MAX629CSLASHD datasheet(6/12 Pages) MAXIM

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a significant loss of accuracy. Begin by selecting R2 to be

in the 10kΩ to 200kΩ range, and calculate R1 using the

applicable equation from the following subsections.

Positive Output Voltages

For positive output voltages, use the typical boost config-

uration shown in Figure 2, connecting POL to GND. This

sets the threshold voltage at FB to equal VREF. Choose

the value of R2 and calculate R1 as follows:

OUT

REF

R1 R2





−









where VREF = 1.25V.

Negative Output Voltages

For negative output voltages, configure R1 and R2 as

shown in Figure 3, connecting POL to VCC. This sets the

FB threshold voltage to GND so that negative voltages

can be regulated. Choose R2 and calculate R1 as follows:

OUT

REF

R1 R2

where VREF = 1.25V.

Figure 3 demonstrates generation of a negative output

voltage by following the MAX629 with an inverting charge

pump. This configuration limits |VOUT| to values between

-|VIN| and -28V. If smaller negative output voltages are

required, D2’s cathode can be connected to VIN. This

alternative configuration permits output voltages smaller

than -|VIN|, but cannot be used for output voltages more

negative than -|28V -VIN|. It produces roughly one-half the

output current as the standard configuration and is typi-

cally 5% less efficient.

Figure 1. Functional Diagram

MAX629

MIN OFF-TIME

GENERATOR

TRIG

ISET

F/F

GND

START-UP

MAX ON-TIME

GENERATOR

(10µs)

CONTROL

REF

VCC

START-UP

COMPARATOR

ERROR

AMP

1.25V

REF

POL

TRIG

SHDN

POLARITY

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Maxim Integrated │ 6

MAX629

28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

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