Datasheet

LTC4366-1/LTC4366-2
436612fd
For more information www.linear.com/LTC4366
20
28V Vehicle Application
The circuit in Figure 8 adds reverse voltage protection to
the standard 28V application shown in Figure 6. There are
three modes to this circuit: pass FET On when the input
is 18V to 41V, clamping the output to 43V when more
than 43V appears at the input and finally reverse voltage
protection when up to –250V DC is present at the input.
The reverse voltage protection consists of the circuitry
inside the dotted box in Figure 8. When a positive voltage
is first applied to the input, D3 and the forward biased
base-collector junction of Q2 allow the gate of M2 to
follow the input voltage minus a two diode drop. During
this condition the body diode of M2 is used to transmit
power to the LTC4366. Once the LTC4366 is powered up
it fully enhances the gate of M1 and M2 (via D1). The M1
and M2 pass FETs then provide a low impedance path to
the load. In an overvoltage condition, D1 blocks excessive
positive voltage from the input supply passing to the GATE
pin of the LTC4366. D4 eliminates current flow through
R6 when the input is positive while D3 prevents emitter
base breakdown of Q2 when the input is powering up.
During negative input voltages Q2 turns on when current
from R6 (via D4) develops a forward diode drop on R5.
Q2 then holds the gate of M2 at the input voltage which
turns M2 off. This blocks negative input voltages from
reaching M1 and the load. D2 prevents damage to the
LTC4366’s GATE pin by clamping it at ground when the
M2’s gate is negative.
Low Voltage Application
The circuit on the last page (Surge Protected Automotive
Supply) starts up with minimum input voltage of 9V. In
order to successfully start up at 9V and clamp the output
voltage at 18V for input voltages up to 100V the value of
R
SS
has to be small (1.91k). The FET used in this case has
a 3V threshold to ease the start-up requirements. The timer
capacitor is sized to give a 2.5ms overvoltage regulation
time that keeps the P
2
t below the 420W
2
s specified for
this MOSFET.
V
DD
SD
R1
470k
R4
270k
R2
100k
OUT
C
G
10nF
GATE
M1
FQA62N25C
M2
FDB33N25
REVERSE VOLTAGE
PROTECTION
SD FB
C
T
10nF
C1
0.47µF
R
FB1
12.4k
V
OUT
1.5A
(43V CLAMP)
V
IN
18V TO 41V
(±250V DC)
R
FB2
422k
436612 F08
R
SS
46.4k
R
G
10Ω
R
IN
324k
Q1
MMBT3904
D4
BAV3004W
D3
BAV3004W
LTC4366-2
TIMER BASEV
SS
R5
470k
Q2
MMBT3904
R6
270k
D2
BAV3004W
D1
BAV3004W
Figure 8. 28V Vehicle Application with Reverse Voltage Protection
applicaTions inForMaTion