+

VCC

5 A

1.5 V

EN/UVLO

UVLO

Comparator

LM5181

1.45 V

:

ª

º

¬

¼

:

˜

ª

º

¬

¼

q

ª

º

¬

¼

FB

TC

PS

Diode

R

k

3

R

k

N

TC

mV C

12

LM5181

SNVSBM6 – APRIL 2020

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Feature Description (continued)

8.3.3.1 Diode Thermal Compensation

The LM5181 employs a unique thermal compensation circuit that adjusts the feedback setpoint based on the

thermal coefficient of the forward voltage drop of the flyback diode. Even though the output voltage is measured

when the secondary current is effectively zero, there is still a non-zero forward voltage drop associated with the

flyback diode. Select the thermal compensation resistor using Equation 9.

(9)

The temperature coefficient of the diode voltage drop may not be explicitly provided in the diode data sheet, so

the effective value can be estimated based on the measured output voltage shift overtemperature when the TC

resistor is not installed.

8.3.4 Control Loop Error Amplifier

The inputs of the error amplifier include a level-shifted version of the FB voltage and an internal 1.21-V reference

set by the resistor at RSET. A type-2 internal compensation network stabilizes the converter. In BCM operation

when the output voltage is in regulation, an on-time interval is initiated when the secondary current reaches zero.

The power MOSFET is subsequently turned off when an amplified version of the peak primary current exceeds

the error amplifier output.

8.3.5 Precision Enable

The precision EN/UVLO input supports adjustable input undervoltage lockout (UVLO) with hysteresis for

application specific power-up and power-down requirements. EN/UVLO connects to a comparator with a 1.5-V

reference voltage and 50-mV hysteresis. An external logic signal can be used to drive the EN/UVLO input to

toggle the output on and off for system sequencing or protection. The simplest way to enable the LM5181 is to

establish a precision UVLO level.

Figure 19. Programmable Input Voltage UVLO With Hysteresis

Use Equation 10 and Equation 11 to calculate the input UVLO voltages turnon and turnoff voltages, respectively.