ANALOG DEVICES LT8292 Dual Phase Synchronous Buck Boost Controller User Guide
EVAL-LT8292-AZ
General Description
Evaluation circuit EVAL-LT8292-AZ features the LT®8292: a parallelable 4-switch synchronous buck-boost controller. The LT8292 regulates the output voltage and output or input current from an input voltage below, equal to, or above the output voltage. With a wide 5.5V to 60V input range and a seamless transition between operating regions, the LT8292 is ideal for automotive, industrial, and telecom systems. The buck-boost peak current mode architecture allows adjustable phase-lockable 100kHz to 650kHz fixed frequency operation or internal spread spectrum operation for low Electromagnetic interference (EMI). Additionally, LT8292 features ISHARE and IGND pins, allowing for multi-IC leaderless current-sharing capabilities for higher-power applications.
The EVAL-LT8292-AZ operates from 9V to 36V input voltage range and generates an output of 12V. A maximum output current of 50A allows up to 600W power delivery while achieving efficiencies that exceed 97%. The board utilizes two LT8292s in parallel, both set to 100kHz, to achieve high power and efficiency.
The output voltage and EN/UVLO are both programmed by resistor dividers. The LT8292 supports an output voltage range from 1V to 60V with a 2% tolerance. EN/UVLO is set so the circuit will turn off when the input voltage falls below 8.1V and will turn on when the input voltage rises above 8.5V.
LT8292 utilizes split pull-up/pull-down gate drivers and four selectable dead time settings. EVAL-LT8292-AZ allows for simple alterations to optimize these features.
The PGOOD status flag indicates when the output voltage is within ±8% of the final regulation voltage.
The EVAL-LT8292-AZ features MOSFETs that comple-ment the 5V gate drive of the LT8392 to achieve high efficiency. 40V MOSFETs are used on the input and output side of the four-switch topology. Ceramic capacitors are used at both the circuit input and output because of their small size and high ripple current capability. In addition to ceramic capacitors, there are bulk aluminum polymer capacitors on the input and output to make input and output stable during the transient period.
The ICTRL input is pulled up to the VREF pin through a 100kΩ resistor to set the output current limit to its maximum, and an external voltage on the ICTRL pin can be used to lower the current limit. A capacitor at the SS pin programs soft-start.
High power operation, parallel capability, 5.5V input voltage operation, 4-switch buck-boost topology, proprietary peak current mode architecture, fault protection, and output current limiting make the LT8292 attractive for high power voltage regulator circuits.
The LT8292AFE is available in a thermally enhanced 38-lead TSSOP package. The LT8292 datasheet gives a complete description of this part, its operation, and applications information. The LT8292 data sheet must be read in conjunction with this user guide to properly use the evaluation circuit EVAL-LT8292-AZ. The evaluation circuit is designed to be easily reconfigured to suit other applications, including the example schematics in the datasheet. Consult the factory for assistance.
Performance Summary (TA = 25C)
Quick Start Procedure
The EVAL-LT8292-AZ is easy to set up to evaluate the performance of the LT8292AFE. See Figure 1 for proper equipment setup and use the following procedure:
- Set JP1 at ENABLE to sync channel 1 and channel 2 180° phasing.
- Connect the EN/UVLO_1 and EN/UVLO_2 turrets to the ground with clip-on leads.
- With the power supply off, connect the positive terminal of the power supply to VIN and the negative terminal to GND.
- Connect the load (<10A) between the OUT and GND terminals.
- Set the power supply to 12V and turn it on.
Note: Ensure that the voltage applied to VIN does not exceed 40V, which is the voltage rating for input side MOSFETs. - Remove the clip-on leads from EN/UVLO_1 and EN/UVLO_2. Verify that the output voltage is 12V.
Note: If the output voltage is low, temporarily disconnect the load to ensure that it is not set too high. - Once the proper output voltage is established, adjust the input voltage and load within the operating ranges and observe the output voltage regulation, ripple voltage, efficiency, and other parameters.
Note: When measuring the input or output voltage ripple, care must be taken to minimize the length of the oscilloscope probe ground lead. Measure the input or output voltage ripple by connecting the probe tip directly across the VIN or VOUT and GND terminals, preferably across the input or output capacitors.
Figure 1. EVAL-LT8292-AZ Board Connections
Adjust Output Voltage
To change the output voltage from the programmed 12V, change R7, R8, R57, and R60. Refer to the Programming Output Voltage and Thresholds section in the datasheet to calculate the VFB resistor divider values for the desired output voltage. All the corresponding components must also be adjusted to handle the desired output voltage.
Thermal Performance
The EVAL-LT8292-AZ features excellent thermal performance across its entire input voltage range due to the high efficiency of the synchronous buck-boost circuit operation. The component temperatures of EVAL-LT8292-AZ with a typical 12V output and 50A load are shown in Figure 6–Figure 8. The six-layer Printed circuit board (PCB) layout features solid copper planes that help spread the heat across the entire board.
Parallel Multiple ICs
The LT8292 is designed to easily be paralleled for even higher output power. This can be achieved by tying all ISHARE pins together and kelvin connecting all IGND pins to a common GND.
The CLKOUT pin of LT8292 IC provides a 180° out-of-phase clock signal fixed at 50% duty cycle. For an interleaved dual-phase operation, the CLKOUT pin of the first phase can be connected to the SYNC/MODE pin of the second IC. In this configuration, it is recommended the first phase be set to fixed-frequency forced continuous mode (FCM) by floating the SYNC/MODE pin.
JP1 on EVAL-LT8292-AZ can be set to either ENABLE or DISABLE. Setting to ENABLE will sync both channels to 180° phasing utilizing the CLKOUT pin. Setting to DISABLE allows the two channels to run independently or be synced with an external clock signal.
Start-Up and Shutdown with a Microcontroller
The circuit enable, and undervoltage lockout is set with a resistor divider to the EN/UVLO pin. The EVAL-LT8292-AZ also has optional circuitry that can be populated to enable and disable the channels via a microcontroller. The circuitry ensures proper start-up and shutdown when using a microcontroller. To utilize, populate Q1-Q4, R77, R79, R81, R82 and R83. Also, add a 0Ω jumper from COM to the desired supply voltage of VIN or VCC. The microcontroller signal can be applied to EN1.
Select Mode of Operation
LT8292 uses a SYNC/MODE pin to select different modes of operation.
- External Clock: For external frequency synchronization and FCM.
- INTVCC: For spread spectrum around internal oscillator frequency and FCM.
- Float: For internal oscillator frequency and FCM at light load.
- 100kΩ to GND: For internal oscillator frequency and pulse-skipping mode at light load.
- GND: For internal oscillator frequency and low ripple Burst® mode at light load.
EVAL-LT8292-AZ allows for easy modification of each channel to select the desired mode of operation.
Select Dead Times
LT8292 has four selectable dead time settings for the boost side switching and the buck side switching. A single resistor is used for each pin, RDT1, and RDT2, to set the dead times.
- 0Ω to INTVCC: 40ns
- 130kΩ to GND: 20ns
- 82kΩ to GND: 10ns
- 51kΩ to GND: 2ns
Performance
(VIN = 12V, TA = 25C, unless otherwise noted.)
Bill of Materials
Schematic
Figure 9. EVAL-LT8292-AZ Schematic (1 of 2)
Revision History
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