MAX1984/MAX1985/MAX1986
for an LED’s presence. The current regulators are
enabled and any regulator with an output voltage less
than 45mV is detected and is ignored, preventing out-
puts left open or shorted to ground from dominating the
step-up regulation loop. Outputs shorted to IN, OUT, or
any voltage above 45mV resemble valid LEDs and are
regulated at the current set point.
As the LEDs draw current, the step-up regulator’s out-
put voltage gradually falls and the voltage drop across
each of the current regulators reduces. Eventually, the
voltage drop across whichever current regulator drives
the LED with the highest forward voltage reaches the
step-up regulator’s threshold (100mV, typ) and step-up
switching starts again (see the Startup Waveform in the
Typical Operating Characteristics).
Step-Up Regulator
The step-up regulator employs a fixed-frequency cur-
rent-mode control method to generate the bias voltage
for the white LEDs. The regulator takes the minimum
value of all the LD_ pin voltages as the feedback signal
to ensure that the output voltage is high enough to
drive all the LEDs. The heart of the controller is a multi-
input, open-loop comparator that sums three signals:
the feedback error signal with respect to the 100mV ref-
erence, the current-sense signal, and the slope com-
pensation ramp (Figure 4).
In normal operation, the controller starts a new cycle by
turning on the N-channel MOSFET and turning off the
P-channel MOSFET on the rising edge of the internal
oscillator if all of the following three conditions are satis-
fied: the summing comparator output is low, the switch
current does not exceed the overcurrent threshold, and
the output voltage does not exceed the overvoltage
threshold. The controller turns off the N-channel
MOSFET and turns on the P-channel MOSFET when
one of the following three conditions occurs: the sum-
ming comparator output becomes high, the switch cur-
rent exceeds the overcurrent threshold, or the falling
edge of the oscillator occurs.
Both the N-channel MOSFET and the P-channel
MOSFET turn off if the output voltage exceeds the over-
voltage rising threshold. Both switches stay off until all
of the following three conditions are satisfied: the out-
put voltage is below the overvoltage falling threshold,
the summing comparator output is low, and the next ris-
ing edge of the oscillator occurs.
Brightness Control Interface
The light intensity of the white LEDs can be easily
adjusted from 15% to 100% of the full-scale LED current
chosen by SETI. The MAX1984/MAX1985/MAX1986
support DPWM control, analog control, and 2-bit or 3-bit
parallel control.
DPWM Control
To use the DPWM control mode, connect MODE and
BITC to IN, leave BITB unconnected, and connect the
DPWM signal to BITA. The LED current is given by the
following equation:
I
LED
= D ✕ I
LED(FS)
where I
LED(FS)
is the full-scale LED current set by SETI,
and D is the duty cycle of the DPWM signal. The aver-
age voltage of the DPWM signal is obtained through an
internal RC filter (Figure 5). The 0.1ms filter time con-
stant allows the use of DPWM frequencies from 10kHz
to 2MHz. If lower frequencies are preferred, an external
Ultra-High-Efficiency White
LED Drivers
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