LT1161 LINER [Linear Technology], LT1161 Datasheet - Page 7

• Manufacturer Part Number: LT1161
• Description: Quad Protected High-Side MOSFET Driver
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
spike can play havoc with the power supply and false trip
the current-sense comparator.
Turn-on V/ t is controlled by the addition of the simple
network shown in Figure 6. This network takes advantage
of the fact that the MOSFET acts as a source follower
during turn-on. Thus the V/ t on the source can be
controlled by controlling the V/ t on the gate:
where V
ing C
spike. For example, if V
1000 F. The diode and second resistor in the network
ensure fast current limit turn-off.
When turning off a capacitive load, the source of the
MOSFET can “hang up” if the load resistance does not
discharge C
If this is the case, a diode may have to be added from
source to gate to prevent V
Adding Current Limit Delay
When capacitive loads are being switched or in very noisy
environments, it is desirable to add delay in the drain
current-sense path to prevent false tripping (inductive
loads normally do not need delay). This is accomplished
by the current limit delay network shown in Figure 6. R
V/ t = 2.2V/ms, resulting in a 2.2A turn-on spike into
V
t
LOAD
LT1161
TH
Figure 6. V/ t Control and Current Limit Delay
V
10
is the MOSFET gate threshold voltage. Multiply-
times this V/ t yields the value of the current
LOAD
5
DS
V
V
G
+
+
V
C
TH
as fast as the gate is being pulled down.
1
U
V/ t CONTROL NETWORK
+
DELAY NETWORK
100k
CURRENT LIMIT
C
= 24V, V
INFORMATION
D
U
GS(MAX)
R
D
1N4148
( 10k)
100k
C1
1N4148
TH
W
from being exceeded.
= 2V, and C1 = 0.1 F,
24V
+
1RFZ24
U
C
1161 F06
LOAD
+
D
and C
approximately 10 I
and provides immediate turn-off (see Figure 7). To ensure
proper operation of the timer, C
Printed Circuit Board Shunts
The sheet resistance of 1oz. copper clad is approximately
5
+0.39%/ C. Since the LT1161 drain sense threshold has a
similar temperature coefficient (+0.33%/ C), this offers
the possibility of nearly zero TC current sensing using
“free” drain sense resistors made out of PC trace material.
A conservative approach is to use 0.02" of width for each
1A of current for 1oz. copper. Combining the LT1161 drain
sense threshold with the 1oz. copper sheet resistance
results in a simple expression for width and length:
The width for 2oz. copper would be halved while the length
would remain the same.
Bends may be incorporated into the resistor to reduce
space; each bend is equivalent to approximately 0.6
width of straight length. Kelvin connections should be
employed by running separate traces from the ends of the
resistors back to the LT1161 V
Application Note 53 for further information on printed
circuit board shunts.
Width (1oz. Cu) = 0.02" I
Length (1oz. Cu) = 2"
10
D
delay the overcurrent trip for drain currents up to
–4
0.01
0.1
10
1
/square with a temperature coefficient of
1
Figure 7. Current Limit Delay Time
MOSFET DRAIN CURRENT (1 = SET CURRENT)
SET
, above which the diode conducts
10
SET
D
+
must be C
and sense pins. See
L1161 F07
100
LT1161
T
.
7