28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

_______________________________________________________________________________________

7

Setting the Current Limit

External current-limit selection provides added control

over the MAX629’s output performance. A higher cur-

rent 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

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 ripple by allowing cheap-

er, 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

inductor 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 com-

parator propagation delay. If input voltages below 2V

will be common, reducing the inductance to 22µH

might improve performance; 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

incremental saturation rating must exceed the selected

current limit. For highest efficiency, use an inductor with

a low DC resistance (under 100m

Ω). See Table 1 for a

list of inductor suppliers.

Figure 3. -20V for a Negative LCD Bias

* FOR SINGLE-SUPPLY OPERATION

MAX629

SHDN

C1

10

µF

35V

C3

0.1

µF

L1

47

µH

R1

576k

1%

R2

35.7k

1%

D1

D1 = D2 = MBR0540L

D2

R3

2

Ω

C5

2.2

µF

C2

10

µF

35V

CF

150pF

C4

0.1

µF

+2.7V

TO +5.5V

+0.8V

TO +15V

-20V

LX

GND

FB

REF

POL

ISET

*

Figure 2. +24V for a Positive LCD Bias

MAX629

SHDN

+24V

+2.7V

TO +5.5V

+0.8V

TO +24V

* FOR SINGLE-SUPPLY OPERATION

*

C3

0.1

µF

C4

0.1

µF

C1

10

µF

35V

C2

10

µF

35V

150pF

R1

576k

1%

R2

31.6k

1%

L1

47

µH

D1

MBR0540L

LX

GND

FB

ISET

REF

POL

*FOR SINGLE-SUPPLY OPERATION

*FOR SINGLE-SUPPLY OPERATION