ADP3000

Rev. A | Page 11 of 16

The delay through the current limiting circuit is approximately

0.3 µs. If the switch-on time is reduced to less than 1.7 µs,

accuracy of the current trip point is reduced as well. An attempt

to program a switch-on time of 0.3 µs or less produces spurious

responses in the switch-on time. However, the ADP3000 still

provides a properly regulated output voltage.

PROGRAMMING THE GAIN BLOCK

The ADP3000’s gain block can be used as a low battery detector,

an error amplifier, or a linear post regulator. It consists of an op

amp with PNP inputs and an open-collector NPN output. The

inverting input is internally connected to the 1.245 V reference,

and the noninverting input is available at the SET pin. The NPN

output transistor sinks in excess of 300 µA.

Figure 25 shows the gain block configured as a low battery

monitor. Set Resistors R1 and R2 to high values to reduce

quiescent current, but not so high that bias current in the SET

input causes large errors. A value of 33 kΩ for R2 is a good

compromise. The value for R1 is then calculated as follows:

R2

V

R1

LOBATT

V

245

.

1

V

245

.

1

−

=

Because the gain block output is an open-collector NPN, a

pull-up resistor should be connected to the positive logic

power supply.

ADP3000

1.245V

REF

GND

AO

5V

47k

Ω

TO

PROCESSOR

R1

SET

1.6M

Ω

R2

33k

Ω

R1 =

37.7

µA

Figure 25. Setting the Low Battery Detector Trip Point

The circuit of Figure 25 may produce multiple pulses when

approaching the trip point due to noise coupled into the SET

input. To prevent multiple interrupts to the digital logic, add

trip point slightly, changing the new value for R1 to

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

−

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

HYS

L

L

LOBATT

R

R

V

R2

V

R1

V

245

.

1

V

245

.

1

V

245

.

1

where:

POWER TRANSISTOR PROTECTION DIODE IN

STEP-DOWN CONFIGURATION

When operating the ADP3000 in step-down mode with the

switch off, the output voltage is impressed across the internal

power switch’s emitter-base junction. When the output voltage

is set to higher than 6 V, a Schottky diode must be placed in a

series with SW2 to protect the switch. Figure 26 shows the

proper way to place D2, the protection diode. The selection of

this diode is identical to the step-down commuting diode (refer

to the Diode Selection section).

+

ADP3000

FB

SW2

GND

C2

R3

L1

R2

D1, D2 = 1N5818 SCHOTTKY DIODES

+

D1

D2

C1

R1

3

4

5

1

2

8

THERMAL CONSIDERATIONS

Power dissipation internal to the ADP3000 can be

approximated with the following equations.

Step-Up

IN

Q

SW

O

O

IN

SW

IN

SW

D

V

I

I

I

V

V

D

β

I

V

R

I

P

+

⎥

⎦

⎤

⎢

⎣

⎡

⎥

⎦

⎤

⎢

⎣

⎡

−

⎥

⎦

⎤

⎢

⎣

⎡

+

=

4

1

2

where:

D is 0.75 (typical duty ratio for a single switching cycle).

β = 30 (typical forced beta).