Datasheet

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)
LM5046
11V
0.1 PF
50
LM5046
VIN
V
PWR
LM5046
www.ti.com SNVS703G –FEBRUARY 2011–REVISED MARCH 2013
Figure 24. Input Transient Protection
FOR APPLICATIONS WITH > 100V INPUT
For applications where the system input voltage exceeds 100V, VIN can be powered from an external start-up
regulator as shown in Figure 25. In this configuration, the VIN and VCC pins should be connected together. The
voltage at the VCC and VIN pins must be greater than 10V (>Max VCC reference voltage) yet not exceed 16V.
To enable operation the VCC voltage must be raised above 10V. The voltage at the VCC pin must not exceed
16V. The voltage source at the right side of Figure 25 is typically derived from the power stage, and becomes
active once the LM5046’s outputs are active.
Figure 25. 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 26 and
Figure 27, or with a three-resistor divider as shown in Figure 28. 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 26. 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:
(8)
Where V
PWR
is the desired turn-on voltage and V
HYS
is the desired UVLO hysteresis at V
PWR
.
For example, if the LM5046 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.
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 25
Product Folder Links: LM5046