MP4655–PURE, SINGLE-STAGE, LLC, LED CURRENT AND SYSTEM VOLTAGE CONTROLLER

MP4655 Rev 1.0

www.MonolithicPower.com

25

2/2/2016

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

© 2016 MPS. All Rights Reserved.

Over-Current

Protection

for

the

System

Output Stage (VOCP)

VOCP implements an over-current protection for

the system output voltage stage. The current of

the system output voltage stage is sensed on

VOCP with a negative polarity. When the voltage

on VOCP is lower than -203mV, the IC triggers

the system output voltage stage protection.

Calculate

the

over-bus

current

protection

threshold with Equation (15):

=

OCP _ Bus

VOCP

203mV

I

R

(15)

Typically, the protection point is around 1.5 to 3

times the normal current of the system output

voltage stage.

Over-Current Protection for the LED Driver

Stage (IOCP/FAULT)

IOCP detects the current through the LED stage

with a negative polarity. When the voltage on

IOCP falls below -320mV, the IC triggers LED

driver stage protection. Calculate the over-LED

current protection threshold with Equation (16):

=

OCP _ LED

IOCP

320mV

I

R

(16)

The over-current protection point for the LED

stage can be set at around 1.5 to 2 times the

total current through the LED strings.

ICOP also functions as the fault indicator for the

system. When either LED driver stage protection,

system bus voltage protection, or capacitive

mode protection are triggered, the fault signal

output is high. Place a 10kΩ resistor between

IOCP and the LED stage current sense point.

Over-LED

Voltage

Protection

and

LED

Voltage Difference Protection (VLED1, VLED2)

VLED1 and VLED2 sense the LED voltages and

function as the over-LED voltage protection. The

voltage divider sets the over-voltage protection

point with Equation (17):

+

=×

OVPH

OVPL

OVP

OVPL

RR

V2.41V

R

(17)

Normally, the OVP point is set about 10% - 30%

higher than the maximum LED voltage.

The MP4655 also implements protection when

the LED string voltages are different from each

other to protect the condition in which several

LEDs in a string are shorted. This protection is

used only for multiple-string applications. The

protection point of the voltage difference between

the LED strings is set with Equation (18):

+

+

Δ=

×

×

×

input

OVPH

OVPL

pro

OVPL

20k R

RR

V2.41V

R16 20k

(18)

VLED2). Adjust the input resistance to program

the protection point.

divider and VLED1 or VLED2 to adjust the input

resistance. See Equation (19):

OVPH

OVPL

input

X

OVPL

OVPH

RR

RR

RR

×

=+

+

(19)

Capacitive

Mode

Protection

(CMODE,

FTH_CMODE)

The MP4655 implements individual capacitive

mode protection for the LLC power stage from

the secondary side. CMODE monitors the

secondary side winding voltage and functions as

the capacitive mode protection. The winding

voltage polarity is positive (high) when GL is on.

When capacitive mode is detected, the IC

decreases the SS voltage and increases the

operating frequency to attempt to move the

power stage to inductive mode. FTH_CMODE

sets the threshold frequency to trigger capacitive

mode protection. If the operating frequency is

higher than that set by FTH_CMODE and

capacitive mode is still detected, the IC triggers

capacitive mode protection. Connect a resistor

on FTH_CMODE to set the threshold frequency.

See Equation (20):

=

9

th_Cmode

Fth _ Cmode

1.2V

f(

) *11.4 *10 (Hz)

R

(20)

PWM Dimming Input (PWM)

PWM is for the PWM dimming input. Apply a

PWM dimming signal with a frequency between

100Hz to 2kHz on PWM. The PWM dimming has

positive polarity.

PWM Dimming Signal Output (DIMO)

DIMO outputs a PWM dimming signal to drive the

external dimming N-channel MOSFET in series

with the LED string and achieves fast PWM

dimming. Connect a resistor in series with DIMO

to adjust the driving speed.