MAX629CSLASHD datasheet(7/12 Pages) MAXIM

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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

VCC. When lower output current is required, the 250mA

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

VOUT

+24V

VCC

+2.7V

TO +5.5V

VIN

+0.8V

TO +24V

0.1µF

10µF

35V

10µF

35V

150pF

576k

31.6k

47µH

MBR0540L

VCC

GND

ISET

REF

POL

MAX629

10µF

35V

0.1µF

47µH

576k

35.7k

D1 = D2 = MBR0540L

2Ω

2.2µF

10µF

35V

150pF

0.1µF

VCC

+2.7V

TO +5.5V

VIN

+0.8V

TO +15V

VOUT

-20V

VCC

GND

REF

POL

ISET

SHDN

www.maximintegrated.com

Maxim Integrated │ 7

MAX629

28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

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