LM5035CMHSLASHNOPB datasheet(29/43 Pages) TI2

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OVP

LM5035C

STANDBY

NTC

THERMISTOR

1.25 V

5 V

23 PA

VREF

OVP

LM5035C

STANDBY

100k

VREF

VOUT

1.25 V

23 µA

LM5035C

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SNVS631D – JANUARY 2010 – REVISED OCTOBER 2016

Product Folder Links: LM5035C

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Remote configuration of the controller’s operational modes can be accomplished with open drain device(s)

connected to the UVLO pin as shown in Figure 23.

9.2.2.7 Fault Protection

The Overvoltage Protection (OVP) comparator of the LM5035C 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

opto-coupler. When VOUT exceeds the conduction threshold of the opto-coupler diode and Zener, the opto-

coupler momentarily turns on Q1 and the LM5035C enters standby mode, disabling the drivers and enabling the

hysteresis current source on the OVP pin. Once the current source is enabled, the OVP voltage will remain at

2.3V (23 µA × 100 k

Ω) without additional drive from the external circuit. If the opto-coupler transistor emitter were

directly connected to the OVP pin, then leakage current in the Zener diode amplified by the opto-coupler’s gain

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

opto-coupler. Using the values of Figure 24, the opto-coupler 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 100-k

Ω thermistor with the β

material constant of 4500 kelvins changes to approximately 2 kΩ 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-kΩ

resistor is selected for R2, then the OVP pin voltage will increase by 22 kΩ × 23 µA = 506 mV. The NTC

temperature must therefore fall by 506 mV / 25 mV per°C = 20°C before the LM5035C switches from the standby

mode to the normal mode.

Figure 25. Remote Thermal Protection

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