lm5045sqx National Semiconductor Corporation, lm5045sqx Datasheet - Page 21

• Manufacturer Part Number: lm5045sqx
• Description: Full-bridge Pwm Controller With Integrated Mosfet Drivers
• Manufacturer: National Semiconductor Corporation

UVLO AND OVP VOLTAGE DIVIDER SELECTION
Two dedicated comparators connected to the UVLO and OVP
pins are used to detect under voltage and over voltage con-
ditions. The threshold values of both these comparators are
set at 1.25V. The two functions can be programmed inde-
pendently with two separate voltage dividers from VIN to
AGND as shown in Figure 13 and 14, or with a three-resistor
divider as shown in Figure 15. Independent UVLO and OVP
pins provide greater flexibility for the user to select the oper-
ational voltage range of the system. When the UVLO pin
voltage is below 0.4V, the controller is in a low current shut-
down mode. For a UVLO pin voltage greater than 0.4V but
less than 1.25V the controller is in standby mode. Once the
UVLO pin voltage is greater than 1.25V, the controller is fully
enabled. Two external resistors can be used to program the
minimum operational voltage for the power converter as
shown in Figure 13. When the UVLO pin voltage falls below
the 1.25V threshold, an internal 20µA current sink is enabled
to lower the voltage at the UVLO pin, thus providing threshold
hysteresis. Resistance values for R
mined from the following equations:
1
and R
FIGURE 12. Start-up Regulator for VPWR>100V
FIGURE 11. Input Transient Protection
2
can be deter-
21
Where V
desired UVLO hysteresis at V
For example, if the LM5045 is to be enabled when VPWR
reaches 33V, and disabled when V
R
the UVLO pin should not exceed 7V at any time.
Two external resistors can be used to program the maximum
operational voltage for the power converter as shown in Fig-
ure 13. When the OVP pin voltage rises above the 1.25V
threshold, an internal 20µA current source is enabled to raise
the voltage at the OVP pin, thus providing threshold hystere-
sis. Resistance values for R
the following equations:
If the LM5045 is to be disabled when V
and enabled when it is decreased to 78V. R
100kΩ, and R
should not exceed 7V at any time.
1
should be 100kΩ, and R
PWR
is the desired turn-on voltage and V
2
30145430
should be 1.5 kΩ. The voltage at the OVP pin
2
1
30145437
should be 4.2kΩ. The voltage at
and R
PWR
.
PWR
2
can be determined from
PWR-OFF
is decreased to 31V,
reaches 80V
1
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should be
HYS
is the