MAX1984ETP 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1984ETP

Ultra-High-Efficiency White LED Drivers

Maxim Integrated 

Ultra-High-Efficiency White

LED Drivers

THRESHOLD

EN_

LD_

CURRENT

SENSE

IREF

SLOPE

OC COMP

COMP

OSCILLATOR

SUMMING

COMPARATOR

OSC

LOGIC

GATE

DRIVERS

1Ω

LDG

FB

100mV

OUT

OV COMP

5.3V

Figure 3. Current Regulator Functional Diagram

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

Figure 4. Step-Up Regulator Functional Diagram

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:

ILED = D ✕ ILED(FS)

where ILED(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

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