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Ultra-High-Efficiency White
LED Drivers
capacitor can be connected from BITB to ground to
increase the total time constant. Use the following
control is from 140mV (15%) to 0.75 ? V REF (100%).
The LED current is given by the following equation:
I LED = ? K 1 + K 2 ×
equation to calculate the total time constant:
τ = 0.2M ? ? C EXT + 0.098ms
?
?
V BITB ?
0 . 75 × V REF ? ?
× I LED ( FS )
where K1 = 0.0465, K2 = 0.953, V BITB is the voltage at
where C EXT is the external capacitance.
The recommended DPWM duty-factor range is from
20% to 100% for DPWM frequencies between 20kHz
and 2MHz, using the internal 0.1ms filter. For lower
DPWM operating frequencies, use C EXT and ensure the
voltage on C EXT (BITB), including ripple, remains
above DF MIN ? 0.75 ? V REF , where DF MIN is the mini-
mum reliable DPWM duty factor of 15%.
In DPWM mode, the MAX1984/MAX1985/MAX1986
enter shutdown mode when the DPWM duty cycle is
below 5% (typ) and BITC is a logical low level.
Analog Control
To use the analog control mode, connect MODE, BITA,
and BITC to IN. Connect BITB to a DC voltage that sets
the LED current. The operational range for the analog
the BITB pin, V REF is the 1.25V internal reference voltage,
and I LED(FS) is the full-scale LED current set by SETI.
In analog mode, the MAX1984/MAX1985/MAX1986
enter shutdown mode when both V BITA and V BITB are
logic low.
Parallel Control
The MAX1984/MAX1985/MAX1986 also support 2-bit or
3-bit parallel control. To use the 3-bit parallel control
mode, connect MODE to ground. BITA is the most sig-
nificant bit and BITC is the least significant bit. To use
the 2-bit parallel control, connect MODE to IN and BITC
to ground. BITA is the most significant bit and BITB is
the least significant bit. In parallel mode, the
MAX1984/MAX1985/MAX1986 enter shutdown mode
when BITA, BITB, and BITC are logic low. Tables 3 and
4 are the truth tables.
SHUTDOWN
MODE
0.75 x V REF
THRESHOLD
V IN
SHUTDOWN
BITA
BITB
DECODER
DAC
1
100
BITC
0.75 x V REF
3-BIT
IREF
LEVEL
SHIFT
200k ?
9.8M ?
10pF
38 ?
TO CURRENT
REGULATORS
SETI
SETI
18mA DEFAULT
SETI
0.5mA TEST
R SETI
300k ?
V IN - 0.7V
0.045V
Figure 5. Brightness Control Equivalent Functional Diagram
200k ?
7.2M ?
______________________________________________________________________________________
13
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