ADP1111
–14–
REV. 0
Voltage-Controlled Positive-to-Negative Converter
By including an op amp in the feedback path, a simple positive-
to-negative converter can be made to give an output that is a
linear multiple of a controlling voltage, Vc. The op amp, an
OP196, rail-to-rail input and output amplifier, sums the
currents from the output and controlling voltage and drives the
FB pin either high or low, thereby controlling the on-board
oscillator. The 0.22 Ω resistor limits the short-circuit current to
about 3 A and, along with the BAT54 Schottky diode, helps
limit the peak switch current over varying input voltages. The
external power switch features an active pull-up to speed up the
turn-off time of the switch. Although an IRF9530 was used in
the evaluation, almost any device that can handle at least 3 A of
peak current at a VDS of at least 50 V is suitable for use in this
application, provided that adequate attention is paid to power
dissipation. The circuit can deliver 2 W of output power with a
+6-volt input from a control voltage range of 0 V to 5 V.
I
LIM
V
IN
SW1
SW2
FB
GNDSETAO
ADP1111
NC
2
1
3
6
7 5 4
8
NC
L1
C
L
47μF
35V
OUTPUT
R
LIM
INPUT
+
+5V TO +12V
0.22Ω
2kΩ
3
4
6
7
2
1kΩ
200kΩ
39kΩ
V
IN
1N4148
IRF9530
20μH
D1
IN5821
–V
OUT
= –5.13 *V
C
2W MAXIMUM OUTPUT
1N5231
CTX20-4
V
C
(0V TO +5V)
2N3904
51Ω
BAT54
Figure 32. Voltage Controlled Positive-to-Negative
Converter
+3 V to –22 V LCD Bias Generator
This circuit uses an adjustable-output version of the ADP1111
to generate a +22.5 V reference output that is level-shifted to
give an output of –22 V. If operation from a +5 volt supply is
desired, change R1 to 47 ohms. The circuit will deliver 7 mA
with a 3 volt supply and 40 mA with a 5 volt supply.
L1
C
L
0.1μF
OUTPUT
25μH
D1
1N4148
1N5818
1N5818
732kΩ
42.2kΩ
22μF
4.7
μF
+3V
2xAA
CELLS
R
LIM
100Ω
–22V OUTPUT
7mA @ 2V INPUT
L1 = CTX25-4
I
LIM
V
IN
SW1
SW2
FB
GNDSETAO
ADP1111
NC
1
2
3
6
7 5 4
8
NC
+
+
Figure 33. 3 V to –22 V LCD Bias Generator
High Power, Low Quiescent Current Step-Down Converter
By making use of the fact that the feedback pin directly controls
the internal oscillator, this circuit achieves a shutdown-like state
by forcing the feedback pin above the 1.25 V comparator
threshold. The logic level at the 1N4148 diode anode needs to
be at least 2 V for reliable standby operation.
The external switch driver circuit features an active pull-up
device, a 2N3904 transistor, to ensure that the power MOSFET
turns off quickly. Almost any power MOSFET will do as the
switch as long as the device can withstand the 18 volt V
GS
and is
reasonably robust. The 0.22 Ω resistor limits the short-circuit
current to about 3 A and, along with the BAT54 Schottky
diode, helps to limit the peak switch current over varying input
voltages.
C
L
220μF
+
IRF9540
D1
IN5821
NC
R
LIM
INPUT
+8V TO +18V
0.22
2k
1N4148
BAT54
2N3904
G
S
D
121kΩ
40.2kΩ
51Ω
1N4148
+5V
500mA
OPERATE/STANDBY
2V ≤ V
IN
≤ 5
I
LIM
V
IN
SW1
SW2
FB
GNDSETAO
ADP1111
1
2
3
6
7 5 4
8
NC
LI
20μH
LI = COILTRONICS CTX20-4
Figure 34. High Power, Low Quiescent Current Step-Down
Converter
NOTES
1. All inductors referenced are Coiltronics CTX-series except
where noted.
2. If the source of power is more than an inch or so from the
converter, the input to the converter should be bypassed with
approximately 10 μF of capacitance. This capacitor should
be a good quality tantalum or aluminum electrolytic.
REV. A
