APW7093 查看數據表（PDF） - Anpec Electronics
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APW7093 Datasheet PDF : 20 Pages
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APW7093
Application Information(Cont.)
Soft-Start
Soft-start allows a gradual increase of the internal
current limit to reduce input surge currents at startup
and at exit from shutdown. A timing capacitor, CSS,
placed from SS to GND sets the rate at which the
internal current limit is changed. Upon power-up, when
the device comes out of under-voltage lockout (2.6V
typ.) or after the SHDN pin is pulled high, a 4.7µA
constant current source charges the soft-start capacitor
and the voltage on SS increases. When the voltage
on SS is less than approximately 0.7V, the current
limit is set to zero. As the voltage increases from
0.7V to approximately VIN, the current limit is adjusted
from 0V to the current-limit threshold. The voltage
across the soft-start capacitor changes with time
according to the equation:
4.7 µA × t
VSS =
C SS
The output current limit during soft-start varies with
the voltage on the soft-start pin, SS, according to the
equation:
ILIIM(SS)= (VSS1−.10V.7V)×ILIMIT ,VSS≤1.8V
where ILIMIT is the current-limit threshold from the
Electrical Characteristics. The constant-current source
stops charging once the voltage across the soft-start
capacitor reaches 1.8V.
SHDN
0
VSS(A)
0
ILIMIT ( A )
0.7V
1.8V
VIN
ILIMIT
Input Source
The output of the APW7093 can accept current due
to the reversible properties of the buck and the boost
converter. When voltage at the output of the APW7093
(low-voltage port) exceeds or equals the output set
voltage the flow of energy reverses, going from the
output to the input (high-voltage port). If the input (high
voltage port) is not connected to a low-impedance
source capable of absorbing energy, the voltage at
the input will rise. This voltage can violate the abso-
lute maximum voltage at the input of the APW7093
and destroy the part. This occurs when sinking current
because the topology acts as a boost converter,
pumping energy from the low-voltage side (the output),
to the high-voltage side (the input). The input (high-
voltage side) voltage is limited only by the clamping
effect of the voltage source connected there. To avoid
this problem, make sure the input to the APW7093 is
connected to a low impedance, two quadrant supply
or that the load (excluding the APW7093) connected
to that supply consumes more power than the amount
being transferred from the APW7093 output to the
input.
Current Limit and Short Circuit Protection
The APW7093 monitors sourcing and sinking current,
and limits the maximum output current to prevent
damages during overload or short-circuit.
Circuit Layout and Grounding
Good layout is necessary to achieve the APW7093’s
intended output power level, high efficiency, and low
noise. Good layout includes the use of ground planes,
careful component placement, and correct routing of
traces using appropriate trace widths. The following
points are in order of decreasing importance:
0
t
Fig6. Soft-Start Current Limit
Copyright © ANPEC Electronics Corp.
14
Rev. A.5 - Jun., 2005
www.anpec.com.tw
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