Advanced Power
Electronics Corp.
APE2901
BLOCK DIAGRAM
OPERATION
APPLICATION INFORMATION
INPUT/OUTPUT CAPACITOR SELECTION
INDUCTOR SELECTION
3
SW
GND
-
+
VFM
CONTROL
Driver
-
+
VREF
OUT
ERROR AMP.
LIMITER AMP.
The APE2901 architecture is built around a VFM CONTROL logic core, switching
frequency is set through a built in oscillator. TON time is fixed (Typ. 5uS) while TOFF time
is determined by the error amplifier output, a logic signal coming from the comparison
made by the Error Amplifier Stage between the signal coming from the output voltage
divider network and the internal Band-Gap voltage reference (Vref). TOFF reaches a
minimum (Typ. 1.7uS) when heavy load conditions are met (Clock frequency 150KHz). An
over current conditions, through the internal power switch, causes a voltage drop
VLX=RDSON x ISW and the VLX limiter block forces the internal switch to be off, so narrowing
TON time and limiting internal power dissipation. In this case the switching frequency may
be higher than the 150KHz set by the internal clock generator.
VFM control ensures very low quiescent current and high conversion efficiency even
with very light loads. Since the Output Voltage pin is also used as the device Supply
Voltage, the versions with higher output voltage present an higher internal supply voltage
that results in lower power switch RDSON, slightly greater output power and higher
efficiency. Moreover, bootstrapping allows the input voltage to sag to 0.6V (at IOUT=1mA)
once the system is started. If the input voltage exceeds the output voltage, the output will
follow the input, however, the input or output voltage must not be forced above 5.5V.
The Output Ripple Voltage, as well as the Efficiency, is strictly related to the behavior
of these elements. The output ripple voltage is the product of the peak inductor current
and the output capacitor Equivalent Series Resistance (ESR). Best performances are
obtained with good high frequency characteristics capacitors and low ESR. The best
compromise for the value of the Output Capacitance is 47F Tantalum Capacitor; Lower
values may cause higher Output Ripple Voltage and lower Efficiency without
compromising the functionality of the device.
An Input Capacitor is required to compensate, if present, the series impedance between
the Supply Voltage Source and the Input Voltage of the Application.
A 47H inductor is recommended for most APE2901 applications. However, the
inductance value is not critical, and the APE2901 will work with inductors in the 33H to
120H.