Datasheet

LT1910
9
1910fc
For more information www.linear.com/LT1910
APPLICATIONS INFORMATION
The turn-on current spike into C
LOAD
is estimated by:
IC•
V–V
R1•C1
PEAK
LOAD
G
TH
=
where V
TH
is the MOSFET gate threshold voltage. V
G
is
obtained by plotting the equation:
I
V
R1
GATE
GATE
=
on the graph of Gate Drive Current (I
GATE
) vs Gate Voltage
(V
GATE
) as shown in Figure 6. The value of V
GATE
at the
intersection of the curves for a given supply is V
G
. For
example, if V
+
= 24V and R1 = 100k, then V
G
= 18.3V. For
V
TH
= 2V, C1 = 0.1µF and C
LOAD
= 1000µF, the estimated
I
PEAK
= 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 C
LOAD
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 V
GS(MAX)
from being exceeded.
R
D
and C
D
delay the overcurrent trip for drain currents up
to approximately 10 • I
SET
, above which the diode conducts
and provides immediate turn-off (see Figure 7). To ensure
proper operation of the timer, C
D
must be ≤C
T
.
GATE VOLTAGE (V)
0
GATE DRIVE CURRENT (µA)
300
400
500
30
50
1910 F06
200
100
0
10 20 40
600
700
800
60
V
+
= 48V
I
GATE
=
V
GATE
/10
5
V
+
= 24V
V
+
= 12V
V
+
= 8V
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.
MOSFET DRAIN CURRENT (1 = SET CURRENT)
1
0.01
TRIP DELAY TIME (1 = R
D
C
D
)
0.1
1
10
10 100
1910 F07
Figure 7. Current Limit Delay Time
Printed Circuit Board Shunts
The sheet resistance of 1oz copper clad is approximately
5 • 10
–4
Ω/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" • I
SET
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.
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