SUM60N04-12LT Vishay Siliconix, SUM60N04-12LT Datasheet - Page 7

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

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The SUM60N04-12LT provides a non-committed diode to
allow temperature sensing of the actual MOSFET chip. The
addition of one simple comparator and a few other
components is all that is required to implement a temperature
protected MOSFET. Since it has a very tight tolerance on
forward voltage, the forward voltage of the diode can be used
to provide to shutdown signal. The diode forward voltage falls
to around 0.4 V with a bias current of 250 mA when the
MOSFET chip is close to the maximum permitted temperature
value. The external comparator used to detect over
temperature can also be used as a driver stage for the
MOSFET, meaning that the on/off input is logic compatible,
and can be driven from a logic gate.
A typical circuit is shown in Figure 1. Here a LMV321
operational amplifier is used to drive the MOSFET, and as a
comparator to when the maximum junction temperature is
reached. The circuit will turn on once more when the chip has
cooled to approximately 110_C, and can cycle on and off until
the fault is cleared or the power is removed. This circuit has
assumed a 5-V rail is available, but the circuit could easily be
adapted for a 12-V rail, for example.
The LMV321 op amp was selected to give reasonable output
current to drive the MOSFET at a reasonable price. The SC-70
package means that the protection circuit uses very little board
space. However the limited output current means that it can
only be used in slow switching applications, where one
microsecond switching time and limited dv/dt immunity can be
Document Number: 71620
S-03830—Rev. A, 28-May-01
INPUT
+5 V
0.1 mF
C2
R3, 18 kW
180 kW
22 kW
Signal Ground
1%
1%
R1
R2
560 pF
IC1, LMV321
R7
10 kW
1%
C1
0.1 mF
C3
New Product
R4, 560 kW, 1%
FIGURE 1.
–
+
accepted. For PWM and other faster applications, a buffer
should be added to drive the MOSFET, or the schematic in
Figure 2 used to give fast switching speed.
The reference voltage for the trip point is derived from the 5-V
rail, which should have reasonable voltage accuracy and
stability (" 0.5 V). A voltage reference could be added if
required, but the circuit is only intended to make the MOSFET
invulnerable to drastic faults that might otherwise cause it to
fail, not to give a precise shutdown point. 1% resistors are
used to provide a reference voltage of 0.545 V, giving a
nominal rising trip point of around 155_C, allowing for the
hysteresis drop over R7.
A 560-pF capacitor across the inputs of the comparator
provides some noise immunity and gives a response time of
around a micro second, just faster than the switching speed of
the MOSFET in this circuit (faster response has diminishing
returns as the turn-off time is fixed). This does have a side
effect of introducing such a delay at turn-on. If this is an issue
(although if this delay is an issue, the switching time should be
reviewed also), a separate driver could be added using a
comparator for over temperature detection only as shown in
Figure 2. The diode is then left biased whenever the power is
applied to the load and there is no turn-on delay. In a very noisy
environment C1 should be increased and additional
capacitors may also be required from each input of the
comparator to ground and on the logic input.
R5, 18 kW
R6, 560 W
Gate
Output
Signal
SUM60N04-12LT
Vishay Siliconix
Power Ground
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