Setting the Output Voltage

The resistor-divider formed by R4 and R3 sets the neg-

ative output voltage; the resistor-divider formed by R1

and R2 sets the positive output voltage. Let R4 be a

value near 100k

Ω to set a resistor-divider current of

approximately 10µA. Determine the value of R3 by the

following:

Let R2 be a value near 100k

Ω to set a resistor-divider

current of approximately 10µA. Determine the value of

R1 with the following formula:

Damping LX

LXN and LXP may ring at the conclusion of each

switching cycle when the inductor current falls to zero.

Typically the ringing waveform appears only on LX_

and has no effect on output ripple and noise. If LX_

ringing is still objectionable, it may be damped by con-

necting a series RC in parallel with L1. Typically 1k

Ω in

series with 100pF provides good damping with only 3%

efficiency degradation. See Figure 5.

R

R

V

V

OUT

3

4

1 24

.

=

−

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

_______________________________________________________________________________________

9

Any manufacturer

0.22µF ceramic

capacitor

C2

Any manufacturer

47pF ceramic cap

C5

Any manufacturer

2.2µF ceramic

capacitor

C3, C4

Murata LHQ4N220J04 or

TDK NLC32522T-220K

22µH, 0.4A

inductor

L1

Motorola MBR0520LT1 (0.5A)

or Central Semiconductor

CMPSH-3

0.1A, 20V

Schottky rectifier

D1, D2

REF

Sprague 595D106X0010A2T or

AVX TAJA106K010R

10µF, 10V

tantalum cap

C1

MANUFACTURER

PART NUMBER

DESCRIPTION

Table 1. Component Values for the

Typical Operating Circuit

Figure 5. LXN and LXP Waveforms with a Series-Connected

1k

Ω Resistor and 100pF Capacitor Connected in Parallel with

L1 to Damp Ringing

+15V

+5V

0V

+5V

0V

-7.5V

LXN

LXP