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.

SW1

SW2

FB

GND

SET

AO

ADP1111

NC

2

1

3

6

7

5

4

8

NC

L1

47µF

35V

OUTPUT

INPUT

+

+5V TO +12V

0.22

Ω

2k

Ω

3

4

6

7

2

1k

Ω

200k

Ω

39k

Ω

1N4148

IRF9530

20µH

D1

IN5821

2W MAXIMUM OUTPUT

1N5231

CTX20-4

2N3904

51

Ω

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

0.1µF

OUTPUT

25µH

D1

1N4148

1N5818

1N5818

732k

Ω

42.2k

Ω

22µF

4.7

µF

+3V

2xAA

CELLS

100

Ω

–22V OUTPUT

7mA @ 2V INPUT

L1 = CTX25-4

SW1

SW2

FB

GND

SET

AO

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

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.

220µF

+

IRF9540

D1

IN5821

NC

INPUT

+8V TO +18V

0.22

2k

1N4148

BAT54

2N3904

G

S

D

121k

Ω

40.2k

Ω

51

Ω

1N4148

+5V

500mA

OPERATE/STANDBY

2V

SW1

SW2

FB

GND

SET

AO

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.