Datasheet

LM9061
www.ti.com SNOS738F –APRIL 1995–REVISED APRIL 1995
This voltage divider arrangement requires a mechanism to raise the ON/OFF pin above the 'On' threshold of
3.5V minimum (3.1V typical) when V
CC
is less than typically 16V. This can be accomplished with a second,
normally open, switch from the ON/OFF pin across R2 (Set), so that closing the switch will short R2 and the
voltage at the ON/OFF pin will be typically one-half of V
CC
. When V
CC
is at the minimum operating voltage of
7.0V this will bias the ON/OFF pin to about 3.5V causing the LM9061 to turn on. When V
CC
is above typically
16.5V the resistor divider will have the ON/OFF pin biased above 3.5V and shorting of the resistor R2 will not be
needed.
While the scaling of the external resistor values between V
CC
and the ON/OFF input pin, against the internal 30
kΩ resistor, can be used to increase the startup voltage, it is important that the resistor ratio always has the
ON/OFF pin biased below the 'Off' threshold (1.5V) when V
CC
falls below the minimum operating voltage of 7.0V.
The accuracy of this voltage divider arrangement is affected by normal manufacturing variations of the 'On' and
'Off' voltage thresholds and the value of internal resistor at the ON/OFF pin. If any application needs to detect
with greater precision when V
CC
is near to 7.0V, an external voltage monitor should be used to drive the ON/OFF
pin. The external voltage monitor would also eliminate both the need for the switch to short R2 to start the
LM9061, as well as R2.
DRIVING MOSFET ARRAYS
The LM9061 is an ideal driver for any application that requires multiple parallel MOSFETs to provide the
necessary load current. Only a few “common sense” precautions need to be observed. All MOSFETs in the array
must have identical electrical and thermal characteristics. This can be solved by using the same part number
from the same manufacturer for all of the MOSFETs in the array. Also, all MOSFETs should have the same style
heat sink or, ideally, all mounted on the same heat sink. The electrical connection of the MOSFETs should get
special attention. With typical R
DS(ON)
values in the range of tens of milli-Ohms, a poor electrical connection for
one of the MOSFETs can render it useless in the circuit.
Also, the MOSFET dissipation during the Normal-OFF discharge of the gate capacitance, 70 µA minimum and
110 µA typical, needs consideration.
One particular caution is that, in the event of a fault condition, the Latch-OFF current sink, 10 µA typical, may not
be able to discharge the total gate capacitance in a timely manner to prevent damage to the MOSFETs.
Figure 21 shows a circuit with four parallel NDP706A MOSFETs. This particular MOSFET has a typical R
DS(ON)
of
0.013Ω with a T
J
of 25°C, and 0.020Ω with a T
J
of +125°C.
With the V
DS
threshold voltage being set to 500 mV, this circuit will provide a typical maximum load current of
150A at 25°C, and a typical maximum load current of 100A at 125°C. The maximum dissipation, per MOSFET,
will be nearly 20W at 25°C, and 12.5W at 125°C. With up to 20W being dissipated by each of the four devices,
an effective heat sink will be required to keep the T
J
as low as possible when operating near the maximum load
currents.
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