LM5035ASQ datasheet(23/30 Pages) NSC

23 / 30 page

30034136

FIGURE 13. Remote Standby and Disable Control

To maintain the threshold’s accuracy, a resistor tolerance of

1% or better is recommended.

The design process starts with the choice of the voltage dif-

ference between the UVLO enabling and disabling thresh-

olds. This will also approximately set the difference between

OVP enabling and disabling regulation:

Next, the combined resistance of R

2 and R3 is calculated by

choosing the threshold for the UVLO disabling threshold:

Then R

3 is determined by selecting the OVP disabling thresh-

old:

Finally, R

3 is subtracted from RCOMBINED to give R2:

2 = RCOMBINED - R3

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 13.

FAULT PROTECTION

The Over Voltage Protection (OVP) comparator of the

LM5035A can be configured for line or load fault protection or

thermal protection using an external temperature sensor or

thermistor. Figure 11 shows a line over voltage shutdown ap-

plication using a voltage divider between the input power

supply, V

PWR, and AGND to monitor the line voltage.

Figure 14 demonstrates the use of the OVP pin for latched

output over-voltage fault protection, using a zener and opto-

coupler. When V

OUT exceeds the conduction threshold of the

opto-coupler diode and zener, the opto-coupler momentarily

turns on Q1 and the LM5035A enters standby mode, disabling

the drivers and enabling the hysteresis current source on the

OVP pin. Once the current source is enabled, the OVP volt-

age 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 opto-coupler.

Using the values of Figure 14, the opto-coupler collector cur-

rent must equal V

BE(Q1) / R1 = 350 µA before OVP latches.

Once the controller has switched to standby mode, the out-

puts no longer switch but the VCC and REF regulators con-

tinue 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.

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