1
Rev. 0
DESCRIPTION
LT8355-1
60V
IN
/120V
OUT
Dual LED
Controller with SSFM
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Evaluation board EVAL-LT8355-1-AZ is a high voltage
dual LED controller featuring the
. Channel 1
is assembled as a boost LED driver and channel 2 as a
buck-boost mode LED driver. Either channel can be altered
to a boost LED driver or buck-boost mode LED driver
by adjusting the FB network and LED connection. EVAL-
LT8355-1-AZ drives a single string of LEDs per channel
when the input is between 8V and 18V. Channel 1 drives
up to 72V of LEDs at 450mA and channel 2 drives up to
30V of LEDs at 1A. EVAL-LT8355-1-AZ has an undervolt-
age lockout (UVLO) set at 6.2V falling and 7.5V rising.
EVAL-LT8355-1-AZ features PWM dimming, analog dim-
ming, shutdown, open LED and short LED fault protection
and reporting.
EVAL-LT8355-1-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 on each channel for up to 32W boost
output for channel 1 and up to 30W buck-boost mode
output for channel 2 as assembled. The open LED over-
voltage protection (OVP) uses the IC’s constant voltage
regulation loop to limit the LED+ to LED– voltage if the
LED string is opened to approximately 81V for channel 1
and 36V for channel 2.
The input and output filters on EVAL-LT8355-1-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-LT8355-1-AZ. For
best efficiency and PWM dimming performance, the EMI
filters can be removed.
The LT8355-1’s integrated PWMTG high-side PMOS driv-
ers assist with PWM dimming of the connected LEDs. The
LED strings can be PWM-dimmed for accurate brightness
control with externally generated PWM signals for highest
achievable dimming ratios. They can also utilize LT8355-
1’s internally generated PWM feature for up to 128:1
exponential dimming. When running PWM dimming, the
SSFM aligns itself with the PWM signal for flicker-free
operation of each channel’s LED string. This applies to
both internal and external PWM dimming. The LT8355-1
uses CTRL1 analog dimming for channel 1, and CTRL2
and IADJ2 pins for two-pin analog dimming for channel 2.
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-LT8355-1-AZ. Modifications can
be made to convert the channels to boost, buck-boost
mode and buck mode LED drivers, and maintain low EMI,
PWM dimming and fault diagnostic features. Buck mode,
buck-boost mode and boost LED driver schematics are
provided in the data sheet. Please consult the data sheet
or the applications team regarding how to customize
EVAL-LT8355-1-AZ.
The LT8355-1 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-LT8355-1-AZ. The
LT8355IUFDM-1 is assembled in a 28-lead side solderable
plastic QFN package with a thermally enhanced exposed
ground pad and is AEC-Q100 qualified for automotive
applications. Proper board layout is essential for maxi-
mum performance. See the data sheet section “Designing
the Printed Circuit Board”.
