Linear Technology LT8391EFE Demo Manual

Buck-Boost Led Controller with Spread Spectrum
DESCRIPTION
Demonstration circuit 2345A is a 60V synchronous
4-switch buck-boost LED controller with spread spectrum
featuring the LT 8391 LED driver. It accepts an input voltage
®
from 4V to 60V and drives a single string of LEDs up to
25V at 2A. DC2345A runs at 400kHz switching frequency
without spread spectrum, but spread spectrum frequency
modulation (SSFM) can be enabled with a simple jumper.
SSFM spreads the switching frequency to fsw ±15% for
reduced EMI.
The LT8391 has a wide input voltage range down to 4V and
up to 60V. It has adjustable switching frequency between
150kHz and 600kHz. There is a simple jumper option for
external frequency synchronization, spread spectrum
frequency modulation, or neither.
The LT8391 can be PWM dimmed with an external PWM
signal and an internally-generated PWM signal. DC2345A
has a jumper that can e set to switch between internally-
generated PWM signal, externally-generated PWM signal,
and no PWM signal (100% on). It can be analog dimmed
with a control voltage on either of its two control pins.
LT8391 features both open LED and short LED (LED+ to
GND) protection as well as a fault output flag.
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 capacitors on
DEMO MANUAL DC2345A
60V Synchronous 4-Switch
both sides of the synchronous switches 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 29.7V if the LED string is
opened although it may reach 32V 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 with long wires.
Undervoltage lockout can be adjusted on the circuit with
a few simple resistor choices.
There is an EMI filter on the input of DC2345A. This filter
has a 3.2µH inductor and two 4.7µF capacitors. It is ef-
fective in reducing the conducted EMI in the AM band to
pass CISPR25 class 5 regulations. When run with SSFM,
DC2345A has low conducted EMI.
The LT8391 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 DC2345A. The LT8391EFE is
assembled in a 28-lead plastic TSSOP (FE) package with
a thermally enhanced ground pad. Proper board layout
is essential for maximum thermal performance. See the
data sheet section 'Layout Considerations'.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2345A
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LT8391EFE
dc2345af
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