Evaluation board EVAL-LT8355-1-AZ is a high voltage dual LED controller featuring the LT8355-1. 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 undervoltage lockout (UVLO) set at 6.2V falling and 7.5V rising. EVAL-LT8355-1-AZ features PWM dimming, analog dimming, shutdown, open LED and short LED fault protection and reporting.
EVAL-LT8355-1-AZ runs at 250kHz switching frequency and features spread spectrum frequency modulation (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 overvoltage 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 drivers 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 resistor 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 the 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 maximum performance. See the data sheet section “Designing the Printed Circuit Board”.
