ADP1108

–10–

REV. 0

Negative-to-Positive Conversion

The circuit of Figure 20 converts a negative input voltage to a posi-

tive output voltage. Operation of this circuit configuration is similar

to the step-up topology of Figure 15, except that 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 would

otherwise be dominated by the

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.

D1

1N5818

1

8

2

SW1

FB

SW2

GND

ADP1108

L1

D2

1N4148

C2

R1

3

Q1

2N3906

6

AO

SET

7

NC

NC

5

4

R2

10k

Ω

POSITIVE

OUTPUT

NEGATIVE

INPUT

Figure 20. A Negative-to-Positive Converter

Limiting the Switch Current

ited 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 ex-

ceeding the maximum switch rating. For example, a particular

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

ceed 1.6 A. The ADP1108 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 Figures 3 and 4.

when the ADP1108 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 ADP1108’s duty cycle. When this relationship

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

ing 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 21).

This increases output ripple and can require a larger inductor

tor, output ripple current can be maintained at the design val-

0%

100

90

10

50µs

200mA

L = 120µH

0%

100

90

10

50µs

200mA

L = 120µH

Q1 is the ADP1108’s internal power switch, which is paralleled

by sense transistor Q2. The relative sizes of Q1 and Q2 are

internal 80

Ω resistor and through the R

resistors parallel the base-emitter junction of the oscillator-

exceeds 0.6 V, Q3 turns on and terminates the output pulse. If

only the 80

Ω internal resistor is used (i.e. the I

OSCILLATOR

SW2

SW1

(EXTERNAL)

DRIVER

Q1

ADP1108

Q2

Q3

R1

80

Ω

(INTERNAL)

Figure 23. ADP1108 Current Limit Operation

The delay through the current limiting circuit is approximately

2

µs. If the switch ON time is reduced to less than 5 µs, accu-

racy of the current trip-point is reduced. Attempting to program

a switch ON time of 2

µs or less will produce spurious responses

in the switch ON time. However, the ADP1108 will still provide

a properly regulated output voltage.