MP24183 – 1A, 55V, PROGRAMABLE FREQUENCY WHITE LED DRIVER

MP24183 Rev. 1.0

www.MonolithicPower.com

8

8/28/2008

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OPERATION

The MP24183 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 1.35MHz

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 0.198V reference. The polarity is a FB pin

voltage lower than 0.198V increases the EA

output voltage. Since the EA output voltage is

proportional to the peak inductor current, an

increase in its voltage also increases the

current delivered to the output.

Open LED Protection

If the LED is open, there is no voltage on the

FB pin. The duty cycle will increase until

V(VDD)-V(VSS) reaches 55V or OVP-VSS

reaches the shutdown threshold set by the

external resistor divider. The top switch will turn

off till the voltage V(VDD)-V(VSS) and OVP-

VSS decreases sufficiently.

Dimming Control

The MP24183 allows both DC and PWM

dimming. When the voltage on EN is less than

0.6V, the chip is turned off. For analog

dimming, when the voltage on EN is from 0.7V

to 1.4V, the LED current will change from 0% to

100% of the maximum LED current. If the

voltage on EN pin is higher than 1.4V,

maximum LED current will be generated. For

PWM dimming, its amplitude (VDIM – VINGND)

must exceed 1.4V. The PWM frequency is

recommended in range of 100Hz to 2kHz to get

a good dimming linearity.

Output Short Circuit Protection

The MP24183 integrates output short circuit

protection. When the output is shorted to VSS,

the voltage on OVP pin which detects the

output voltage gets smaller than 0.4V, and FB

pin senses no voltage (<0.1V) as no current

goes through the WLED. At this condition, the

operating frequency is folded back to decrease

the power consumption.

In Buck-boost application, when there is

possibility that the LED+ short circuit to VSS, it

is recommended to add a diode from VSS to

INGND to protect the IC, as shown in below

Figure 2 .

Figure 2—Buck-boost Application When

LED+ Possibly Short to VSS