ADP1110

–11–

REV. 0

POSITIVE-TO-NEGATIVE CONVERSION

The ADP1110 can convert a positive input voltage to a negative

output voltage as shown in Figure 23. This circuit is essentially

identical to the step-down application of Figure 19, except that

the “output” side of the inductor is connected to power ground.

When the ADP1110’s internal power switch turns off, current

potential. The ADP1110 will continue to turn the switch on

until its FB pin is 220 mV above its GND pin, so the output

voltage is determined by the formula:

R1

R2









ILIM

VIN

SW1

SW2

FB

GND

SET

AO

ADP1110

NC

NC

L1

R1

R2

D1

1N5818

OUTPUT

INPUT

6

7

8

1

2

4

5

3

NEGATIVE

OUTPUT

Figure 23. A Positive-to-Negative Converter

The design criteria for the step-down application also apply to

the positive-to-negative converter. The output voltage should be

limited to |6.2 V| unless a diode is inserted in series with the

SW2 pin (see Figure 21.) Also, D1 must again be a Schottky

diode to prevent excessive power dissipation in the ADP1110.

NEGATIVE-TO-POSITIVE CONVERSION

The circuit of Figure 24 converts a negative input voltage to a

positive output voltage. Operation of this circuit configuration is

similar to the step-up topology of Figure 19, except the current

through feedback resistor R1 is level-shifted below ground by a

diode D2 level-shifts the base of Q1 about 0.6 V below ground

the circuit’s output voltage sensitivity to temperature, which other-

The output voltage for this circuit is determined by the formula:

R1

R2









Unlike the positive step-up converter, the negative-to-positive

converter’s output voltage can be either higher or lower than the

input voltage.

SW1

SW2

FB

GND

SET

AO

ADP1110

NC

NC

L1

R2

D1

6

7

8

1

2

4

5

3

POSITIVE

OUTPUT

10K

NEGATIVE

INPUT

Q1

2N3906

D2

1N4148

R1

Figure 24. A Negative-to-Positive Converter

LIMITING THE SWITCH CURRENT

limited with a single resistor. This current limiting action occurs

on a pulse by pulse basis. This feature allows the input voltage

to vary over a wide range without saturating the inductor or

exceeding the maximum switch rating. For example, a particular

design may require peak switch current of 800 mA with a 2.0 V

exceed 1.6 A. The ADP1110 limits switch current to 1.5 A and

thereby protects the switch, but the output ripple will increase.

Selecting the proper resistor will limit the switch current to

and maximum switch current is shown in Figure 6.

when the ADP1110 goes into continuous-conduction mode.

This occurs in the step-up mode when the following condition is

met:









<

1

1– DC

where DC is the ADP1110’s duty cycle. When this relationship

exists, the inductor current does not go all the way to zero

during the time that the switch is OFF. When the switch turns

on for the next cycle, the inductor current begins to ramp up

from the residual level. If the switch ON time remains constant,

the inductor current will increase to a high level (see Figure 25).

This increases output ripple and can require a larger inductor

resistor, output ripple current can be maintained at the design

10

0%

100

90

10µs

10mV

200mA/div.

10

0%

100

90

10µs

10mV

200mA/div.