MAX1985ETP datasheet(13/17 Pages) MAXIM

13 / 17 page

capacitor can be connected from BITB to ground to

increase the total time constant. Use the following

equation to calculate the total time constant:

τ = 0.2MΩ ✕ CEXT + 0.098ms

where CEXT 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 CEXT and ensure the

voltage on CEXT (BITB), including ripple, remains

above DFMIN ✕ 0.75 ✕ VREF, where DFMIN 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

control is from 140mV (15%) to 0.75 ✕ VREF (100%).

The LED current is given by the following equation:

where K1 = 0.0465, K2 = 0.953, VBITB is the voltage at

the BITB pin, VREF is the 1.25V internal reference voltage,

and ILED(FS) is the full-scale LED current set by SETI.

In analog mode, the MAX1984/MAX1985/MAX1986

enter shutdown mode when both VBITA and VBITB 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.

LED

BITB

REF

LED FS









075

()

Ultra-High-Efficiency White

LED Drivers

______________________________________________________________________________________

VIN

Ω

7.2M

Ω

DAC

DECODER

0.75 x VREF

3-BIT

LEVEL

SHIFT

10pF

200k

Ω

9.8M

Ω

MODE

BITA

BITB

BITC

SHUTDOWN

THRESHOLD

SHUTDOWN

TO CURRENT

REGULATORS

IREF

SETI

0.045V

0.5mA TEST

200k

Ω

300k

Ω

RSETI

SETI

VIN - 0.7V

18mA DEFAULT

100

Figure 5. Brightness Control Equivalent Functional Diagram

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