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AP1603WA

Step-Up DC/DC Converter

Anachip Corporation 

Step-Up DC/DC Converter

AP1603

„ Function Descriptions

General Description

AP1603 PFM (pulse frequency modulation)

converter IC series combine a switch mode

converter, power MOSFET, and precision voltage

reference in a single monolithic device. They offer

both extreme low quiescent current, high efficiency,

and very low gate threshold voltage to ensure

start-up with low battery voltage (0.9V typ.).

Designed to maximize battery life in portable

products, and minimize switching losses by only

switching as needed service the load. PFM

converters transfer a discrete amount of energy

per cycle and regulate the output voltage by

modulating switching frequency with the constant

turn-on time. Switching frequency depends on load,

input voltage, and inductor value, and it can range

up to 150KHz. The SW on resistance is typically 1

to 1.5 W to minimize switch losses. When the

output voltage drops, the error comparator enables

150KHz oscillator that turns on the MOSFET

around 7.5us and 2.5ms off time. Turning on the

MOSFET allows inductor current to ramp up,

storing energy in a magnetic field and when

MOSFET turns off that force inductor current

through diode to the output capacitor and load. As

the stored energy is depleted, the current ramp

down until the diode turns off. At this point,

inductor may ring due to residual energy and stray

capacitance. The output capacitor stores charge

when current flowing through the diode is high, and

release it when current is low, thereby maintaining

a steady voltage across the load. As the load

increases, the output capacitor discharges faster

and the error comparator initiates cycles sooner,

increasing the switching frequency. The maximum

duty cycle ensure adequate time for energy

transfer to output during the second half each

cycle. Depending on circuit, PFM converter can

operate in either discontinuous mode or

continuous conduction mode. Continuous

conduction mode means that the inductor current

does not ramp to zero during each cycle.

Inductor Selection

To operate as an efficient energy transfer element,

the inductor must fulfill three requirements. First, the

inductance must be low enough for the inductor to

store adequate energy under the worst case condition

of minimum input voltage and switch ON time. Second,

the inductance must also be high enough so

maximum current rating of AP1603 and inductor are

not exceed at the other worst case condition of

maximum input voltage and ON time. Lastly, the

inductor must have sufficiently low DC resistance so

excessive power is not lost as heat in the windings.

But unfortunately this is inversely related to physical

size. Minimum and Maximum input voltage, output

voltage and output current must be established before

and inductor can be selected.

Capacitor Selection

A poor choice for a output capacitor can result in poor

efficiency and high output ripple. Ordinary aluminum

electrolyzers, while inexpensive may have

unacceptably poor ESR and ESL. There are low ESR

aluminum capacitors for switch mode DC-DC

converters which work much better than general

propose unit. Tantalum capacitors provide still better

performance at more expensive. OS-CON capacitors

have extremely low ESR in a small size. If

capacitance is reduced, output ripple will increase.

Most of the input supply is supplied by the input

bypass capacitor. The capacitor voltage rating should

be at least 1.25 times greater than a maximum input

voltage.

Anachip Corp.

www.anachip.com.tw

Rev.1.0 Jun. 3, 2004

7/8

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