Datasheet

Overvoltage Protection
To protect the step-up regulator when the load is open,
or if the output voltage becomes excessive for any
reason, the MAX17127 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. When the OVP pin voltage (V
OVP
)
exceeds 1.25V (typ), a comparator turns off the
internal power MOSFET. This step-up regulator switch is
reenabled after the V
OVP
drops 90mV (typ hyster-
esis) 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.
LED Current Sources
Maintaining uniform LED brightness and dimming
capability is critical for backlight applications. The
MAX17127 is equipped with a bank of six matched
current sources. These specialized current sources are
accurate within ≤ 3% and match each other within 2%.
They can be switched on and off at PWM frequencies
of up to 25kHz. LED full-scale current is set through the
ISET pin (10mA < I
LED
< 30mA).
The minimum voltage drop across each current source
is 480mV (typ) 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 AGND at startup. When the
IC is enabled, the controller scans settings for all FB_
pins. If an FB_ pin is not connected to AGND, an internal
circuit pulls this pin high, and the controller enables the
corresponding current source to regulate the string
current. If the FB_ pin is connected to AGND, the
controller disables the corresponding current regulator.
The current regulator cannot be disabled by connecting
the respective FB_ pin to AGND 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
current source is disabled. The remaining LED strings are
still operated normally. The LED open/short detection is
not executed when LED on-time is less than 2µs.
The MAX17127 can tolerate a slight mismatch between
LED strings. When severe mismatches or WLED
shorts occur, the FB_ voltages are uneven because of
mismatched 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 LED turn-on, an LED short is detected if at least one
FB_ voltage is lower than the minimum FB_ regulation
voltage, +460mV (typ). The remaining LED strings can
still operate normally. The LED short protection is dis-
abled during the soft-start phase of the step-up regulator.
Open Current-Source Protection
The MAX17127 step-up regulator output voltage is
regulated according to the minimum FB_ voltages on all
the strings in use. If one or more strings are open, the
respective FB_ pins are pulled to ground. For any FB_
lower than 180mV, the corresponding current source is
disabled. The remaining LED strings can still operate
normally. If all strings in use are open, the MAX17127
shuts the step-up regulator down.
FPO Function
The fault conditions trigger FPO function and pull the
FPO pin low. Table 3 shows the state of the FPO pin with
different fault conditions.
Dimming Control
The MAX17127 performs brightness control with a PWM
input signal. Dimming duty cycle and frequency of current
sources follow the signal at the PWM pin directly.
Table 3. FPO Function Table
FAULT CONDITION
THERMAL FAULT OUTPUT OVERVOLTAGE INPUT OVERCURRENT
LATCHED Yes No (stop switching) Yes (after time expires)
FPO PIN STATE Low Low Low
www.maximintegrated.com
Maxim Integrated │ 15
MAX17127 Six-String WLED Driver with
Integrated Step-Up Converter