Analog Devices EVAL-LT8356-1-AZ Demo Manual

DESCRIPTION
Evaluation board EVAL-LT8356-1-AZ is a high voltage
LED controller featuring the
as a buck-boost mode LED driver but can be altered to a
boost LED driver by adjusting the FB network and LED-
connection. EVAL-LT8356-1-AZ drives a single string of
LEDs up to 36V at 1A when the input is between 8V and
36V. It has an undervoltage lockout (UVLO) set at 6.5V
falling and 7.5V rising. The evaluation board features
PWM dimming, analog dimming, shutdown, open LED
and short LED fault protection and reporting.
EVAL-LT8356-1-AZ runs at 250kHz switching frequency
and features spread spectrum frequency modula-
tion (SSFM) modulating its switching frequency from
250kHz to 310kHz to reduce EMI emissions. Small ceramic
input and output capacitors are used to save space and
cost. A high voltage 100V external power switch and 100V
catch diode are used for up to 36W buck-boost mode
output as assembled. The open LED overvoltage protec-
tion (OVP) uses the IC's constant voltage regulation loop
to limit the LED+ to LED– voltage to approximately 41V if
the LED string is opened. The V
will jump to V + 41V if the LED string is opened.
IN
The input and output filters on EVAL-LT8356-1-AZ help
further reduce its EMI. These filters consist of a small
ferrite bead or inductor and high frequency ceramic
capacitors. A small resistor on the gate pin of the power
MOSFET is used to reduce high frequency EMI. These
filters, combined with proper board layout and SSFM, are
very effective in reducing EMI to comply with CISPR25
class 5 limits. Please follow the recommended layout and
the four-layer PCB thickness of EVAL-LT8356-1-AZ. For
best efficiency and PWM dimming performance, the EMI
filters can be removed.
DEMO MANUAL EVAL-LT8356-1-AZ
LT 8356-1. It is assembled
®
, if referenced to GND,
OUT
100V
Controller with SSFM
The LT8356-1's integrated PWMTG high-side PMOS
driver assists with PWM dimming of the connected LEDs.
The LED string can be PWM-dimmed for accurate bright-
ness control with an externally generated PWM signal
for highest achievable dimming ratio. It can also utilize
LT8356-1's internally generated PWM feature for up to
128:1 exponential dimming. When running PWM dim-
ming, the SSFM aligns itself with the PWM signal for
flicker-free operation of the LED string. This applies to
both internal and external PWM dimming. The LT8356-1
uses CTRL and IADJ pins for two-pin analog dimming.
The input undervoltage lockout (UVLO), LED current,
output overvoltage protection (OVP), and switching
frequency, can all be easy adjusted with simple resis-
tor changes to EVAL-LT8356-1-AZ. Modifications can be
made to convert the board from buck-boost mode LED
driver to boost and buck mode LED Driver, and maintain
low EMI, PWM dimming and fault diagnostic features.
Buck mode and boost LED Driver schematics are pro-
vided in the data sheet. Please consult the data sheet
or the applications team regarding how to customize
EVAL-LT8356-1-AZ.
The LT8356-1 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 evaluation board EVAL-LT8356-1-AZ. The
LT8356IUDCM-1 is assembled in a 20-lead side sol-
derable plastic QFN package with a thermally enhanced
exposed ground pad. Proper board layout is essential
for maximum performance. See the data sheet section
"Designing the Printed Circuit Board".
Design files for this circuit board are
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LT8356-1
/120V LED
IN OUT
available.
Rev. A
1