AP1522
Step-Up DC/DC Converter
Typical Application Circuit (Continued)
+20V Dual Output Converter with Output Disconnect
D3
C4
1uF
D2
C5
1uF D1
-20V
4mA
C3
1uF
20V
4mA
150K 4.7pF
C2
10uF
10K
V
IN
2.5V to 5V
5
V
IN
C1
4.7uF
4
L1
10uH
1
SW
FB
3
AP1522
D4
SHDN
GND
2
Applications Information
Inductor Selection
A 10
µ
H inductor is recommended for most AP1522
applications.
Although small size and high
efficiency are major concerns, the inductor should
have low core losses at 1MHz and low DCR
(copper wire resistance).
Capacitor Selection
The small size of ceramic capacitors makes them
ideal for AP1522 applications. X5R and X7R types
are recommended because they retain their
capacitance over wider voltage and temperature
ranges than other types such as Y5V or Z5U. A
4.7
µ
F input capacitor and a 4.7
µ
F output capacitor
are sufficient for most AP1522 applications.
Diode Selection
Schottky diodes, with their low forward voltage drop
and fast reverse recovery, are the ideal choices for
AP1522 applications. The forward voltage drop of
a Schottky diode represents the conduction losses
in the diode, while the diode capacitance (C
T
or C
D
)
represents the switching losses.
For diode
selection, both forward voltage drop and diode
capacitance need to be considered.
Schottky
diodes with higher current ratings usually have
lower forward voltage drop and larger diode
capacitance, which can cause significant switching
losses at the 1MHz switching frequency of the
AP1522. A Schottky diode rated at 100mA to
200mA is sufficient for most AP1522 applications.
Anachip Corp.
www.anachip.com.tw
5/8
Open-Circuit Protection
In the cases of output open circuit, when the R1 are
disconnected from the circuit, the feedback voltage
will be zero. The AP1522 will then switch at a high
duty cycle resulting in a high output voltage, which
may cause the SW pin voltage to exceed its
maximum 36V rating. A zener diode can be used
at the output to limit the voltage on the SW pin
(Figure
1).
The zener voltage should be larger
than the maximum forward voltage of the V
OUT
.
The current rating of the zener should be larger
than 0.1mA.
V
IN
L
22uH
D
C
OUT
2uF
4.7pF
C
IN
1uF
V
IN
SHDN
GND
SW
R2
1K
R3
14K
AP1522
FB
R1
200K
Figure 1.
With Open-Circuit Protection
Rev. 1.0
May. 5, 2005
ANACHIP [ ANACHIP CORP ]
