Datasheet

LM9061
SNOS738F –APRIL 1995–REVISED APRIL 1995 www.ti.com
The resistor R
SENSE
is optional, but is strongly recommended to provide transient protection for the Sense pin,
especially when driving inductive type loads. A minimum value of 1 kΩ will protect the pin from transients ranging
from −25V to +60V. This resistor should be equal to, or less than, the resistor used for R
THRESHOLD
. Never set
R
SENSE
to a value larger than R
THRESHOLD
. When the protection comparator output goes high , the total bias
current for the input stage transfers from the Sense pin to the Threshold pin, thereby changing the voltages
present at the inputs to the comparator. For consistent switching of the comparator right at the desired threshold
point, the voltage drop that occurs at the non-inverting input (Threshold) should equal, or exceed, the rise in
voltage at the inverting input (Sense).
A bypass capacitor across R
REF
is optional and is used to help keep the reference voltage constant in
applications where the V
CC
supply is subject to high levels of transient noise. This bypass capacitor should be no
larger than 0.1 µF, and is not needed for most applications.
DELAY TIMER
To allow the MOSFET to conduct currents beyond the protection threshold for a brief period of time, a delay
timer function is provided. This timer delays the actual latching OFF of the MOSFET for a programmable interval.
This feature is important to drive loads which require a surge of current in excess of the normal ON current upon
start up, or at any point in time, such as lamps and motors. Figure 18 details the delay timer circuitry. A capacitor
connected from the Delay pin 8, to ground sets the delay time interval. With the MOSFET turned ON and all
conditions normal, the output of the protection comparator is low and this keeps the discharge transistor ON. This
transistor keeps the delay capacitor discharged. Should a surge of load current trip the protection comparator
high, the discharge transistor turns OFF and an internal 10 µA current source begins linearly charging the delay
capacitor.
If the surge current, with excessive V
DS
voltage, lasts long enough for the capacitor to charge to the timing
comparator threshold of typically 5.5V, the output of the comparator will go high to set a flip-flop and immediately
latch the MOSFET OFF. It will not re-start until the ON/OFF Input is toggled low then high.
The delay time interval is set by the selection of C
DELAY
and can be found from:
where
• Typically, V
TIMER
= 5.5V.
• I
DELAY
= 10 µA. (6)
Charging of the delay capacitor is clamped at approximately 7.5V which is the internal bias voltage for the 10 µA
current source.
Figure 18. Delay Timer
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