MAX17127 Maxim Integrated Products, MAX17127 Datasheet - Page 15

• Manufacturer Part Number: MAX17127
• Description: Six-String WLED Driver
• Manufacturer: Maxim Integrated Products

To protect the step-up regulator when the load is open,
or if the output voltage becomes excessive for any rea-
son, 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
exceeds 1.25V (typ), a comparator turns off the internal
power MOSFET. This step-up regulator switch is reen-
abled after the V
the protection threshold. This overvoltage-protection
feature ensures the step-up regulator fail-safe operation
when the LED strings are disconnected from the output.
Maintaining uniform LED brightness and dimming capa-
bility is critical for backlight applications. The MAX17127
is equipped with a bank of six matched current sources.
These specialized current sources are accurate within
P 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
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 connect-
ing 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 inter-
nal 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 con-
troller 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.
Table 3. FPO Function Table
LED
FPO PIN STATE
LATCHED
< 30mA).
OVP
______________________________________________________________________________________
drops 90mV (typ hysteresis) below
Six-String WLED Driver with Integrated
Overvoltage Protection
THERMAL FAULT
LED Current Sources
Low
Yes
OVP
OUTPUT OVERVOLTAGE
,
No (stop switching)
FAULT CONDITION
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. The LED open/short detection is
not executed when LED on-time is less than 2Fs.
The MAX17127 can tolerate a 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 regula-
tion 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 protec-
tion is disabled during the soft-start phase of the step-up
regulator.
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.
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.
The MAX17127 performs brightness control with a PWM
input signal. Dimming duty cycle and frequency of cur-
rent sources follow the signal at the PWM pin directly.
Low
Step-Up Converter
Current-Source Fault Protection
Open Current-Source Protection
INPUT OVERCURRENT
Yes (after time expires)
Dimming Control
Low
FPO Function
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