AP1601

Step-Up DC/DC Converter

Anachip Corp.

www.anachip.com.tw

Rev. 1.1 Apr. 14, 2005

7/9

Function Descriptions

General Description

AP1601

PFM

(pulse

frequency

modulation)

converter IC series combine a switch mode

converter, N-channel power MOSFET, precision

voltage reference, and voltage detector 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 100KHz. The

SW on resistance is typically 1 to 1.5 W to

minimize switch losses. When the output voltage

drops, the error comparator enables 100KHz

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.

Diode Selection

Speed, forward drop, and leakage current are the

three main considerations in selecting a rectifier diode.

Best performance is obtained with Schottky rectifier

diode, such as 1N5819. Motorola makes MBR0530 in

surface mount. For lower output power a 1N4148 can

be used although efficiency and start up voltage will

suffer substantially.

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 AP1601 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.