˜

˜

SET

FB

OUT

D

PS

REF

R

R

V

V

N

V

MAG

PRI-PK(DCM)

SW(DCM)

DCM

IN

L

I

F

D

V

˜

˜

OUT

OUT

D

PRI-PK(DCM)

MAG

SW(DCM)

2 I

V

V

I

L

F

˜

˜

˜

2

MAG

PRI-PK(DCM)

OUT(DCM)

SW(DCM)

L

I

P

F

2

˜

˜

OUT

D

OUT

PRI-PK(BCM)

IN

2 V

V

I

I

V

D

˜

˜

˜

SW(BCM)

MAG

MAG

PRI-PK(BCM)

IN

PS

OUT

D

1

F

L

L

I

V

N

V

V

§

·

˜ ¨

¸

¨

¸

˜

©

¹

2

MAG

PRI-PK(BCM)

OUT(BCM)

SW(BCM)

L

I

P

F

2

˜

˜

˜

˜

OUT

D

PS

IN

OUT

D

PS

V

V

N

D

V

V

V

N

11

LM25183

www.ti.com

SNVSBJ3 – APRIL 2020

Product Folder Links: LM25183

Submit Documentation Feedback

Copyright © 2020, Texas Instruments Incorporated

Feature Description (continued)

where

•

(1)

Equation 2 gives the output power in BCM, where the applicable switching frequency and peak primary current

are specified by Equation 3 and Equation 4, respectively.

(2)

(3)

(4)

As the load decreases, the LM25183 clamps the maximum switching frequency to 350 kHz, and the converter

enters discontinuous conduction mode (DCM). The power delivered to the output in DCM is proportional to the

peak primary current squared as given by Equation 5 and Equation 6. Thus, as the load decreases, the peak

current reduces to maintain regulation at 350-kHz switching frequency.

(5)

(6)

(7)

At even lighter loads, the primary-side peak current set by the internal error amplifier decreases to a minimum

level of 0.5 A, or 20% of its 2.5-A peak value, and the MOSFET off-time extends to maintain the output load

requirement. The system operates in frequency foldback mode (FFM), and the switching frequency decreases as

the load current is reduced. Other than a fault condition, the lowest frequency of operation of the LM25183 is 12

kHz, which sets a minimum load requirement of approximately 0.5% full load.

7.3.3 Setting the Output Voltage

To minimize output voltage regulation error, the LM25183 senses the reflected secondary voltage when the

secondary current reaches zero. The feedback (FB) resistor, which is connected between SW and FB is

(8)

7.3.3.1 Diode Thermal Compensation

The LM25183 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.