LTC2913 datasheet(20/24 Pages) LINER

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LTC4366

436612fe

For more information www.linear.com/LTC4366

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

RSS has to be small (1.91k). The FET used in this case has

a3Vthresholdtoeasethestart-uprequirements.Thetimer

capacitor is sized to give a 2.5ms overvoltage regulation

time that keeps the P2t below the 420W2s specified for

this MOSFET.

VDD

470k

270k

100k

OUT

10nF

GATE

FQA62N25C

FDB33N25

REVERSE VOLTAGE

PROTECTION

8.2nF

0.47µF

RFB1

12.4k

VOUT

1.5A

(43V CLAMP)

VIN

18V TO 41V

(±250V DC)

RFB2

422k

436612 F08

RSS

46.4k

RIN

324k

MMBT3904

BAV3004W

LTC4366-2

TIMER

BASE

VSS

470k

MMBT3904

270k

BAV3004W

Figure 8. 28V Vehicle Application with Reverse Voltage Protection

APPLICATIONS INFORMATION

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