ADP3000

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REV. 0

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 (boost or step-up

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

device uses a gated oscillator technique to provide high perfor-

mance with low quiescent current.

A functional block diagram of the ADP3000 is shown in

Figure 3a. The internal 1.245 V reference is connected to one

input of the comparator, while the other input is externally

connected (via the FB pin) to a resistor divider connected to

the regulated output. When the voltage at the FB pin falls below

1.245 V, the 400 kHz oscillator turns on. 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 is shut 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 can be set

the maximum current is exceeded, the switch is turned OFF.

The current limit circuitry has a time delay of about 0.3

µs. If

yields the maximum feasible current limit. Further information

sheet. The ADP3000 internal oscillator provides typically 1.7

µs ON and 0.8 µs OFF times.

An uncommitted gain block on the ADP3000 can be con-

nected 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

connected 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 emitter of its internal power switch, which permits

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 is connected

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.

APPLICATIONS INFORMATION

COMPONENT SELECTION

Inductor Selection

For most applications the inductor used with the ADP3000 will

fall in the range between 4.7

µH to 33 µH. Table I shows

recommended inductors and their vendors.

When selecting an inductor, it is very important to make sure

that the inductor used with the ADP3000 is able to handle a

current that is higher than the ADP3000’s current limit without

saturation.

As a rule of thumb, powdered iron cores saturate softly, whereas

Ferrite cores saturate abruptly. Rod or “open” drum core

geometry inductors saturate gradually. Inductors that saturate

gradually are easier to use. Even though rod or drum core

inductors are attractive in both price and physical size, these

types of inductors must be handled with care because they have

high magnetic radiation. Toroid or “closed” core geometry

should be used when minimizing EMI is critical.

In addition, inductor dc resistance causes power loss. It is best

to use low dc resistance inductors so that power loss in the

inductor is kept to the minimum. Typically, it is best to use an

inductor with a dc resistance lower than 0.2

Ω.

Table I. Recommended Inductors

Series

Core Type

Phone Numbers

Coiltronics

OCTAPAC

Toroid

(407) 241-7876

Coiltronics

UNIPAC

Open

(407) 241-7876

Sumida

CD43, CD54

Open

(847) 956-0666

Sumida

CDRH62, CDRH73, Semi-Closed

(847) 956-0666

CDRH64

Geometry

Capacitor Selection

For most applications, the capacitor used with the ADP3000

will fall in the range between 33

µF to 220 µF. Table II shows

recommended capacitors and their vendors.

For input and output capacitors, use low ESR type capacitors

for best efficiency and lowest ripple. Recommended capacitors

include AVX TPS series, Sprague 595D series, Panasonic HFQ

series and Sanyo OS-CON series.

When selecting a capacitor, it is important to make sure the

maximum capacitor ripple current rms rating is higher than the

ADP3000’s rms switching current.

It is best to protect the input capacitor from high turn-on cur-

rent charging surges by derating the capacitor voltage by 2:1.

For very low input or output voltage ripple requirements,

Sanyo OS-CON series capacitors can be used since this type of

capacitor has very low ESR. Alternatively, two or more tanta-

lum capacitors can be used in parallel.