Datasheet

LM9061
www.ti.com SNOS738F –APRIL 1995–REVISED APRIL 1995
The threshold voltage is set by the voltage drop across resistor R
THRESHOLD
. A reference current is fixed by a
resistor to ground at I
REF
, pin 6. To precisely regulate the reference current over temperature, a stable band gap
reference voltage is provided to bias a constant current sink. The reference current is set by:
(4)
The reference current sink output is internally connected to the threshold pin. I
REF
then flows from the load supply
through R
THRESHOLD
. The fixed voltage drop across R
THRESHOLD
is approximately equal to the maximum value of
V
DS
across the MOSFET before the protection comparator trips.
It is important to note that the programmed reference current serves a multiple purpose as it is used internally for
biasing and also has a direct effect on the internal charge pump switching frequency. The design of the LM9061
is optimized for a reference current of approximately 80 µA, set with a 15.4 kΩ ±1% resistor for R
REF
. To obtain
the ensured performance characteristics it is recommended that a 15.4 kΩ resistor be used for R
REF
.
The protection comparator is configured such that during normal operation, when the output of the comparator is
low, the differential input stage of the comparator is switched in a manner that there is virtually no current flowing
into the non-inverting input of the comparator. Therefore, only I
REF
flows through resistor R
THRESHOLD
. All of the
input bias current, 20 µA maximum, for the comparator input stage (twice the I
SENSE
specification of 10 µA
maximum, defined for equal potentials on each of the comparator inputs) however flows into the inverting input
through resistor R
SENSE
. At the comparator threshold, the current through R
SENSE
will be no more than the I
SENSE
specification of 10 µA.
Figure 17. Protection Comparator Biasing
To tailor the V
DS (MAX)
threshold for any particular application, the resistor R
THRESHOLD
can be selected per the
following formula:
where
• R
REF
= 15.4 kΩ.
• I
SENSE
is the input bias current to the protection comparator.
• R
SENSE
is the resistor connected to pin 1.
• V
OS
is the offset voltage of the protection comparator (typically in the range of ±10 mV). (5)
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