OVP

LM5035C

STANDBY

R2

100k

Q1

R1

2k

5V

1.25V

23 PA

LM5035C

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SNVS631C – JANUARY 2010 – REVISED MARCH 2013

Fault Protection

The Over Voltage 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 23 shows a line over

monitor the line voltage.

Figure 26 demonstrates the use of the OVP pin for latched output over-voltage fault protection, using a zener

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 x 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

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 6.2V

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

Figure 26. Latched Load Over-Voltage Protection

Figure 27 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

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 x 23 µA. For example, if a 22k

Ω

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

Ω x 23 µA = 506 mV. The NTC

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

mode to the normal mode.

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Product Folder Links: LM5035C