DESCRIPTION
Demonstration circuit DC2286A is a 36V, 2A synchronous
2MHz buck LED driver featuring the LT®3932. It drives
one or two series LEDs at 1A when VIN is between 9V
and 36V, and can run down to 4VIN with a single LED
(if UVLO is adjusted). DC2286A runs at 2MHz switching
frequency. Spread spectrum frequency modulation
(SSFM) can be enabled with a simple jumper, reducing
EMI. DC2286A comes with low EMI features including
optimized layout, SSFM, and input and output filters. 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 LT3932 has an input voltage range from 9V to 36V. It
has internal, synchronous 40V switches for high efficiency
and small size. It has an adjustable switching frequency
between 200kHz and 2MHz. It can be synchronized to an
external source or programmed with SSFM for low EMI.
The PWMTG high-side PWM MOSFET driver assists with
short-circuit protection and versatility. Although the demo
circuit is optimized for one or two LEDs, it can be altered
to have as high as 33V of LEDs if VIN is high-enough and
circuit components are changed appropriately.
The LT3932 can be PWM dimmed with an external PWM
signal or an internally-generated PWM signal. DC2286A
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.
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 7V if the LED
string is opened. 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 lockout can be adjusted on the circuit with
a few simple resistor choices.
There is an EMI filter on the input of DC2286A. There is
also a small ferrite bead output filter. These filters, combined
with proper board layout and SSFM are effective
to help the PCB pass CISPR 25 class 5 conducted and
radiated EMI. Please follow the recommend layout and
four-layer thickness of DC2286A for low EMI applications.
The LT3932 data sheet gives a complete description of
the part, operation and applications information. The data
sheet must be read in conjunction with this Demo Manual
for demonstration circuit DC2286A. The LT3932EUFD 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".