LT1910 Linear Technology, LT1910 Datasheet - Page 9

• Manufacturer Part Number: LT1910
• Description: Protected High Side MOSFET Driver
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
The turn-on current spike into C
where V
obtained by plotting the equation:
on the graph of Gate Drive Current (I
(V
intersection of the curves for a given supply is V
example, if V
V
I
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 15V zener may be added from gate to
source 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 5. R
PEAK
TH
GATE
I
I
PEAK
GATE
= 2V, C1 = 0.1 F and C
= 1.6A. The diode and the second resistor in the
) as shown in Figure 6. The value of V
TH
Figure 6. Gate Drive Current vs Gate Voltage
400
200
800
700
600
500
300
100
C
is the MOSFET gate threshold voltage. V
V
LOAD
0
LOAD
GATE
+
R
0
= 24V and R1 = 100k, then V
1
V
as fast as the gate is being pulled down.
+
10
•
= 24V
V
U
V
GS(MAX)
+
R C
G
V
= 12V
+
• 1 1
GATE VOLTAGE (V)
20
–
= 8V
V
U
TH
LOAD
30
from being exceeded.
V
+
= 48V
LOAD
= 1000 F, the estimated
40
W
GATE
I
V
GATE
GATE
is estimated by:
50
=
/10
1910 F06
) vs Gate Voltage
5
60
G
= 18.3V. For
GATE
U
G
at the
. For
G
is
D
Printed Circuit Board Shunts
The sheet resistance of 1oz copper clad is approximately
5 • 10
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:
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
Application Note 53 for further information on printed
circuit board shunts.
and C
approximately 10 • I
and provides immediate turn-off (see Figure 7). To ensure
proper operation of the timer, C
Width (1oz Cu) = 0.02" • I
Length (1oz Cu) = 2"
D
delay the overcurrent trip for drain currents up to
–4
0.01
0.1
10
/square with a temperature coefficient of
1
Figure 7. Current Limit Delay Time
1
MOSFET DRAIN CURRENT (1 = SET CURRENT)
SET
, above which the diode conducts
10
SET
D
+
must be C
and SENSE pins. See
1910 F07
100
LT1910
T
.
sn1910 1910fs
9