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LT3760
APPLICATIONS INFORMATION
For OVP = 49.2V, OVP SET =
= 0.863V
ProgrammingOvervoltageProtection(OVP)Level
The LT3760 LED driver provides optimum protection to
the LEDs and the external MOSFET by providing a pro-
grammable maximum regulated output voltage limit using
the OVP SET pin. The Overvoltage Protection (OVP) level
is programmed as:
OVP(MAXIMUM REGULATED V OUT ) = 57 ? OVP SET
If every LED string fails open or the multiple string LED
display becomes disconnected the LT3760 LED driver loop
regulates to the programmed OVP level. The OVP level
should be programmed to a level high enough to regulate
the LED strings but low enough to prevent damage to the
power switch and to minimize the voltage across the LED
pins upon reconnection of the LED strings. Recommended
OVP level is given by:
OVP(RECOMMENDED ) = 1.2 ? (( N ? V F ) + 1V)
where:
N = number of LEDs in each string,
V F = maximum LED forward voltage drop
and the scaling factor of 1.2 accounts for variation in the
generation of OVP from OVP SET pin voltage and startup
logic requirements.
Example: For a converter operating with 10 LEDs per string
at a maximum forward voltage of 4V per LED, the OVP
level should be programmed to:
OVP(RECOMMENDED) = 1.2 ? ( (10 ? 4) + 1V ) = 49.2V
49.2
57
The OVP SET pin voltage can be generated using a resistor
divider from the REF pin.
LED Open Circuit and PWM Dimming Ratios
The LT3760 monitors each LED pin voltage to determine if
the LED string has an open fault (LED pin voltage < 0.5V).
If an open LED fault occurs, the FAULT flag is pulled low.
To avoid false detection of faults during the initial converter
startup when V OUT is low, the LT3760 ignores low LED
pin voltages until V OUT reaches 90% of its maximum al-
lowed OVP level. Once this condition is met, the LT3760
monitors all LED pins for open LED faults. To avoid false
detection of faults during PWM dimming edges (where
LED pins can possibly ring and trip fault detection levels)
the LT3760 only monitors/updates fault conditions during
PWM high (and only after a blank duration of 2μs following
each PWM rising edge).
LED Short Circuit
A short circuit fault between the positive terminal of an
LED string (V OUT ) and the negative terminal of the LED
string (LEDx pin) causes the channel to be disabled in
order to protect the internal current source. A resistive
short is allowed as long as (V OUT -V LEDx ) < 6V.
Loop Compensation
Be sure to check the stability of the loop with the LEDs
connected (LED regulation loop) and disconnected
(Overvoltage Protection (OVP) regulation loop). Various
application circuits are shown in the data sheet which
cover a range of V IN , V OUT , f OSC , output power and inductor
current ripple values. For application requirements which
deviate from the circuits shown in the data sheet be sure
to check the stability of the final application over the full
V IN range, LED current range (if analog dimming) and
temperature range. Be aware that if the V C pin components
represent a dominant pole for the converter loop and they
have been adjusted to achieve stability, the V C pin might
move more slowly during load transient conditions such
as an all-LEDs-open fault. A slower moving V C pin will
add to V OUT overshoot during an all-LEDs-open fault.
An alternative compensation approach is to place the
dominant pole of the converter loop at the output. This
requires an increased output capacitor value but will allow
a much reduced Vc capacitor. The combination will allow
V C to move more quickly and V OUT to move more slowly
resulting in less overshoot during an all-LEDs-open fault.
3760fc
20
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