Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD
Table 1. Component Values for the
Typical Operating Circuit
REF
C1
C2
C3, C4
C5
D1, D2
DESCRIPTION
10µF, 10V
tantalum cap
0.22µF ceramic
capacitor
2.2µF ceramic
capacitor
47pF ceramic cap
0.1A, 20V
Schottky rectifier
22µH, 0.4A
inductor
MANUFACTURER
PART NUMBER
Sprague 595D106X0010A2T or
AVX TAJA106K010R
Any manufacturer
Any manufacturer
Any manufacturer
Motorola MBR0520LT1 (0.5A)
or Central Semiconductor
CMPSH-3
Murata LHQ4N220J04 or
TDK NLC32522T-220K
LXP
LXN
+5V
0V
+5V
0V
-7.5V
+15V
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 1kΩ in
series with 100pF provides good damping with only 3%
efficiency degradation. See Figure 5.
MAX685
L1
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 100kΩ to set a resistor-divider current of
approximately 10µA. Determine the value of R3 by the
following:
R 3
=
R4
V
OUT
−
1.24 V
Figure 5. LXN and LXP Waveforms with a Series-Connected
1k
Ω
Resistor and 100pF Capacitor Connected in Parallel with
L1 to Damp Ringing
Let R2 be a value near 100kΩ to set a resistor-divider
current of approximately 10µA. Determine the value of
R1 with the following formula:
R1 = R2 x (V
OUT
+ - 1.24V) / 1.24
_______________________________________________________________________________________
9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
