Setting the Current Limit

External current-limit selection provides added control

over the MAX629’s output performance. A higher current

limit increases the amount of energy stored in the inductor

during each cycle, which provides a higher output current

capability. For higher output current applications, choose

the 500mA current-limit option by connecting ISET to

current limit can provide several advantages. First, a

smaller inductor can be used, which saves board area

and cost. Second, the smaller energy transfer per cycle

reduces output ripple for a given capacitor, providing

design flexibility between board area, cost, and output rip-

ple by allowing cheaper, higher-ESR capacitors. Connect

ISET to GND to select the 250mA current-limit option.

Inductor Selection

The MAX629’s high switching frequency allows for the

use of a small inductor. The 47μH inductor shown in

the Typical Operating Circuit is recommended for most

applications. Larger inductances reduce the peak induc-

tor current, but may limit output current capability at low

input voltages and provide slower start-up times. Smaller

inductances require less board space, but may cause

greater peak current due to current-sense comparator

propagation delay. If input voltages below 2V will be com-

mon, reducing the inductance to 22μH might improve per-

formance; however, maximum load current and efficiency

may decline. It is important to thoroughly test operation

under all input and output conditions to ensure proper

component selection.

Inductors with a ferrite core or equivalent are recom-

mended; powder iron cores are not recommended for

use with high switching frequencies. The inductor’s incre-

mental saturation rating must exceed the selected current

limit. For highest efficiency, use an inductor with a low DC

resistance (under 100mΩ). See Table 1 for a list of induc-

tor suppliers.

Figure 2. +24V for a Positive LCD Bias

Figure 3. -20V for a Negative LCD Bias

MAX629

SHDN

+24V

+2.7V

TO +5.5V

+0.8V

TO +24V

*

C3

0.1µF

C4

0.1µF

C1

10µF

35V

C2

10µF

35V

150pF

R1

576k

1%

R2

31.6k

1%

L1

47µH

D1

MBR0540L

LX

GND

FB

ISET

REF

POL

MAX629

C1

10µF

35V

C3

0.1µF

L1

47µH

R1

576k

1%

R2

35.7k

1%

D1

D1 = D2 = MBR0540L

D2

R3

2Ω

C5

2.2µF

C2

10µF

35V

CF

150pF

C4

0.1µF

+2.7V

TO +5.5V

+0.8V

TO +15V

-20V

LX

GND

FB

REF

POL

ISET

*

SHDN

www.maximintegrated.com

Maxim Integrated │ 7

MAX629

28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter