lm5035bsqx National Semiconductor Corporation, lm5035bsqx Datasheet - Page 21

• Manufacturer Part Number: lm5035bsqx
• Description: Pwm Controller With Integrated Half-bridge And Syncfet Drivers
• Manufacturer: National Semiconductor Corporation

The impedance seen looking into the resistor divider from the
UVLO and OVP pins determines the hysteresis level. UVLO
and OVP enable and disable thresholds are calculated using
The typical operating ranges of undervoltage and overvoltage
thresholds are calculated from the above equations. For ex-
ample, for resistor values R1 = 86.6kΩ, R2 = 2.10kΩ and R3
= 1.40kΩ the computed thresholds are:
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
choosing the threshold for the UVLO disabling threshold:
Then R
old:
Outputs disabled due to VIN falling below UVLO threshold
Outputs enabled due to VIN rising above UVLO threshold
Outputs disabled due to VIN rising above OVP threshold
Outputs enabled due to VIN falling below OVP threshold
3
is determined by selecting the OVP disabling thresh-
2
and R
FIGURE 13. Remote Standby and Disable Control
3
TABLE 1. UVO/OVP Divider Formulas
is calculated by
21
UVLO
OVP
the equations in the table below for the three-resistor divider
illustrated in Figure 12.
UVLO turn-off = 32.2V
UVLO turn-on = 34.2V
OVP turn-on = 78.4V
OVP turn-off = 80.5V
Finally, R
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
LM5035B 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
Figure 14 demonstrates the use of the OVP pin for latched
output over-voltage fault protection, using a zener and opto-
coupler. When V
opto-coupler diode and zener, the opto-coupler momentarily
turns on Q1 and the LM5035B 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 kΩ) without additional
drive from the external circuit. If the opto-coupler transistor
emitter were directly connected to the OVP pin, then leakage
on
on
= OVP
= UVLO
PWR
3
off
is subtracted from R
- [23 µA x (R1 + R2)]
, and AGND to monitor the line voltage.
off
+ (23 µA x R
OUT
R
exceeds the conduction threshold of the
2
= R
COMBINED
1
)
COMBINED
- R
30091336
3
to give R
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