Rainbow Electronics MAX17127 - Manual (Page 15)

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MAX17127
Six-String WLED Driver with Integrated
Step-Up Converter
Overvoltage Protection
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
OVP ,
exceeds 1.25V (typ), a comparator turns off the internal
power MOSFET. This step-up regulator switch is reen-
abled after the V
OVP drops 90mV (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.
LED Current Sources
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
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 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.
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. 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.
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 cur-
rent 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