AP1603

STEP-UP DC/DC CONVERTER

AP1603 Rev. 1

7 of 8

NOVEMBER 2006

www.diodes.com

Function 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.

Marking Information

(1) SOT26

Part Number

Package

Identification Code

AP1603W

SOT26

EY

XX : Identification code

1

2

3

6

4

XX Y M

5

Y : Year: 0-9

M : Month: A~L