MAX17061AETI+T【PDF 14/24 页】IC LED DRVR WHITE BCKLGT 28-TQFN

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8-String White LED Driver with

SMBus for LCD Panel Applications

IN UVLO

The MAX17061A implements an IN UVLO function to

Table 1. Frequency Selection

safeguard the operation in the specified range. When

IN pin voltage drops below its UVLO threshold (6.98V

typ), the converter is shut off and the BL_Stat bit of the

fault/status register is cleared. The BL_CTL bit of con-

trol register is not altered. When IN pin voltage rises

OSC PIN CONNECTION

GND

Open

V CC

SWITCHING FREQUENCY (kHz)

500

750

1000

above the UVLO rising edge threshold (7.2V typ) the

backlight resumes automatically if the BL_CTL bit is set

to 1.

Startup

At startup, the MAX17061A checks the OV pin to see if

the Schottky diode is open. If the OV voltage is lower

than 84mV (typ), the boost converter does not start.

After the OV test is done, the MAX17061A performs a

diagnostic test of the LED array. The test is divided to

two phases; each phase takes approximately 1.024ms.

In the first test phase, all FB_ inputs are quickly dis-

charged down to 5.6V (typ) and then continuously dis-

charged by 800μA (typ) current sources. If a given FB_

voltage remains higher than 5.6V (typ), the string is

considered to be shorted. Otherwise, if a given FB_

voltage is higher than 3V (typ), the string is considered

to be unused. Unused strings should be tied to V CC . In

the second phase, each FB_ is precharged by an inter-

nal 400μA (typ) current source. If a given FB_ voltage

remains lower than 1V (typ), the FB_ is considered to

be a short to GND and the device is disabled. After the

LED string diagnostic phases are finished, the boost

converter starts. The total startup time is less than

10ms, including 4.2ms (typ) soft-start.

Shutdown

The MAX17061A can be placed into shutdown by

clearing bit 0 of the device control register (0x01).

When a critical failure is detected, the IC also enters

shutdown mode. In shutdown mode, all functions of the

IC are turned off. The fault/status register is set accor-

dingly in shutdown. When bit 0 of the device control

register (0x01) is recycled to 1, the MAX17061A exits

shutdown mode and starts. The fault/status register is

reset at startup.

Frequency Selection

A tri-level OSC input sets the internal oscillator frequen-

cy for step-up converter, as shown in Table 1. High-

frequency (1MHz) operation optimizes the regulator for

the smallest component size, at the expense of efficien-

cy due to increased switching losses. Low-frequency

(500kHz) operation offers the best overall efficiency but

requires larger components and PCB area.

Overvoltage Protection

To protect the step-up converter when the load is open,

or the output voltage becomes excessive for any rea-

son, the MAX17061A features a dedicated overvoltage

feedback input (OV). The OV pin is tied to the center

tap of a resistive voltage-divider from the high-voltage

output. When the OV pin voltage, V OV , exceeds

1.236V, a comparator turns off the internal power MOS-

FET. This step-up converter switch is reenabled after

the V OV drops 60mV (typ) hysteresis below the protec-

tion threshold. This overvoltage-protection feature

ensures the step-up converter fail-safe operation when

the LED strings are disconnected from the output.

LED Current Sources

Maintaining uniform LED brightness and dimming capa-

bility are critical for LCD backlight applications. The

MAX17061A is equipped with a bank of eight matched

current sources. These specialized current sources are

accurate to within ±1.5% between strings and can be

switched on and off within 15μs, enabling PWM fre-

quencies of up to 2kHz. All LED full-scale currents are

identical and are set through the ISET pin (15mA < I LED

< 30mA). When ISET is connected to V CC , the LED full-

scale current is set at the 25mA default value.

The minimum voltage drop across each current source

is approximately 625mV when the LED current is 25mA.

The low-voltage drop helps reduce dissipation while

maintaining sufficient compliance to control the LED

current within the required tolerances.

The LED current sources can be disabled by tying the

respective FB_ pin to V CC at startup. When the IC is

powered up, the controller scans settings for all FB_

pins. If a FB_ pin is not tied to V CC , an internal circuit

pulls this pin low, and the controller enables the corre-

sponding current source to regulate the string current.

If the FB_ pin is tied to V CC , the controller disables the

corresponding current regulator. The current regulator

cannot be disabled by connecting the respective FB_

pin to V CC after the IC is powered up.

All FB_ pins in use are combined to extract a lowest

FB_ voltage (LVC) (see Figure 2). LVC is fed into the

step-up converter’s error amplifier and is used to set

the output voltage.

14

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