SC4250 데이터 시트보기 (PDF) - Semtech Corporation
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SC4250 Datasheet PDF : 12 Pages
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SC4250
POWER MANAGEMENT
Applications Information
Insertion of a power circuit board into a live backplane
would draw enormous inrush currents. This is mostly due
to the charging of the bulk electrolytic capacitors at the
input of the power module being plugged in.
The transient currents would send glitches all over the
power system and could cause corruption of the signals
and even a power down if the source isn’t able to handle
these high surges.
This section describes the components selection needed
for a typical application utilizing the SC4250. Let’s assume
the following requirements for a representative system:
Input voltage range: 36V to 72V
Resistors R1, R2 and R3 make up a voltage divider to
set the Under-Voltage (UV) and Over-Voltage (OV) trip
points.
When the input power supply ramps up the UV trips at
1.273V and OV trips at 1.223V; during the ramp down
transition the UV trips at 1.223V and OV trips at 1.198V.
The 50mV hysteresis for UV and 25mV hysteresis for OV
provide the necessary guard-bands to prevent false
tripping during power up and power down conditions.
As an additional noise killing and stabilizing measure, the
capacitor C1 should be placed at the OV terminal with
the value in range from 1,000 to 10,000pF.
Nominal current: 2A typ.
Over-current condition: 5A
Bulk capacitance: Cload = 150µF
The schematic in Figure 2 combines internal function
blocks along with the external components of the
application circuit.
For the UV=38V and OV=70V the values of the resistor
can be calculated as follows:
Vuv = 1.273V · (R1+R2+R3) ÷ (R2+R3)
Vov = 1.223V · (R1+R2+R3) ÷ R3
With the input bias current of the UV and OV comparators
in the range of 20-30nA, let’s choose the R1 to be
562kΩ. This yields the values of R2=9.31kΩ and R3 =
10.2kΩ. With these values the accuracy is about 1%
which is quite acceptable for those functions.
+48V
Vcc
PWRGD
R1
R2
C1
R3
12.5V Reg
UV
_
+
_
OV
+
_
60mV
+
1.223V
_ +7V
+
Delay
50uA
_+
1.75V
-48V
2005 Semtech Corp.
Vee
SENSE
R4
GATE
R5
C3
DRAIN
R6
C2
Q1
Figure 2
6
Cload
150uF
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