Demonstration circuit DC2857A is a 36V 2A synchronous
2MHz boost LED driver featuring the LT3922-1. It drives
a single string of LEDs at 330mA up to 34V when VIN is
between 7V and 28V. It runs down to 4VIN with reduced
ILED and can withstand VIN as high as 36V, but has overvoltage
lockout (OVLO) set for 28V for this application.
DC2857A runs at 2MHz switching frequency. Spread
spectrum frequency modulation (SSFM) can be turned
on with a simple jumper, reducing EMI. DC2857A comes
with low EMI features including optimized layout, SSFM
and input EMI filter. It passes CISPR 25 class 5 conducted
and radiated EMI. It is protected against both open and
short LED conditions and reports the faults.
The LT3922-1 has an input voltage range from 2.8V to
36V. Its internal synchronous 2A 40V switches allow up
to 34V of LEDs on the output with room for over voltage
protection and overshoot during an open LED event. It
has adjustable switching frequency between 200kHz and
2.5MHz. It can be synchronized to an external source or
programmed with SSFM for low EMI. The PWMTG highside
PWM MOSFET driver assists with short-circuit protection
and versatility. LT3922-1 can be configured as a
boost, boost-buck, or buck mode LED driver and maintain
all of its low EMI, PWM dimming, and fault diagnostic
features.
The LT3922-1 can be PWM dimmed with an external PWM
signal or an internally-generated PWM signal. DC2857A
has a jumper that can be set to switch between internallygenerated
PWM signal, externally-generated PWM signal,
and no PWM signal (100% on). It can be analog dimmed
with a control voltage on its control pin (CTRL). When run with both PWM dimming and spread spectrum,
the spread spectrum aligns itself with the PWM signal for
flicker-free operation.
Small ceramic input and output capacitors are used to
save space and cost. The board is designed with tiny, high
frequency capacitors on both sides of the VOUT pins for
a reduction in radiated EMI. The open LED overvoltage
protection uses the IC’s constant voltage regulation loop
to regulate the output to approximately 37.5V if the LED
string is opened, although it may reach almost 40V peak
during transient from running LEDs to open. There is a
protection diode from LED+ to GND to prevent negative
ringing during a short-circuit. The output current can be
monitored through the ISMON output pin.
Undervoltage and overvoltage lockout can be adjusted on
the circuit with a few simple resistor choices.
There is an EMI filter on the input of DC2857A. Please
follow the recommend layout and four-layer thickness of
DC2857A for low EMI applications.
The LT3922-1 data sheet gives a complete description of
the part, operation and applications information. The LT3922EUFD-1 is
assembled in a 28-lead plastic 4mm × 5mm QFN package
with a thermally enhanced ground pad. Proper board layout
is essential for maximum thermal performance. See
the data sheet section Layout Considerations.
