MAX17061A
IN UVLO
The MAX17061A implements an IN UVLO function to
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
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 VCC. 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, VOV, exceeds
1.236V, a comparator turns off the internal power MOS-
FET. This step-up converter switch is reenabled after
the VOV 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 < ILED
< 30mA). When ISET is connected to VCC, 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 VCC at startup. When the IC is
powered up, the controller scans settings for all FB_
pins. If a FB_ pin is not tied to VCC, 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 VCC, the controller disables the
corresponding current regulator. The current regulator
cannot be disabled by connecting the respective FB_
pin to VCC 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.
8-String White LED Driver with
SMBus for LCD Panel Applications
14 ______________________________________________________________________________________
Table 1. Frequency Selection