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8-String WLED Driver with Integrated Step-Up
Regulator and SMBus/PWM Dimming Capability
In standby or fault shutdown, the FAULT pin is pulled
up to IN with an internal resistor of 1M I (typ), and the
p-channel MOSFET is turned off. An external resistor
could be used as well (R EXT > 200k I ).
Frequency Selection
The step-up regulator switching frequency can be
adjusted through the external resistor-connected OSC
pin. The switching frequency adjustable range is 500kHz
to 2MHz. High-frequency (2MHz) operation optimizes
the regulator for the smallest component size, at the
expense of efficiency due to increased switching losses.
Low-frequency (500kHz) operation offers the best over-
all efficiency, but requires larger components and PCB
area. The maximum step-up regulator duty is limited by
the 50ns (typ) minimum on-time.
Overvoltage Protection
To protect the step-up regulator when the load is open,
or the output voltage becomes excessive for any rea-
son, the MAX17105 features a dedicated overvoltage
feedback input (OVP). The OVP pin is connected to the
center tap of a resistive voltage-divider from the high-
voltage output. The OVP pin has two levels of threshold.
When the OVP pin voltage, V OVP , exceeds 1.25V (typ),
a comparator turns off the internal power MOSFET and
prevents excessive voltage from damage. This step-
up regulator switch is reenabled after the V OVP drops
60mV (typ) hysteresis below the protection threshold.
This overvoltage-protection feature ensures the step-up
regulator fail-safe operation when the LED strings are
disconnected from the output. When the OVP pin voltage
exceeds 1.35V (typ), the IC is in fault shutdown.
LED Current Sources
Maintaining uniform LED brightness and dimming capa-
bility are critical for backlight applications. The MAX17105
is equipped with a bank of eight matched current sourc-
es. These specialized current sources are accurate to
within Q 2%, and can be switched on- and off-PWM fre-
quencies of up to 30kHz in direct-PWM mode and 5kHz in
SMBus mode. All LED full-scale currents are identical and
are set through the ISET pin (0mA < I LED < 30mA). The
LED current source has high-speed capability and allows
400ns minimum on-time within 400ns rise and fall time.
The minimum voltage drop across each current source
is 500mV (max) when the LED current is 20mA. 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 connecting
the respective FB_ pin to SGND at startup. When the IC
is enabled, the controller scans settings for all FB_ pins.
If a FB_ pin is not connected to SGND, an internal circuit
pulls this pin high, and the controller enables the cor-
responding current source to regulate the string current.
If the FB_ pin is connected to SGND, the controller dis-
ables the corresponding current regulator. The current
regulator cannot be disabled by connecting the respec-
tive FB_ pin to SGND after the IC is enabled.
All FB_ pins in use are combined to extract a lowest FB_
voltage (LVC). (See Figure 2.) LVC is fed into the step-up
regulator’s error amplifier and is used to set the output
voltage.
Current Source Fault Protection
LED fault open/short is detected after startup. When one
or more strings fail after startup, the corresponding cur-
rent source is disabled. The remaining LED strings are
still operated normally. When a fault is detected, bit 4
or/and bit 5 of the Fault/Status resister are set in SMBus
mode. (See the Fault Status Register description in the
Dimming Control Register Descriptions section.)
LED Short and String Mismatch Protection
The MAX17105 can tolerate slight mismatch between
LED strings. When severe mismatches or WLED shorts
occur, the FB_ voltages are uneven because of mis-
matched voltage drops across strings. At each LED turn-
on, the FB_ voltage is brought down to the regulation
voltage quickly. When FB_ voltage is higher than 8V (typ)
after the LED turns on, the LED short is detected. When
the LED short condition is continued for 2ms, the strings
are disabled. The remaining LED strings can still operate
normally. If only one string is used, the output voltage
decreases and regulates the needed current during
the operation of LED short. The LED short protection is
not triggered since FB_ voltage is never higher than the
threshold. The LED short protection is disabled during
the soft-start phase of the step-up regulator.
Open-Current Source Protection
The MAX17105 step-up regulator output voltage is regu-
lated according to the minimum FB_ voltages on all of
the strings in use. If one or more strings are open, the
respective FB_ pins are pulled to ground. For any FB_
lower than 220mV, the corresponding current source is
disabled. The remaining LED strings can still operate
normally. If all strings in use are open, the MAX17105
shuts the step-up regulator down.
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