LI

f

and I

IN ON

OUT

OUT

pk

pk

VT

L

IN ON

=

=

=

()

sin

:

:

2

2

2

2

R

VV

A

IC

S

≤

+−

μ

13

4

.

10

2

1

1

2

131

131

10

4

1

3

4

131

131

ΩΩ

ΩΩ

≤≤

=

−

≤≤

=

−

.

.

.

.

RM

R

R

x

VV

RM

R

R

x

VV

OUT

LB

CMOS Micropower Step-Up

Switching Regulator

6

_______________________________________________________________________________________

EJ

=

=μ

2

2

525

.