AP1603

STEP-UP DC/DC CONVERTER

AP1603

Document number: DS31131 Rev. 3 - 2

7 of 10

www.diodes.com

June 2010

© Diodes Incorporated

Functions Description

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 to 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 the 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 the 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 flow through the diode is high, and release it when the current flow 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 ensures adequate time for energy transfer to output during the second half of each cycle. Depending on

the 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 the maximum

current rating of AP1603 and inductor are not exceeded 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 an inductor can be selected.

Capacitor Selection

A poor choice for an output capacitor can result in poor efficiency and high output ripple. Ordinary aluminum

electrolyzers, while inexpensive, may have unacceptably poor ESR and ESL. There is a low ESR aluminum

capacitor for switch mode DC-DC converters which work much better than the general purpose unit. Tantalum

capacitors provide still better performance at more expense. OS-CON capacitors have extremely low ESR in a

small size. If capacitance is reduced, the 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.