Datasheet
OVP
LM5035
STANDBY
R2
100k
Q1
R1
2k
V
REF
V
OUT
5V
1.25V
23 PA
1.25V x (R
1
+
R
COMBINED
)
OVP
off
R
3
=
R
COMBINED
=
1.25V x R
1
UVLO
off
± 1.25V
LM5035
www.ti.com
SNVS428G –JANUARY 2006–REVISED MARCH 2013
Next, the combined resistance of R
2
and R
3
is calculated by choosing the threshold for the UVLO disabling
threshold:
(14)
Then R
3
is determined by selecting the OVP disabling threshold:
(15)
Finally, R
3
is subtracted from R
COMBINED
to give R
2
:
R
2
= R
COMBINED
- R
3
(16)
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.
FAULT PROTECTION
The Over Voltage 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, V
PWR
, and AGND to monitor the
line voltage.
Figure 24 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 LM5035 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 (23µA x 100kΩ) 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 V
BE(Q1)
/ R1 = 350µA
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 24. Latched Load Over-Voltage 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 100kΩ thermistor with the β
material constant of 4500 kelvins changes to approximately 2kΩ at 130°C. Setting R1 to one-third of this
resistance (665Ω) establishes 130°C as the desired trip point (for V
REF
= 5V). In a temperature band from 20°C
below to 20°C above the OVP threshold, the voltage divider is nearly linear with 25mV per°C sensitivity.
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