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

MP4658 Rev. 1.0

MonolithicPower.com

12

10/13/2021

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OPERATION

The MP4658 is a 4-string, secondary-side LED

driver for LED backlighting, and is ideal for

monitor and TV LED backlighting. It controls the

flyback power stage to regulate the LED current

(ILED) loop during the PWM on time and regulate

system supply voltage loop during the PWM off

time. Powered by a 6V to 36V input voltage

(VIN)

supply,

the

MP4658

outputs

a

compensation signal to control the primary-side

flyback (or other power stage) through an

optocoupler.

Start-Up

When the flyback is on, VIN is charged until it

reaches its UVLO threshold, at which point the

system logic starts to work.

LED Current and Voltage Regulation

The MP4658 accurately regulates the ILED loop

or the output voltage (VOUT) with a single flyback

power stage.

When pulse-width modulation (PWM) dimming

is on, the MP4658 regulates ILED through the

flyback power stage control (see Figure 2).

EA_V

FB

COMP2

1.2V

EA_I

Min LEDx

COMP1

Ref

Flyback

Control

Total

Power

DC/DC

LED Stage

Min VLEDx

Figure 2: MP4658 Control Scheme when PWM

Dimming Is On

When PWM dimming is off, the LED stage is

disabled and the MP4658 regulates the output

voltage (VOUT) using the feedback (FB) voltage

(VFB) from the FB pin (see Figure 3).

EA_V

FB

COMP2

1.2V

EA_I

Min LEDx

COMP1

Ref

Flyback

Control

Total

Power

VOUT

DC/DC

VSYS

Figure 3: MP4658 Control Scheme when PWM

Dimming Is Off

LED

Current

Regulation

when

PWM

Dimming Is On

For the ILED regulation loop, the minimum LEDx

voltage (VLEDx) is fed back and compared to the

internal

reference

voltage.

This

reference

changes with the ILED setting. The internal error

amplifier (EA) regulates the average minimum

VLEDx to the internal reference voltage. The EA’s

output is connected to the external current-loop

compensation network on the COMP1 pin

through an inner switch (S1). During the PWM

on time, S1 is on and the minimum VLEDx is

regulated by this control loop. The flyback

power is set by the COMP1 voltage (VCOMP1).

During the PWM off period, S1 is turned off, the

compensation network is disconnected from the

EA’s output, and VCOMP1 remains steady until

the next PWM on interval. The

EA’s output is

also pulled low during the PWM off period.

Output

Voltage

Regulation

when

PWM

Dimming Is Off

When PWM dimming is off, the output voltage

(VOUT) is fed back to the FB pin and the internal

VREF is equal to 1.2V. During the PWM off

period,

the

MP4658

regulates

VOUT

by

controlling the flyback power stage. The internal

voltage-loop EA regulates the average VFB to its

internal

reference.

The

EA’s

output

is

connected

to

the

external

voltage-loop

compensation network on the COMP2 pin. The

flyback power is set by the COMP2 voltage

(VCOMP2).

When PWM dimming is on, the flyback power is

controlled by VCOMP1 and VOUT varies with

different dimming conditions. The voltage-loop

EA’s output is also pulled low.

Dimming Control

The MP4658 provides analog dimming and

PWM dimming for ILED through the separate

ADIM and PDIM pin. For analog dimming, apply

a pulse signal with a frequency above 5kHz to

the ADIM pin and pull the PDIM pin high. The

pulse signal’s duty cycle sets the ILED amplitude.

For PWM dimming, the duty cycle of input

signal at the ADIM pin sets the peak ILED value.

ILED is chopped by the input PWM signal at the

PDIM pin. The brightness of the LED string is

proportional to the external PWM signal

’s duty

cycle, which helps achieve fast and high-

contrast ratio PWM dimming. For analog PWM

dimming, apply a <5kHz pulse signal to the

ADIM pin and pull the PDIM pin high. The ADIM

pin’s input impedance determines the internal

analog dimming RC filter and the rising/falling

speed during PWM dimming. If the input