LT4256-1/LT4256-2
13
425612fa
Figure 13. V
GATE
vs V
CC
Supply Transient Protection
The LT4256-1/LT4256-2 is 100% tested and guaranteed
to be safe from damage with supply voltages up to 80V.
However, voltage transients above 100V may cause per-
manent damage. During a short-circuit condition, the
large change in currents flowing through the power supply
traces can cause inductive voltage transients which could
exceed 100V. To minimize the voltage transients, the
power trace parasitic inductance should be minimized by
using wider traces or heavier trace plating and a 0.1礔
bypass capacitor should be placed between V
CC
and GND.
A surge suppressor, as shown in the application diagrams,
(Transzorb) at the input can also prevent damage from
voltage transients.
GATE Pin
A curve of gate drive vs V
CC
is shown in Figure 13. GATE
is clamped to a maximum voltage of 12.8V above V
CC
. This
clamp is designed to sink the internal charge pump cur-
rent. An external Zener diode must be used as shown in all
applications. At a minimum input supply voltage of 12V,
the minimum gate drive voltage is 4.5V. When the input
supply voltage is higher than 20V, the gate drive voltage is
at least 10V and a standard threshold MOSFET can be
used. In applications from 12V to 15V range, a logic level
MOSFET must be used.
In some applications it may be possible for the V
OUT
pin to
ring below ground (due to the parasitic trace inductance).
V
CC
(V)
10
8
9
10
4256 F13
7
6
5
20 30
70
60
50
40
80
12
11
APPLICATIO S I FOR ATIO
U
U
U
Higher current applications, especially where the output
load is physically far away from the LT4256-1/LT4256-2
will be more susceptible to these transients. This is normal
and the LT4256-1/LT4256-2 have been designed to allow
for some ringing below ground. However, if the applica-
tion is such that V
OUT
can ring more than 10V below
ground, damage may occur to the LT4256-1 and an
external diode from ground (anode) to V
OUT
(cathode)
must be added to the circuit as shown in Figure 14 (it is
critical that the reverse breakdown voltage of the diode be
higher than the highest expected V
CC
voltage). A capacitor
placed from ground to V
OUT
directly at the LT4256-1/
LT4256-2 can help reduce the amount of ringing on V
OUT
but it may not be enough for some applications.
During a fault condition, the LT4256-1/LT4256-2 pulls
down on GATE with a switch capable of sinking about
60mA. Once GATE drops below the output voltage by a
diode forward voltage, the external Zener will forward bias
and V
OUT
will also be discharged to GND. In addition to the
GATE capacitance, the output capacitance will be dis-
charged through the LT4256-1/LT4256-2.
In applications utilizing very large external N-channel
MOSFETs, the possibility exists for the MOSFET to turn on
when initially inserted into a live backplane (before the
LT4256-1/LT4256-2 becomes active and pulls down on
GATE). This is due to the drain to gate capacitance forcing
current into R7 and C1 when the drain voltage steps up
from ground to V
IN
with an extremely fast rise time. To
alleviate this situation, a diode, D3, should be put
across R7 with the cathode connected to C1 as shown in
Figure 15.
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