MP2481 –36V, 1.2A, 1.4MHz WHITE LED DRIVER
MP2481 Rev. 1.43 www.MonolithicPower.com 8
7/10/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 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
