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LT3754 Datasheet(PDF) 21 Page - Linear Technology

Part No. LT3754
Description  16-Channel × 50mA LED Driver
Download  28 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT3754 Datasheet(HTML) 21 Page - Linear Technology

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LT3754
21
3754f
APPLICATIONS INFORMATION
LED Open Circuit and PWM Dimming Ratios
The LT3754 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 VOUT is low, the LT3754 ignores low LED
pin voltages until VOUT reaches 90% of its maximum
allowed OVP level. Once this condition is met, the LT3754
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 LT3754 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 (VOUT) 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 (VOUT-VLEDx) < 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 datasheet which cover a
range of VIN, VOUT, fOSC, output power and inductor current
ripple values. For application requirements which deviate
from the circuits shown in the datasheet be sure to check
the stability of the final application over the full VIN range,
LED current range (if analog dimming) and temperature
range. Be aware that if the VC pin components represent
the dominant pole for the converter loop and they have
been adjusted to achieve stability, the VC pin might move
more slowly during load transient conditions such as an
all-LEDs-open fault. A slower moving VCpinwilladdtoVOUT
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 VC to move more
quickly and VOUT to move more slowly resulting in less
overshoot during an all-LEDs-open fault.
Thermal Considerations
The internal power dissipation of the LT3754 comes from 3
main sources: VIN quiescent current (IQ total), VIN current
for GATE switching (IGATE) and the LT3754 LED current
sources. Since the maximum operational VIN voltage is
40V, care should be taken when selecting the switching
frequency and type of external power MOSFET since the
current required from VIN for GATE switching is given by,
IGATE = fOSC • Qg
where Qg is the gate charge (at VGS = INTVCC) specified
for the MOSFET and fOSC is the programmed switching
frequency for the LT3754. A low Qg MOSFET should
always be used when operating the LT3754 from high VIN
voltages. The internal junction temperature of the LT3754
can be estimated as:
TJ=TA+[VIN•(IQTOTAL+(fOSC•Qg))+(16•I(LEDX)•1.1V)]
θJA
where, TA is the ambient temperature for the LT3754
IQTOTAL represents the VIN quiescent current for the LT3754
(not switching, PWM = 1.5V and CTRL = 0.1V) - illustrated
in the Typical Characteristics Graphs – plus the base
currents of active channels (typically 16 • I(LED)/75).
θJA
is the thermal resistance of the package (34°C/W for the
5mm
× 5mm QFN package).


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