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