AP1601

STEP-UP DC/DC CONVERTER

AP1601 Rev. 5

9 of 12

FEBRUARY 2009

www.diodes.com

Functional Description

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 1.5W

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 B0520LW 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.