APL3521 datasheet(12/18 Pages) ANPEC

12 / 18 page

Rev. A.1 - Aug., 2012

APL3521

www.anpec.com.tw

Application Information

Input Capacitor

A 1

µF ceramic bypass capacitor from V

IN to GND, located

near the APL3521, is strongly recommended to suppress

the ringing during short-circuit fault event. Without the by-

pass capacitor, the output short may cause sufficient ring-

ing on the input (from supply lead inductance) to damage

internal control circuitry.

Output Capacitor

A low-ESR 10

µF MLCC, aluminum electrolytic or tanta-

lum between VOUT and GND is strongly recommended

to reduce the voltage droop during hot-attachment of

downstream peripheral. Higher-value output capacitor is

better when the output load is heavy. Additionally, bypass-

ing the output with a 0.1

µF ceramic capacitor improves

the immunity of the device to short-circuit transients.

Layout Consideration

The PCB layout should be carefully performed to maxi-

mize thermal dissipation and to minimize voltage drop,

droop and EMI. The following guidelines must be

considered:

1. Please place the input capacitors near the VIN pin as

close as possible.

2. Output decoupling capacitors for load must be placed

near the load as close as possible for decoupling high-

frequency ripples.

3. Locate APL3521 and output capacitors near the load to

reduce parasitic resistance and inductance for excel-

lent load transient performance.

4. The negative pins of the input and output capacitors

and the GND pin must be connected to the ground plane

of the load.

5. Keep V

IN and VOUT traces as wide and short as possible.

During soft-start process, the output bulk capacitor draws

inrush current from V

IN. If the inrush current reaches

foldback current-limit threshold, namely 0.8A, the output

current will be clamped in 0.8A level. It will take longer to

complete the soft-start process since the soft-start rate

is not controlled by internal soft-start anymore. When the

OUT meets the following formula, the soft-start will be

controlled by foldback current-limiting:

OUT > (0.8xtSS)/VIN

Where,

SS is 2ms.

If the soft-start rate is controlled by the foldback current-

limiting, the soft-start time can be got by the following

equation:

SS_Foldback = (COUT x VIN)/0.8

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