6
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 isnt able to handle
these high surges.
This section describes the components selection needed
for a typical application utilizing the SC4250. Lets assume
the following requirements for a representative system:
Input voltage range: 36V to 72V
Nominal current: 2A typ.
Over-current condition: 5A
Bulk capacitance: Cload = 150礔
The schematic in Figure 2 combines internal function blocks
along with the external components of the application
circuit.
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.
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, lets choose the R1 to be 562k&.
This yields the values of R2=9.31k& and R3 = 10.2k&.
With these values the accuracy is about 1% which is quite
acceptable for those functions.
Resistor R4 sets the over-current trip. To choose R4, the
Figure 2
60mV
+
_
_
+
_
+
_
+
+
_
+7 V
Delay
Q1
DRAIN
GATE
R 4
SENSE
Vee
Vc c
1 2 .5 V Reg
1 .2 2 3 V
1.75V
PW RGD
5 0 uA
Cload
150uF
C 3
R5
R1
R2
R3
C2
UV
OV
- 48V
48V
R 6
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