APL5934 datasheet(15/24 Pages) ANPEC

15 / 24 page

Rev. A.1 - Mar., 2016

www.anpec.com.tw

APL5934

Application Information

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

Power Sequencing

Output Capacitor

Input Capacitor

Setting Output Voltage







 +

⋅

0.8

VOUT

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 APL5934 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 APL5934 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 APL5934 requires proper input capacitors to supply

current surge during stepping load transients to prevent

the input voltage rail from dropping. Because the para-

sitic inductor from the voltage sources or other bulk ca-

pacitors 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 capaci-

tance of VIN is 10

µF at least.

However, if the drop of the input voltage is not cared, the

input capacitance can be less than 10

µF. More capaci-

tance 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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