Evaluation circuit EVAL-LT8391D-AZ is a 4-switch synchronous buck-boost LED driver featuring the LT8391D. It drives a single string of LEDs at up to 1.5A and up to 50V when VIN is between 6V and 45V. EVAL-LT8391D-AZ runs at 400kHz switching frequency and efficiency can exceed 97%. It comes with low EMI features including optimized layout, spread spectrum frequency modulation (SSFM) and input EMI filter. SSFM can be turned on or off with a simple jumper.
The LT8391D 4-swich buck-boost controller is the new member of the LT8391 family (see Table 1 in the user guide). It has an input voltage range of 4V to 60V and output voltage range of 0V to 60V. It has input under voltage lockout (UVLO) which can also be used to enable or disable the controller. On the output side, it is protected against both open and short LED conditions.
The LT8391D allows adjustable switching frequency between 150kHz and 650kHz. It can be PWM dimmed with an external PWM signal. When run with PWM dimming and either fixed switching frequency or SSFM, the internal oscillator aligns itself with the PWM signal for flicker-free operation. On EVAL-LT8391D-AZ, the PWM pin is pulled up to INTVCC through a 100k resistor. Therefore, the LED string is 100% ON when there is no external PWM signal. It can also be analog dimmed with a control voltage on its control pin (CTRL).
EVAL-LT8391D-AZ features MOSFETs that complement the 5V gate drive of the LT8391D to achieve high efficiency. All MOSFETs used on the 4-switch topology are rated at 60V and AEC-Q101 qualified. Ceramic capacitors are used at both input and output side due to their small size and high ripple current capability. In addition to ceramic capacitors, there is one aluminum electrolytic capacitor at the input side to mitigate the effects of the input and output transients.
Sense resistor for LED current can be placed either on high side (LED+) or low side (GND). With high side sense resistor, LED– is directly connected to GND. This gives the option to run only one wire (LED+) to LED string and use chassis ground as return path in automotive or similar applications. LT8391D guarantees ±4% current regulation accuracy with high side sense resistor. Low side sense resistor gives the most accurate LED current regulation. EVAL-LT8391D-AZ implemented both high side and low side sense resistors, with high side sense resistor as default and low side sense resistor as optional.
The LT8391D data sheet gives a complete description of the part, operation and applications information. The data sheet must be read in conjunction with the user guide for evaluation circuit EVAL-LT8391D-AZ. The LT8391DJUFDM is assembled in a 28-lead plastic 4mm × 5mm QFN package with a thermally enhanced GND pad. Proper board layout is essential for maximum thermal performance and EMI performance. Please see the evaluation kit design files for details, including top layer layout, inner traces and grounding.
Refer to the Emission Results section of the user guide for the excellent EMI performance of this evaluation board.
