ADP1173

–10–

REV. 0

until its FB pin is 1.245 V above its GND pin, so the output

voltage is determined by the formula:

R1

R2









FB

ADP1173

SW2

GND

5

1

2

8

SW1

3

R3

+

C2

L1

D1

1N5818

+

C1

R2

R1

4

Figure 17. 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 16). Also, D1 must again be a Schottky

diode to prevent excessive power dissipation in the ADP1173.

NEGATIVE-TO-POSITIVE CONVERSION

The circuit of Figure 18 converts a negative input voltage to a

positive output voltage. Operation of this circuit configuration is

similar to the step-up topology of Figure 14, except that the current

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

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

also reduces the circuit’s output voltage sensitivity to tempera-

ture, which otherwise would be dominated by the –2 mV/

°C V

BE

contribution of Q1. The output voltage for this circuit is deter-

mined by the formula:

V

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

ADP1173

L1

FB

SW2

GND

4

5

1

2

3

8

SET

7

AO

6

+

C2

NEGATIVE

INPUT

NC

NC

R1

1N5818

D1

R2

10k

Ω

Q1

2N3906

1N4148

D2

POSITIVE

OUTPUT

Figure 18. 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

1.6 A. The ADP1173 limits switch current to 1.5 A and thereby

protects the switch, but increases the output ripple. Selecting

the proper resistor will limit the switch current to 800 mA, even

switch current is shown in Figures 4 and 5.

when the ADP1173 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 ADP1173’s duty cycle. When this relationship

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

ing the time 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 in-

ductor current will increase to a high level (see Figure 19). This

increases output ripple, and can require a larger inductor and

output ripple current can be maintained at the design values.