MP4658 – 4-STRING, 80V RETURN LED DRIVER WITH AC/DC FEEDBACK

MP4658 Rev. 1.0

MonolithicPower.com

14

10/13/2021

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2021 MPS. All Rights Reserved.

APPLICATION INFORMATION

Open LED Protection (OVP)

The OVP pin monitors the LED output voltage

and controls open LED protection. Connect this

pin to the LED output voltage through a voltage

divider, and set the OVP point using Equation

(1):

OVPH

OVPL

OV_LED

OVPL

2.4V (R

+R

)

V=

R



(1)

Where ROVPH and ROVPL are the high-side and

low-side

resistors

of

the

voltage

divider,

respectively.

Output Voltage Feedback (FB)

The FB pin feeds back the output voltage (VOUT).

Connect this pin to the output voltage through a

voltage divider. This voltage divider determines

VOUT, which can be calculated with Equation (2):

VFBH

VFBL

OUT

VFBL

1.2V (R

+R

)

V=

R



(2)

Where RVFBH and RVFBL are the high-side and

low-side

resistors

of

the

voltage

divider,

respectively.

LED Current Setting

The LED current (ILED) is set by the resistor on

the ISET pin. The ISET pin outputs a 1.6V

voltage and its sourcing current determines ILED.

Estimate ILED with Equation (3):

LED

ISET

80k

Ω×200mA

I

(mA)=

R

(k

Ω)

(3)

Selecting the Turn Ratios of the Power

Transformer

The power transformer includes three power

windings: the primary-side winding, the LED

secondary-side winding and the output voltage

winding. Follow these steps to design the turn

ratios of the power transformer:

Design the turn ratio between the primary-side

winding and LED secondary-side winding using

NP:NLED. The maximum voltage stress on the

primary-side switch occurs at the maximum

input AC voltage and maximum output LED

voltage.

Calculate the maximum voltage stress on the

primary-side switch (VDS_PRI) with Equation (4):

P

DS_PRI

IN_AC_MAX

OV_LED

DIODE

LED

N

V

=1.414 V

+(V

+V

)

N



(4)

Considering the leakage inductance of the

flyback transformer, assume a 60V spike

voltage on the primary-side switch and a 10%

derating

of

the

switch

voltage

capability.

Calculate

the

primary-side

switch

’s rating

voltage with Equation (5):

DS_PRI

RATING_PRI

V

+ 60 0.9 V



(5)

Calculate the turn ratio (NP:NLED) between the

primary-side winding (NP) and the secondary

LED winding (NLED) with Equation (6):

RATING_PRI

IN_AC_MAX

P

LED

OV_LED

DIODE

(0.9 V

-60V-1.414 V

)

N :N

=

V

+V



(6)

Where VRATING_PRI is the rating voltage of the

primary-side switch, VIN_AC_MAX is the maximum

input AC voltage, VDIODE is the forward voltage

of the rectifier diode, and VOV_LED is the OVP

point of the LED output voltage.

Design the turn ratio between the LED winding

and the output voltage winding: NLED:NSYS.

When the LED stage is working, the LED

winding voltage (VLED) clamps the system

winding voltage via the turn ratio. Ensure that

VOUT exceeds the voltage set with Equation (2)

when the LED stage is on with the minimum

VLED. Estimate this relationship with Equation

(7):

LED_MIN

DIODE

OUT

DIODE

LED

SYS

VV

V

+ V

N

N

+



(7)

Where VLED_MIN is the minimum LED output

voltage. Choose a 10% margin for production

design, calculated with Equation (8):

LED_MIN

DIODE

LED

SYS

OUT

DIODE

0.9 (V

V

)

N

: N

VV

+



+

(8)