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LT1910IS8-PBF Datasheet PDF : 14 Pages
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LT1910
Applications Information
The turn-on current spike into CLOAD is estimated by:
IPEAK
=
CLOAD
•
VG – VTH
R1• C1
where VTH is the MOSFET gate threshold voltage. VG is
obtained by plotting the equation:
IGATE
=
VGATE
R1
on the graph of Gate Drive Current (IGATE) vs Gate Voltage
(VGATE) as shown in Figure 6. The value of VGATE at the
intersection of the curves for a given supply is VG. For
example, if V+ = 24V and R1 = 100k, then VG = 18.3V. For
VTH = 2V, C1 = 0.1µF and CLOAD = 1000µF, the estimated
IPEAK = 1.6A. The diode and the 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 CLOAD as fast as the gate is being pulled down.
If this is the case, a 15V zener may be added from gate to
source to prevent VGS(MAX) from being exceeded.
800
700
600
V+ = 24V
500
V+ = 48V
V+ = 12V
400
V+ = 8V
300
IVGGAATTEE=/105
200
100
0
0 10 20 30 40 50 60
GATE VOLTAGE (V)
1910 F06
Figure 6. Gate Drive Current vs Gate Voltage
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 5.
RD and CD delay the overcurrent trip for drain currents up
to approximately 10 • ISET , above which the diode conducts
and provides immediate turn-off (see Figure 7). To ensure
proper operation of the timer, CD must be ≤CT .
10
1
0.1
0.01
1
10
100
MOSFET DRAIN CURRENT (1 = SET CURRENT)
1910 F07
Figure 7. Current Limit Delay Time
Printed Circuit Board Shunts
The sheet resistance of 1oz copper clad is approximately
5 • 10–4Ω/square with a temperature coefficient of
0.39%/°C. Since the LT1910 drain-sense threshold has a
similar temperature coefficient (0.33%/°C), this offers the
possibility of nearly zero TC current sensing using the “free”
drain-sense resistor 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 LT1910 drain
sense threshold with the 1oz copper resistance results in
a simple expression for width and length:
Width (1oz Cu) = 0.02" • ISET
Length (1oz Cu) = 2"
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 •
the width of a straight length. Kelvin connection should
be employed by running a separate trace from the ends
of the resistor back to the LT1910’s V+ and SENSE pins.
See Application Note 53 for further information on printed
circuit board shunts.
For more information www.linear.com/LT1910
1910fb
9
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