AP1602
Step-Up DC/DC Converter
Anachip Corp.
www.anachip.com.tw Rev. 1.0 Feb. 22, 2005
6/7
Function Descriptions
General Description
AP1602 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 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 AP1602 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.
Not Recommended for New Design
