APL5331
Application Information
Power Inputs
Input power sequence are not required for VIN and
VCNTL. However, do not apply a voltage to VOUT
when there is not voltage VCNTL. This is due to the
internal parasitic diodes between VOUT to VIN and
VOUT to VCNTL which will be forward bias. The
APL5331 can source few current or sinks current up
to 3A for load when the input Voltage at VIN is not
present.
Reference Voltage
A reference voltage is applied at the VREF pin by a
resistor divider between VIN and GND pins. Normally
the bias current of the VREF pin flows out of the IC
and is about 150nA(typ.), creating voltage offset at
the resistor divider and affecting the output voltage
accuracy. The recommended resistor is <5kΩ to
maintain the accuracy of the output voltage. An ex-
ternal bypass capacitor is also connected to VREF.
The capacitor and the resistor divider form a low-
pass filter to reduce the inherent reference noise from
VIN. A ceramic capacitor can be use and is selected
to be greater than 0.1µF. Connected the capacitor
as close to VREF as possible for optimal effect. More
capacitance and large resistor divider will increase
the soft-start interval. Do not place any additional load-
ing on this reference input pin.
Output Capacitor
The APL5331 requires a proper output capacitor to
maintain stability over full temperature and current
ranges, and improve transient response. The output
capacitor selection is dependent upon the ESR
(equivalent series resistance) and capacitance of the
output capacitor over full temperature range. The fol-
lowing chart shows the stable region of the output
capacitor for APL5331. The stable region is above
the curve, indicating minimum required ESR and
capacitance to maintain stability. However, the out
Copyright
ANPEC Electronics Corp.
Rev. A.8 - Oct., 2003
11
put capacitor should have an ESR less than 1Ω.
25
20
15
10
5
0
10
100
Capacitance(µF)
1000
Ultra-low-ESR capacitors, such as ceramic chip
capacitors, may promote under-damped transient
response, but proper ceramic chip capacitors placed
near loads can be used as decoupling capacitors. A
low-ESR solid tantalum and aluminum electrolytic ca-
pacitor (ESR<1Ω) works extremely well and provides
good transient response and stability over temperature.
The output capacitors are also used to reduce the
slew rate of load current and help the APL5331 to
minimize variations of the output voltage, improving
transient response. For this purpose, the low-ESR
capacitors are recommended and depend on the step-
ping and slew rate of load current.
Input Capacitor
The input capacitors of VCNTL and VIN pins are not
required for stability but for supplying surge currents
during large load transients, This will prevent the in-
put rail from drooping and improve the performance
of the APL5331. Because of parasitic inductors from
voltage sources or other bulk capacitors to the VCNTL
and VIN pins will limit the slew rate of the surge cur-
rents during large load transients, resulting in voltage
drop at VIN and VCNTL pins.
ESR (m
Ω
)
ESR (m
Ω
)
Stable Region
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
