APW7301
Application Information
T=1/FOSC
Setting Output Voltage
The regulated output voltage is determined by:
VLX
DT
I
R
1
IOUT
VOUT= 0.925´ (1+
) × (V)
R
IL
2
To prevent stray pickup, please locate resistors R1 and
R2 close to APW7301.
IOUT
IQ1
I
ICOUT
Input Capacitor Selection
VOUT
Use small ceramic capacitors for high frequency
decoupling and bulk capacitors to supply the surge cur-
rent needed each time the N-channel power MOSFET
(Q1) turns on. Place the small ceramic capacitors physi-
cally close to the VIN and between the VIN and GND.
The important parameters for the bulk input capacitor are
the voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and
current ratings above the maximum input voltage and
largest RMS current required by the circuit. The capacitor
voltage rating should be at least 1.25 times greater than
the maximum input voltage and a voltage rating of 1.5
times is a conservative guideline. The RMS current (IRMS)
of the bulk input capacitor is calculated as the following
equation:
VOUT
Figure 1. Converter Waveforms
In critical condition, like input voltage plug in, it will cause
the high spike voltage. It is recommended to place large
capacitance and higher voltage rating to reduce the spike
voltage. In general, to parallel a electrolytic capacitor with
large capacitance can reduce the spike voltage in critical
condition. This electrolytic capacitor must also be short
pin wire to make it as close as to the power plane or trace.
VIN
VIN
CIN1
CIN2
10uF/
MLCC
IRMS = IOUT D ´ (1- D) ×(A)
A P W 7301
where D is the duty cycle of the power MOSFET.
For a through hole design, several electrolytic capacitors
may be needed. For surface mount designs, solid tanta-
lum capacitors can be used, but caution must be exer-
cised with regard to the capacitor surge current rating.
Output Capacitor Selection
An output capacitor is required to filter the output and sup-
ply the load transient current. The filtering requirements
are the function of the switching frequency and the ripple
current (DI). The output ripple is the sum of the voltages,
having phase shift, across the ESR and the ideal output
capacitor. The peak-to-peak voltage of the ESR is calcu-
ated as the following equations:
VIN
VIN
IQ1
CIN
Q1
Q2
IOUT
IL
VOUT
VOUT
........... (1)
D =
L
LX
VIN
ESR
COUT
ICOUT
VOUT ´ (1 - D )
FOSC ´ L
........... (2)
DI =
........... (3)
VESR = DI ´ ESR
Copyright ã ANPEC Electronics Corp.
13
www.anpec.com.tw
Rev. A.2 - Apr., 2013
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
