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LTM4636-1

46361fa

For more information www.linear.com/LTM4636-1

APPLICATIONS INFORMATION

Enhanced Overvoltage OVP and Overtemperature

Protection OTP

TheLTM4636-1hastwoenhancedprotectionfeaturesthat

can be used to detect overtemperature and overvoltage.

The BIAS pin is supplied to power the protection circuitry

off of an auxiliary supervisory supply separate from the

highpowercircuitbreakerpath.Thisisrequiredtoproperly

bias the protection circuitry with the proper headroom at

powerup,andcircuitbreakerretry.Thisseparatebiassup-

ply needs to supply 1mA per module, and 6mA per module

during an overvoltage and overtemperature event. If the

CROWBAR signal is used then an instantaneous current

will be need to drive the crow bar mosfets. This is current

is ~ 50mA. The bias voltage can be used to program the

OTP and OVP set points.

TheTMONpinvoltagevariesfrom1.0Vat25°Cupto1.45V

at 150°C. The TMON pin can be compared to a program-

mable set point utilizing the 24.9k resistor to ground on

the OTP_SET pin. See the block diagram. By adding an

external resistor from this pin to the BIAS pin, an OTP

trip point can be set below the 150°C (1.45V) level as an

overtemperature detection, and the OVER_TEMP open-

collector pin will pull low to provide a system alert, or can

be used to trip off power. This feature can be ignored by

floating the OVER_TEMP and OTP_SET pins.

The equation:

ROTP 24.9k •

BIAS

OTP _ SET

1 ;

OTP _ SET 1.0V(25 C) 3.52mV/°C • 105 C

For 130°C, OTP_SET = 1.37V

Where BIAS is the voltage applied to BIAS pin, Ex: 5V.

OTP_SET is the voltage equivalent to temperature on the

TMON pin. For example, BIAS = 5V, and the overtem-

perature trip point is set to 130°C. TMON has a range of

440mVfrom25°Cto150°C,thereforetheTMONpinmoves

3.52mV/°C above 25°C. 130°C minus 25°C = 105°C rise.

The ROTP resistor calculates out to a close 1% value of

66.5k.

The BIAS supply can be between 4V to 5.5V, the OTP_SET

trip point depends on the BIAS supply voltage to be accu-

rate, otherwise a more accurate reference can be derived

from the BIAS supply to be utilized to program the set

point. See application schematics.

The LTM4636-1 controller has overvoltage protection

feedback voltage referred that trips at ~7.5% above VOUT.

This feature will try to correct an overvoltage transient

issue, but does not protect for a shorted input power

source in the event of a top power switch short while the

bottom switch is on trying to protect the load. The input

source has a direct path to ground in this condition. Nei-

ther will this feature protect an overvoltage issue caused

by an open feedback path. The enhanced overvoltage

protection consists of a fast comparator that compares

VOUT to a programmed value set at the OVP_SET pin. An

OVP_TRIP point can be set using the BIAS supply and an

external resistor (ROVP) with the internal 24.9k resistor.

The equation:

ROVP 24.9k •

BIAS

OVP _ TRIP

– 1

where OVP_TRIP is the maximum overvoltage level set

point. The OVP_TRIP open-collector output pulls low and

can be used to trip off a circuit breaker or alert the system

of an OVP fault. The CROWBAR output drive can be used

todriveanexternalN-channelpowerMOSFETtoclampthe

output voltage and discharge the inductor stored energy.

Depending on the power design, the OVP trip point and

the output capacitance, the CROWBAR clamp may not be

needed. Testing will need to be performed to characterize

the behavior of the clamping with and without the crow-

bar. Proper crowbar power MOSFET SOA (safe operating

area) selection will be required to make sure the MOSFET

can handle the discharge current. A small surface mount

resistor can be placed in series from VOUT to the drain of

the CROWBAR FET to limit the CROWBAR but still clamp

the output. To disable the function, float the OVP_SET,

OVP_TRIP and CROWBAR pins.

The BIAS supply can be between 4.0V to 5.5V, while the

OVP_TRIP point depends on the BIAS supply voltage ac-

curacy. Otherwise an accurate reference can be derived

from the BIAS supply.

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