Datasheet

R
2
=
1.25V x R
1
V
PWR
-1.25V + (20 PA x R1)
V
HYS
20 PA
R
1
=
R
2
=
1.25V x R
1
V
PWR-OFF
-1.25V - (20 PA x R1)
R
1
=
V
HYS
20 PA
V
PWR
VIN
VCC
10V - 16V
(from aux winding)
LM5045
11V
LM5045
SNVS699G –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
Figure 22. Start-up Regulator for VPWR>100V
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 conditions. The threshold values of both these comparators are set at 1.25V. The two functions can be
programmed independently with two separate voltage dividers from VIN to AGND as shown in Figure 23 and
Figure 24, or with a three-resistor divider as shown in Figure 25. Independent UVLO and OVP pins provide
greater flexibility for the user to select the operational voltage range of the system. When the UVLO pin voltage is
below 0.4V, the controller is in a low current shutdown 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 23. 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
1
and R
2
can be determined from the following equations:
where
• V
PWR
is the desired turn-on voltage
• V
HYS
is the desired UVLO hysteresis at V
PWR
(6)
For example, if the LM5045 is to be enabled when VPWR reaches 33V, and disabled when V
PWR
is decreased to
31V, R
1
should be 100kΩ, and R
2
should be 4.2kΩ. The voltage at 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 Figure 24. 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 hysteresis. Resistance values for R
1
and
R
2
can be determined from the following equations:
(7)
If the LM5045 is to be disabled when V
PWR-OFF
reaches 80V and enabled when it is decreased to 78V. R
1
should
be 100kΩ, and R
2
should be 1.5 kΩ. The voltage at the OVP pin should not exceed 7V at any time.
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