LTC2913 datasheet(13/24 Pages) LINER

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LTC4366

436612fe

For more information www.linear.com/LTC4366

APPLICATIONS INFORMATION

Followinganovervoltagefault,puttingthepartinshutdown

will clear the fault, allowing operation to resume once the

LTC4366 leaves shutdown.

Output Short

A sudden short on the output can result in excessive cur-

rent into the LTC4366 GATE pin supplied from the gate

capacitor, CG. The GATE pin is internally clamped to OUT

with a 10V to 12V clamp. If the OUT pin is pulled low

while the GATE pin is held up with CG, then the clamp will

be damaged trying to discharge CG when clamp voltage

is exceeded. One solution is to add a 1k RS resistor in

series with CG with a bypass diode as shown in Figure 3.

The diode allows the capacitor to function as a bypass for

energy coming from the MOSFET drain to gate capacitor

during an supply overvoltage.

GATE

DBYPASS

436612 F03

LTC4366

OUT

10V TO

12V

Figure 3. Output Short Protection

LTC4366

BASE

436612 F04

RSS

VSS

Figure 4. External PNP Option

Resistor Power Ratings

The proper rating for the RSS resistor in Figure 1 must be

considered. During an overvoltage event the OUT pin is

at regulation voltage (VREG), so the voltage across RSS is

VREGminus5.7V.Asmallminimumsupplyvoltagereduces

the value of RSS. Therefore, large differences between

minimum supply voltage and the regulation voltage may

require a large power resistor for RSS.

The full supply voltage minus 12V can appear across

RIN during the overvoltage cool-down period. Normally

the value for RIN is several times larger than RSS which

lowers the power and size requirements for this resistor.

External PNP

InsomecasesthepowerresistorforRSSmaybephysically

large. A large value RSS (with lower power and size) may

be used in conjunction with a PNP as shown in Figure 4.

In addition to the 0.8µA sourced from the BASE pin, the

base current from the PNP must flow through RSS which

will limit the maximum RSS value. In some cases the

minimum PNP Beta is as low as 35. The base current

becomes 10µA when the VSS current is 350µA. One can

see this allows a 35 (Beta) times larger RSS than the ap-

plication without the PNP.

Minimum Supply Start-Up

When designing for the minimum supply condition, it is

important that RSS and RIN are chosen to provide enough

current to sufficiently charge C1 to 4.75V. The parameters

that determine the minimum supply voltage include: C1

voltage,MOSFETthresholdvoltage,aseriesSchottkydiode

voltage drop, resistance of RSS and RIN, current in the VDD

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