ADP3000

Rev. A | Page 9 of 16

THEORY OF OPERATION

The ADP3000 is a versatile, high frequency, switch mode power

supply (SMPS) controller. The regulated output voltage can be

greater than the input voltage (in boost or step-up mode) or less

than the input voltage (in buck or step-down mode). This

device uses a gated oscillator technique to provide high

performance with low quiescent current.

Figure 7 is a functional block diagram of the ADP3000. The

internal 1.245 V reference is connected to one input of the

comparator, and the other input is externally connected (via the

FB pin) to a resistor divider, which is connected to the regulated

output. When the voltage at the FB pin falls below 1.245 V, the

400 kHz oscillator turns on. The ADP3000 internal oscillator

typically provides a 1.7 µs on time and a 0.8 µs off time. A driver

amplifier provides base drive to the internal power switch, and

the switching action raises the output voltage. When the voltage

at the FB pin exceeds 1.245 V, the oscillator shuts off. While the

oscillator is off, the ADP3000 quiescent current is only 500 µA.

The comparator’s hysteresis ensures loop stability without

requiring external components for frequency compensation.

The maximum current in the internal power switch is set by

maximum current is exceeded, the switch is turned off. The

current limit circuitry has a time delay of about 0.3 µs. If an

included in the Applications Information section.

An uncommitted gain block on the ADP3000 can be connected

as a low battery detector. The inverting input of the gain block

is internally connected to the 1.245 V reference. The

noninverting input is available at the SET pin. A resistor divider,

to the SET pin, causes the AO output to go low when the low

battery set point is exceeded. The AO output is an open

collector NPN transistor that can sink in excess of 300 µA.

The ADP3000 provides external connections for both the

collector and the emitter of its internal power switch,

permitting both step-up and step-down modes of operation.

For the step-up mode, the emitter (Pin SW2) is connected to

GND, and the collector (Pin SW1) drives the inductor. For step-

down mode, the emitter drives the inductor, while the collector

The output voltage of the ADP3000 is set with two external

resistors. Three fixed voltage models are also available:

ADP3000-3.3 (3.3 V), ADP3000-5 (5 V), and ADP3000-12

(12 V). The fixed voltage models include laser-trimmed,

voltage-setting resistors on the chip. On the fixed voltage

models of the ADP3000, simply connect the feedback pin

(Pin 8) directly to the output voltage.