AP5101
1.5A Step-Down Converter with 1.4MHz Switching
Frequency
AP5101
Document number: DS32258 Rev. 1 - 2
11 of 15
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July 2010
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NEW PRODUCT
Applications Information (Continued)
Input Capacitor
The input capacitor reduces the surge current drawn from the input supply and the switching noise from the device. The input
capacitor has to sustain the ripple current produced during the on time on the upper MOSFET. It must hence have a low ESR
to minimize the losses.
Due to large dI/dt through the input capacitors, electrolytic or ceramics should be used. If a tantalum must be used, it must be
surge protected. Otherwise, capacitor failure could occur. For most applications, a 4.7µF ceramic capacitor is sufficient.
Output Capacitor
The output capacitor keeps the output voltage ripple small, ensures feedback loop stability and reduces the overshoot of the
output voltage. The output capacitor is a basic component for the fast response of the power supply. In fact, during load
transient, for the first few microseconds it supplies the current to the load.
The converter recognizes the load transient and sets the duty cycle to maximum, but the current slope is limited by the inductor
value.
Maximum capacitance required can be calculated from the following equation:
2
out
V
2
)
out
V V(Δ
2
)
2
inductor
ΔI
out
L(I
o
C
−+
+
=
Where
V is the maximum output voltage overshoot.
ESR of the output capacitor dominates the output voltage ripple. The amount of ripple can be calculated from the equation
below:
ESR
inductor
ΔI
capacitor
Vout ×=
An output capacitor with ample capacitance and low ESR is the best option. For most applications, a 22µF ceramic capacitor
will be sufficient.
External Diode
The external diode’s forward current must not exceed the maximum output current. Since power dissipation is a critical factor
when choosing a diode, it can be calculated from the equation below:
0.3V
OUT
I)
IN
V
OUT
V
(1
diode
P ××−=
Note: 0.3V is the voltage drop across the Schottky diode. A diode that can withstand this power dissipation must be chosen.
PC Board Layout
This is a high switching frequency converter. Hence attention must be paid to the switching currents interference in the layout.
Switching current from one power device to another can generate voltage transients across the impedances of the
interconnecting bond wires and circuit traces. These interconnecting impedances should be minimized by using wide, short
printed circuit traces. The input capacitor needs to be as close as possible to the IN and GND pins. The external feedback
resistors should be placed next to the FB pin.
