LM5045 datasheet(24/38 Pages) TI

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R2 =

1.25V x R1

VPWR -1.25V + (20 PA x R1)

VHYS

20 PA

R1 =

R2 =

1.25V x R1

VPWR-OFF -1.25V - (20 PA x R1)

R1 =

VHYS

20 PA

VPWR

VIN

VCC

10V - 16V

(from aux winding)

LM5045

11V

LM5045

SNVS699G – FEBRUARY 2011 – REVISED MARCH 2013

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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 R1 and R2 can be determined from the following equations:

where

•

VPWR is the desired turn-on voltage

•

VHYS is the desired UVLO hysteresis at VPWR

(6)

For example, if the LM5045 is to be enabled when VPWR reaches 33V, and disabled when VPWR is decreased to

31V, R1 should be 100kΩ, and R2 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 R1 and

R2 can be determined from the following equations:

(7)

If the LM5045 is to be disabled when VPWR-OFF reaches 80V and enabled when it is decreased to 78V. R1 should

be 100k

Ω, and R2 should be 1.5 kΩ. The voltage at the OVP pin should not exceed 7V at any time.

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