SUM60N04-12LT Vishay Siliconix, SUM60N04-12LT Datasheet - Page 8

• Manufacturer Part Number: SUM60N04-12LT
• Description: N-Channel MOSFET
• Manufacturer: Vishay Siliconix

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SUM60N04-12LT
Vishay Siliconix
The bias current of 250-mA nominal is derived from the input
signal. In this manner, a simple comparator can be used as a
driver for normal on/off operation and a fault detector circuit.
The circuit used to provide the input signal must therefore be
able to source 0.25 mA with no significant voltage drop.
The LMV321 can provide a output current of 60-mA typical,
which provides reasonable switching time for non-PWM
applications. A 560-W resistor is added in series to protect the
op amp and to prevent instability, but will result in switching
times of several micro seconds. A lower value may be possible
depending
recommended by the op amp manufacturer.
Hysteresis is added by means of a resistor network around the
comparator. Approximately 40_C hysteresis is added using
the components shown. This hysteresis could be reduced if
necessary by increasing the value of R4. Another means of
implementing hysteresis is to use the output of the comparator
to provide some of the bias current for the sensing diode. When
the comparator output is low (tripped/off), the bias current is
reduced by, say, 150 mA, causing the forward voltage to drop
by around 50 mV. This concept would also allow a lower
sourcing capability in the logic circuit providing the on/off signal
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8
+5 V
on
layout,
0.1 mF
C2
R3, 18 kW
but
180 kW
22 kW
Signal Ground
may
1%
1%
R1
R2
violate
560 pF
IC1, LMV331
C1
R6
10 kW
1%
0.1 mF
conditions
C3
New Product
R4, 560 kW, 1%
FIGURE 2.
–
+
and therefore should be used if input current requirements
become a problem.
With the input high, bias current flows and as long as the
forward voltage of the diode is higher than 0.465 V, the
comparator output is high and the MOSFET is on. If the forward
voltage of the diode drops below 0.465 V, the comparator
output goes low and the MOSFET is turned off. The gate drive
voltage can also be used as an output signal (if required) for
logic to interpret and to signify that there is a fault. Note the
cathode of the sensing diode should NOT be connected
directly to the source of the MOSFET as the noise introduced
by high currents in the source loop could affect operation of the
sensing circuit. A separate signal ground should be used and
connect to power ground at one point only.
A variation on this schematic is shown in Figure 2. Here a low
cost comparator (again in a SOT-23 or SC-70) is used to
provide a fault output signal only. The diode bias current is
taken from the 5 V. In this manner the diode bias is applied at all
times, so the noise filtering capacitor, C1 will not introduce a
turn-on delay. The fault output signal could be used to enable
the gate driver as shown, or fed to larger monitoring circuit to
shutdown the MOSFET.
R5
10 kW
ENABLE
IN
DRIVER
S-03830—Rev. A, 28-May-01
Document Number: 71620
Power Ground