+

-

STANDBY

OFF

5.0V

0.4V

1.25V

23 PA

LM5039

-

+

R1

R2

VIN

UVLO

23 PA

LM5039

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SNVS621D – FEBRUARY 2010 – REVISED MARCH 2013

to assure that there is never a time where both SR MOSFETs are enabled AND one of the primary side

MOSFETs is enabled. The time period T1 should be set so that the SR MOSFET has turned off before the

primary MOSFET is enabled. Conversely, T1 and T2 should be kept as low as tolerances allow to optimize

efficiency. The SR body diode conducts during the time between the SR MOSFET turns off and the power

transformer begins supplying energy. Power losses increase when this happens since the body diode voltage

drop is many times higher than the MOSFET channel voltage drop. The interval of body diode conduction can be

observed with an oscilloscope as a negative 0.7V to 1.5V pulse at the SR MOSFET drain.

UVLO Divider Selection

A dedicated comparator connected to the UVLO pin is used to detect under-voltage condition. 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. When the UVLO pin voltage rises above 1.25V threshold, an internal 23µA current

source is activated thus providing threshold hysteresis. The 23µA current source is deactivated when the voltage

at the UVLO pin falls below 1.25V. Resistance values for R1 and R2 can be determined from the following

equations:

where

•

•

(10)

For example, if the LM5039 is to be enabled when VPWR reaches 33V, and disabled when VPWR is decreased

to 30V, R1 should be 130k

Ω, and R2 should be 5.11kΩ. The voltage at the UVLO pin should not exceed 7V at

any time. Be sure to check both the power and voltage rating (0603 resistors can be rated as low as 50V) for the

selected R1 resistor. To maintain the threshold’s accuracy, a resistor tolerance of 1% or better is recommended.

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 22.

Figure 22. Basic UVLO Configuration

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