APL5950 datasheet(13/22 Pages) ANPEC

13 / 22 page

Rev. A.1 - Oct., 2012

www.anpec.com.tw

APL5950/A

Application Information

........... (V)

Power Sequencing

Output Capacitor

Input Capacitor

Setting Output Voltage

The power sequencing of VIN and VCNTL is not neces-

sary to be concerned. However, do not apply a voltage to

VOUT for a long time when the main voltage applied at

VIN is not present. The reason is the internal parasitic

diode from VOUT to VIN conducts and dissipates power

without protections due to the forward-voltage.

The APL5950/A requires a proper output capacitor to main-

tain stability and improve transient response. The output

capacitor selection is dependent upon ESR (equivalent

series resistance) and capacitance of the output capaci-

tor over the operating temperature.

Ultra-low-ESR capacitors (such as ceramic chip

capacitors) and low-ESR bulk capacitors (such as solid

tantalum, POSCap, and Aluminum electrolytic capacitors)

can all be used as output capacitors.

During load transients, the output capacitors, depending

on the stepping amplitude and slew rate of load current,

are used to reduce the slew rate of the current seen by

the APL5950/A and help the device to minimize the varia-

tions of output voltage for good transient response. For

the applications with large stepping load current, the low-

ESR bulk capacitors are normally recommended.

Decoupling ceramic capacitors must be placed at the

load and ground pins as close as possible and the im-

pedance of the layout must be minimized.

The APL5950/A requires proper input capacitors to sup-

ply current surge during stepping load transients to pre-

vent the input voltage rail from dropping. Because the

parasitic inductor from the voltage sources or other bulk

capacitors to the VIN pin limit the slew rate of the surge

currents, more parasitic inductance needs more input

capacitance.

Ultra-low-ESR capacitors (such as ceramic chip capaci-

tors) and low-ESR bulk capacitors (such as solid

tantalum, POSCap, and Aluminum electrolytic capacitors)

can all be used as an input capacitor of VIN. For most

applications, the recommended input capacitance of VIN

is 22

µF at least. However, if the drop of the input voltage

is not cared, the input capacitance can be less than 22

µF.

More capacitance reduces the variations of the supply

voltage on VIN pin.

The output voltage is programmed by the resistor divider

connected to FB pin. The preset output voltage is calcu-

lated by the following equation :

Where R1 is the resistor connected from VOUT to FB with

Kelvin sensing connection and R2 is the resistor con-

nected from FB to GND. A bypass capacitor(C1) may be

connected with R1 in parallel to improve load transient

response and stability.

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REF

OUT

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