APL3546 Datasheet, PDF (10/17 Pages) Anpec Electronics Coropration – Power -Distribution Switches with Soft Start

APL3546
Application Information
Input Capacitor
A
1µF
ceramic
bypass
capacitor
from
V
IN
to
GND,
located
near the APL3546, is strongly recommended to suppress
the ringing during short-circuit fault event. Without the
bypass capacitor, the output short may cause sufficient
ringing on the input (from supply lead inductance) to dam-
age internal control circuitry.
Output Capacitor
A low-ESR 10µF 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 0.1µF ceramic capacitor improves
the immunity of the device to short-circuit transients.
During soft-start process, the output bulk capacitor draws
inrush current from VIN. If the inrush current reaches fold
back current-limit threshold, namely 1A, the output cur-
rent will be clamped in 1A 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 fold back cur-
rent-limiting:
COUT > (I × tss) / VIN
where
t is 2ms when SS is open or tied to V , or obtained by
SS
IN
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 fold back current limiting,
the soft-start time can be got by the following equation:
Soft-Start Capacitor
The APL3546 has a built-in adjustable soft-start control
for user to set an optimum soft-start time for the
application. 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 , t =2ms when V =12V, for example. If a soft
SS SS
IN
start capacitor is used, please make sure the C_SS is in the
recommended operating range.
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 APL3546 and output capacitors near the load to
reduce parasitic resistance and inductance for excellent
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 V_OUT traces as wide and short as possible.
tss _ Foldback = (COUT × VIN) / I
Current Limit Protection
The APL3546 power switch provides the current limit pro-
tection function. During current limit, the devices limit out-
put current at current limit threshold. For reliable
operation, the device should not be operated in current
limit for extended period time.
Copyright © ANPEC Electronics Corp.
10
Rev. A.3 - Jul., 2013
www.anpec.com.tw