MP2481 –36V, 1.2A, 1.4MHz WHITE LED DRIVER

MP2481 Rev. 1.42

www.MonolithicPower.com

8

4/28/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

The MP2481 is a current mode regulator. The EA

output voltage is proportional to the peak inductor

current.

At the beginning of a cycle, M1 is off. The EA

output voltage is higher than the current sense

amplifier output, and the current comparator’s

output is low. The rising edge of the 1.4MHz CLK

signal sets the RS Flip-Flop. Its output turns on

M1 thus connecting the SW pin and inductor to

the input supply.

The increasing inductor current is sensed and

amplified by the Current Sense Amplifier. Ramp

compensation is summed to the Current Sense

Amplifier output and compared to the Error

Amplifier output by the PWM Comparator. When

the sum of the Current Sense Amplifier output

and the Slope Compensation signal exceeds the

EA output voltage, the RS Flip-Flop is reset and

M1 is turned off. The external Schottky rectifier

diode (D1) conducts the inductor current.

If the sum of the Current Sense Amplifier output

and the Slope Compensation signal does not

exceed the EA output for a whole cycle, then the

falling edge of the CLK resets the Flip-Flop.

The output of the Error Amplifier integrates the

voltage difference between the feedback and the

0.2V reference. The polarity is such that a FB pin

voltage lower than 0.2V increases the EA output

voltage. Since the EA output voltage is

proportional to the peak inductor current, an

increase in its voltage also increases current

delivered to the output.

Open Strings Protection

If LED strings are open, there is no voltage on

the FB pin. The duty cycle increases until V(VDD)

- V(VSS) reaches to 38V or V(OVP) - V(VSS)

reaches the shutdown threshold 1.23V. Then the

top switch is kept off. It will be switching again

when the voltage V(VDD) - V(VSS) and V(OVP) -

V(VSS) decreases sufficiently.

Dimming Control

The MP2481 allows both DC and PWM dimming.

When V(EN/DIM) – V(INGND) is less than 0.4V,

the chip is turned off. For analog dimming, when

V(EN/DIM) – V(INGND) rises from 0.7V to 1.4V,

the LED current will change from 0% to 100% of

the maximum LED current. If V(EN/DIM) –

V(INGND) is higher than 1.4V, maximum LED

current is generated. If a PWM signal is used, its

amplitude V (EN/DIM) – V (INGND) must exceed

1.4V.

The MP2481 is designed with an internal EN

shut-down delay (see Figure 2). If EN is pulled

down,

the

FB

reference

drops

to

zero

immediately and the LED current decreases. But

the IC keeps internal logic on for about 2.5ms. In

this period, the IC may switch with very short

pulses in pulse-skipping mode, while the LED

current is zero since the FB reference is zero. It

has benefit that the IC will start up more quickly

without delay when next PWM duty comes. This

function offers the possibility to extend the PWM

dimming frequency and also the minimum PWM

on time for further dimming depth.

Figure 2— Internal EN Shut-down Delay Function Waveform Diagram