28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

6

_______________________________________________________________________________________

without a significant loss of accuracy. Begin by select-

ing R2 to be in the 10k

Ω 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 con-

figuration shown in Figure 2, connecting POL to GND.

Choose the value of R2 and calculate R1 as follows:

Negative Output Voltages

For negative output voltages, configure R1 and R2 as

the FB threshold voltage to GND so that negative volt-

ages can be regulated. Choose R2 and calculate R1 as

follows:

Figure 3 demonstrates generation of a negative output

voltage by following the MAX629 with an inverting

charge pump. This configuration limits

between -

ages are required, D2’s cathode can be connected to

ages smaller than -

voltages more negative than -

roughly one-half the output current as the standard con-

figuration and is typically 5% less efficient.

R1 = R2 x

V

V

|

|

OUT

REF

R1 = R2 x

V

V

OUT

REF

−









1

MAX629

MIN OFF-TIME

GENERATOR

TRIG

Q

S

LX

ISET

Q

F/F

GND

R

START-UP

MAX ON-TIME

GENERATOR

(10

µs)

CONTROL

FB

REF

1V

START-UP

COMPARATOR

ERROR

AMP

1.25V

REF

POL

TRIG

Q

SHDN

POLARITY

Figure 1. Functional Diagram