MP24183 Datasheet, PDF (8/12 Pages) Monolithic Power Systems – 1A, 55V, Programmable Frequency White LED Driver

MP24183 – 1A, 55V, PROGRAMABLE FREQUENCY WHITE LED DRIVER
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
MP24183 Rev. 1.0
www.MonolithicPower.com
8
8/28/2008
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