ANALOG DEVICES LT8640A Synchronous Step Down Silent Switcher User Manual

ANALOG DEVICES LT8640A Synchronous Step Down Silent Switcher

Product Information

Specifications

VIN and Thermal
Conditions| | | 5| A
fSW| Switching Frequency| R2 = 17.8k, JP1 = FCM/SYNC| 1.85| 2| 2.15| MHz
EFF| Efficiency at DC| VIN = 12V, IOUT = 3A| 94| | | %

Product Usage Instructions

Quick Start Procedure

1. Place JP1 on the BURST position.
2. With power off, connect the input power supply to VEMI and GND. If EMI performance is not important, bypass the input EMI filter by connecting the input power supply to VIN and GND. With power off, connect the load from VOUT to GND.
3. To read the input voltage and output voltage accurately, connect the voltage meters to VIN_SENSE and VOUT_SENSE turret pins.
4. Turn on the power at the input. Note: Ensure that the input voltage does not exceed 42V.
5. Check for the proper output voltage (VOUT = 5V). Note: If there is no output, temporarily disconnect the load to check if it is set too high or shorted.
6. Once the proper output voltage is established, adjust the load within the operating ranges and observe the output voltage regulation, ripple voltage, efficiency, and other parameters.
7. JP1 can also be used to set LT8640A in spread spectrum mode
8. (JP1 on the SPREAD-SPECTRUM position) or forced continuous mode
9. (JP1 on the FCM/SYNC position). An external clock can be added to the SYNC terminal when the SYNC function is used (JP1 on the FCM/SYNC position). Ensure that R2 is chosen to set the LT8640A switching frequency equal to or below the lowest SYNC frequency.

DESCRIPTION
Demonstration circuit 3099A is a 42V, 5A synchronous step-down Silent Switcher® with spread spectrum fre-quency modulation featuring the LT®8640A. The demo board is designed for 5V output from a 5.7V to 42V input. Meanwhile, the LT8640A can operate down to 3.4V inputs. The wide input range allows a variety of input sources, such as automotive batteries and industrial supplies. The LT8640A is a compact, ultralow emission, high efficiency, and high speed synchronous monolithic step-down switching regulator. The integrated power switches and inclusion of all necessary circuitry reduce the component count and solution size. Special Silent Switcher architecture minimizes EMI emissions. Selectable spread spectrum mode can further improve EMI performance. Ultralow 2.5μA quiescent current in Burst Mode® operation achieves high efficiency at very light loads. Fast minimum on-time of 30ns enables high VIN to low VOUT conversion at high frequency. The LT8640A switching frequency can be programmed either via an oscillator resistor or external clock over a 200kHz to 3MHz range. The default frequency of demo circuit 3099A is 2MHz. The LT8640A SYNC/MODE pin on the demo board is grounded (JP1 at BURST position) by default for low ripple Burst Mode operation. Spread spectrum mode and forced continuous mode can be selected respectively by moving the JP1 shunt. To synchronize to an external clock, move JP1 to FCM/SYNC and apply the external clock to the SYNC terminal.
The LT8640A data sheet gives a complete description of the part, operation, and application information. The data sheet must be read in conjunction with this demo manual for demo circuit 3099A. The LT8640A is assembled in a 3mm × 4mm plastic QFN package with an exposed pad for low thermal resistance. The layout recommendations for low EMI operation and maximum thermal performance are available in the datasheet section Low EMI PCB Layout and Thermal Considerations and Peak Output Current.

Design files for this circuit board are available.

All registered trademarks and trademarks are the property of their respective owners.

PERFORMANCE SUMMARY

Specifications are at TA = 25°C

SYMBOL| PARAMETER| CONDITIONS| MIN| TYP| MAX| UNITS
---|---|---|---|---|---|---
VIN| Input Supply Range|  | 5.7|  | 42| V
VOUT| Output Voltage| R4 = 1MΩ, R5 = 243kΩ| 4.85| 5| 5.15| V
ABOUT| Maximum Continuous Output Current| Derating Is Necessary for Certain VIN and Thermal Conditions| 5| A
fSW| Switching Frequency| R2 = 17.8kΩ, JP1 = FCM/SYNC| 1.85| 2| 2.15| MHz
EFF| Efficiency at DC| VIN = 12V , IOUT = 3A| 94| %

QUICK START PROCEDURE

Demonstration circuit 3099A is easy to set up to evaluate the performance of the LT8640A. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below:

1. Place JP1 on BURST position.
2.  With power off, connect the input power supply to VEMI and GND. If the EMI performance is not important, the input EMI filter can be bypassed by connecting the input power supply to VIN and GND.
3. With power off, connect the load from VOUT to GND.
4. To read the input voltage and output voltage accurately, the voltage meters should be connected to VIN_SENSE and VOUT_SENSE turret pins.
5. Turn on the power at the input. NOTE : Make sure that the input voltage does not exceed 42V.
6. Check for the proper output voltage (VOUT = 5V). NOTE: If there is no output, temporarily disconnect the load to make sure that the load is not set too high or is shorted.
7. Once the proper output voltage is established, adjust the 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 avoid a long ground lead on the oscilloscope probe. Measure the output voltage ripple by touching the probe tip directly across the output capacitor. See Figure 2 for the proper scope technique.
8. JP1 can also set LT8640A in spread spectrum mode (JP1 on the SPREAD-SPECTRUM position) or forced continuous mode (JP1 on the FCM/SYNC position). An external clock can be added to the SYNC terminal when the SYNC function is used (JP1 on the FCM/SYNC position). Please make sure that R2 should be chosen to set the LT8640A switching frequency equal to or below the lowest SYNC frequency.

Figure 3 shows the efficiency of demo circuit 3099A at 12V input and 24V input in Burse Mode Operation (input from VIN terminal, JP1 = BURST).
The demo board has an EMI filter installed. The EMI performance of the board (with EMI filter) is shown on Figure 4. The red line in Figure 4 is CISPR25 Class 5 peak limit. The figure shows that the circuit passes the test with a wide margin. To achieve EMI performance as shown in Figure 4, the input EMI filter is required, and the input voltage should be applied at VEMI.

• Conducted EMI Performance CISPR25 Conducted Emission Test with Class 5 Peak Limits)

• Radiated EMI Performance (CISPR25 Radiated Emission Test with Class 5 Peak Limits)

Figure 4. LT8640A Demo Circuit 3099A EMI Performance (12V Input from VEMI, with EMI filter, IOUT = 5A)

PARTS LIST

ITEM| QTY| REFERENCE| PART DESCRIPTION| MANUFACTURER/PART NUMBER

---|---|---|---|---

Required Circuit Components

LT8640AJUDCM#PBF

Additional Demo Board Circuit Components

74279226101
6| 0| L2| IND., OPT, XAL60XX|
7| 1| Q1| MOSFET, N-CH, 40V, 14A, DPAK (TO-252)| VISHAY, SUD50N04-8M8P-4GE3
8| 0| R6 (OPT)| RES., OPTION, 0603|
9| 2| R7, R12| RES., CHIP, 0Ω, 1/10W, 1%, 0603| VISHAY, CRCW06030000Z0EA
10| 1| R10| RES., CHIP, 10k, 1/10W, 1% 0603| VISHAY, CRCW060310K0FKEA
11| 1| R11| RES., 0.1Ω, 1%, 3W, 2512, SHORT-SIDE TERMINAL| SUSUMU, KRL3264E-C-R100-F-T1

Hardware: For Demo Board Only

1| 9| E1, E5-E9, E15-E17| TESTPOINT, TURRET, 0.094″| MILL-MAX, 2501-2-00-80-00-00-07-0
---|---|---|---|---
2| 4| E2, E3, E4, E10| TESTPOINT, TURRET, 0.064″| MILL-MAX, 2308-2-00-80-00-00-07-0
3| 4| J1-J4| JACK BANANA| KEYSTONE, 575-4
4| 1| JP1| 2X3, 0.079 DOUBLE ROW HEADER| WURTH ELEKTRONIK, 62000621121
5| 4| MP1-MP4| STANDOFF, NYLON, SNAP-ON, 11.1mm| WURTH ELEKTRONIK, 702934000
6| 1| XJP1| SHUNT, 0.079″ CENTER| WURTH ELEKTRONIK, 60800213421

SCHEMATIC DIAGRAM

ESD Caution
ESD (electrostatic discharge) sensitive device. Charged devices and circuit boards can discharge without detection. Although this product features patented or proprietary protection circuitry, damage may occur on devices subjected to high-energy ESD. Therefore, proper ESD precautions should be taken to avoid performance degradation or loss of functionality.

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