APL3514 datasheet(13/21 Pages) ANPEC

13 / 21 page

Rev. A.6 - Aug., 2012

APL3512

www.anpec.com.tw

Application Information

Input Capacitor

A 1

µF ceramic bypass capacitor from V

IN to GND, located

near the APL3512, 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 APL3512 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 controlled neither by internal soft-start nor by external

soft-start circuitry. When the C

OUT meets the following

formula, the soft-start will be controlled by foldback cur-

rent-limiting:

OUT > (0.8xtSS)/VIN

Where,

SS is 1ms when SS is open or tied to VIN, or obtained by

the tss equation, described in the paragraph of Soft-Start

in Functional Description section when C

SS is used.

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

Soft-Start Capacitor

The APL3512 has a built-in adjustable soft-start control

for user to set an optimum soft-start time for the app-

lication. The soft-start time can be calculated by the

equation, described in the paragraph of Soft-Start in Func-

tional Description section. Please note that there are mini-

mum and maximum limitations of soft-start capacitor. If

the value of soft-start capacitor is less than the minimum

limitation or higher than the maximum limitation (please

refer to the Recommended Operating Conditions), the

soft-start time will become internally controlled as if there

is no C

SS, tSS=2ms when VIN=5V, for example. If a soft-

start capacitor is used, please make sure the C

SS is in the

recommeded operating range.

0.02

0.037

0.075

Unit : Inch

Recommended Minimum Footprint

SOT-23-5

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