OVP

LM5035

STANDBY

NTC

THERMISTOR

T

R2

R1

1.25V

5V

23 2A

VREF

OVP

LM5035

STANDBY

R2

100k

Q1

R1

2k

VREF

VOUT

5V

1.25V

23 2A

LM5035

SNVS428H – JANUARY 2006 – REVISED OCTOBER 2015

www.ti.com

Remote configuration of the operational modes of the controller can be accomplished with open drain devices

connected to the UVLO pin as shown in Figure 23.

8.2.2.7 Fault Protection

The overvoltage protection (OVP) comparator of the LM5035 can be configured for line or load fault protection or

thermal protection using an external temperature sensor or thermistor. Figure 21 shows a line over voltage

shutdown application using a voltage divider between the input power supply, VPWR, and AGND to monitor the

line voltage.

Figure 24 demonstrates the use of the OVP pin for latched output overvoltage fault protection, using a zener and

optocoupler. When VOUT exceeds the conduction threshold of the optocoupler diode and zener, the optocoupler

momentarily turns on Q1 and the LM5035 enters standby mode, disabling the drivers and enabling the hysteresis

current source on the OVP pin. When the current source is enabled, the OVP voltage will remain at 2.3 V (23 µA

× 100 k

Ω) without additional drive from the external circuit. If the optocoupler transistor emitter were directly

connected to the OVP pin, then leakage current in the zener diode amplified by the gain of the optocoupler could

falsely trip the protection latch. R1 and Q1 are added reduce the sensitivity to low level currents in the

optocoupler. Using the values of Figure 24, the optocoupler collector current must equal VBE(Q1) / R1 = 350 µA

before OVP latches. Once the controller has switched to standby mode, the outputs no longer switch but the

VCC and REF regulators continue functioning and supply bias to the external circuitry. VCC must fall below 6.2 V

or the UVLO pin must fall below 0.4 V to clear the OVP latch.

Figure 24. Latched Load Overvoltage Protection

Figure 25 shows an application of the OVP comparator for Remote Thermal Protection using a thermistor (or

multiple thermistors) which may be located near the main heat sources of the power supply. The negative

temperature coefficient (NTC) thermistor is nearly logarithmic, and in this example a 100k

Ω thermistor with the β

material constant of 4500 kelvins changes to approximately 2 k

Ω at 130°C. Setting R1 to one-third of this

resistance (665

Ω) establishes 130°C as the desired trip point (for VREF = 5 V). In a temperature band from 20°C

below to 20°C above the OVP threshold, the voltage divider is nearly linear with 25 mV per°C sensitivity.

R2 provides temperature hysteresis by raising the OVP comparator input by R2 × 23 µA. For example, if a 22-k

Ω

resistor is selected for R2, then the OVP pin voltage will increase by 22 k

Ω × 23 µA = 506 mV. The NTC

temperature must therefore fall by 506 mV / 25 mV per°C = 20°C before the LM5035 switches from the standby

mode to the normal mode.

Figure 25. Remote Thermal Protection

28

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