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LT1161 Datasheet(PDF) 6 Page - Linear Technology

Part No. LT1161
Description  Quad Protected High-Side MOSFET Driver
Download  12 Pages
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT1161 Datasheet(HTML) 6 Page - Linear Technology

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6
LT1161
APPLICATIONS INFORMATION
Drain Sense Configuration
The LT1161 uses supply-referenced current sensing. One
input of each channel’s current-sense comparator is con-
nected to a drain sense pin, while the second input is offset
65mV below the supply bus inside the device. For this
reason, pins 11 and 20 of the LT1161 must be treated not
only as supply pins, but as the reference inputs for the
current-sense comparators.
Figure 4 shows the proper drain sense configuration for
the LT1161. Note that the sense pin goes to the drain end
of the sense resistor, while the two V+ pins are tied to each
other and connected to supply at the same point as the
positive ends of the sense resistors. Local supply
decoupling at the LT1161 is important at high input
voltages (see Protecting Against Supply Transients).
The drain sense threshold voltage has a positive tempera-
ture coefficient, allowing PTC sense resistors to be used
(see Printed Circuit Board Shunts). The selection of RS
should be based on the minimum threshold voltage:
R
mV
I
S
SET
=
50
Thus the 0.02
Ω drainsenseresistorinFigure4wouldyield
a minimum trip current of 2.5A. This simple configuration
is appropriate for resistive or inductive loads which do not
generate large current transients at turn-on.
Automatic Restart Period
The timing capacitor CT shown in Figure 4 determines the
length of time the power MOSFET is held off following a
current limit trip. Curves are given in the Typical Perfor-
mance Characteristics to show the restart period for
various values of CT. For example, CT = 0.33µF yields a
50ms restart period.
Defeating Automatic Restart
Some applications are required to remain off after a fault
occurs. When the LT1161 is being driven from CMOS
logic, this can be easily implemented by connecting
resistor R1 between the input and timer pins as shown in
Figure 5. R1 supplies the sustaining current for an SCR
which latches the timer pin low. This prevents the MOSFET
gate from turning ON until the input has been recycled.
LT1161
ON = 5V
OFF = 0V
TIMER
R1
2k
1161 F05
INPUT
5V
CMOS
LOGIC
Figure 5. Latch-Off Input Network (Auto-Restart Defeated)
Inductive vs Capacitive Loads
Turning on an inductive load produces a relatively benign
ramp in MOSFET current. However, when an inductive
load is turned off, the current stored in the inductor needs
somewhere to decay. A clamp diode connected directly
across each inductive load normally serves this purpose.
If a diode is not employed the LT1161 clamps the MOSFET
gate 0.7V below ground. This causes the MOSFET to
resume conduction during the current decay with (V+ +
VGS + 0.7V) across it, resulting in high dissipation peaks.
Capacitive loads exhibit the opposite behavior. Any load
that includes a decoupling capacitor will generate a cur-
rent equal to CLOAD × (∂V/∂t) during capacitor in-rush.
With large electrolytic capacitors, the resulting current
LT1161
T1
V+
V+
1161 F04
24V
10
µF
100
µF
50V
24V, 2A
SOLENOID
IRFZ34
RS
0.02
(PTC)
CT
1
µF
GND
G1
DS1
GND
+
+
Figure 4. Drain Sense Configuration


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