Features and Benefits
- Silent Switcher®2 Architecture
- Ultralow EMI Emissions on Any PCB
- Eliminates PCB Layout Sensitivity
- Internal Bypass Capacitors Reduce Radiated EMI
- Optional Spread Spectrum Modulation
- High Efficiency at High Frequency
- Up to 96% Efficiency at 1MHz, 12VIN to 5VOUT
- Up to 94% Efficiency at 2MHz, 12VIN to 5VOUT
- Wide Input Voltage Range: 2.8V to 42V
- 10A Maximum Continuous, 12A Peak Transient Output
- Reference Accuracy: ±1% Over Temperature
- Fast Transient Response with External Compensation
- Low Quiescent Current Burst Mode® Operation
- 90µA IQ Regulating 12VIN to 5VOUT
- Output Ripple < 10mVP-P
- Fast Minimum Switch On-Time: 25ns
- PolyPhase® Operation: Up to 12 Phases
- Low Dropout Under All Conditions: 45mV at 1A
- Adjustable and Synchronizable: 200kHz to 3MHz
- Output Soft-Start and Power Good
- Safely Tolerates High Reverse Current
- 28-Lead 5mm × 4mm LQFN Package
- AEC-Q100 Qualified for Automotive Applications
The LT8638S synchronous step-down regulator features second generation Silent Switcher architecture designed to minimize EMI emissions while delivering high efficiency at high switching frequencies. This includes the integration of input capacitors to optimize all the fast current loops inside and make it easy to achieve advertised EMI performance by reducing layout sensitivity. This performance makes the LT8638S ideal for noise sensitive applications and environments.
The fast, clean, low overshoot switching edges enable high efficiency operation even at high switching frequencies, leading to a small overall solution size. Peak current mode control with a 25ns minimum on-time allows high step down ratios even at high switching frequencies. External compensation via the VC pin allows for fast transient response. PolyPhase operation allows multiple LT8638S regulators to run with interleaving phase shift to provide more output current.
Burst Mode operation enables low standby current consumption, forced continuous mode can control frequency harmonics across the entire output load range, or spread spectrum operation can further reduce EMI emissions. Soft-start and tracking functionality is accessed via the SS pin, and an accurate input voltage UVLO threshold can be set using the EN/UV pin.
Product Lifecycle Recommended for New Designs
This product has been released to the market. The data sheet contains all final specifications and operating conditions. For new designs, ADI recommends utilization of these products.
Evaluation Kits (1)
Demonstration circuit 2929A is a 42V, 10A (12A peak) synchronous step-down Silent Switcher 2 with spread spectrum frequency modulation featuring the LT8638S. The demo board is designed for 3.3V output from a 3.9V to 42V input. The wide input range allows a variety of input sources, such as automotive batteries and industrial supplies. The LT8638S is a compact, ultralow emission, high efficiency, and high speed synchronous monolithic step-down switching regulator. The integrated bypass capacitors optimize the fast current loops and make it easier to minimize EMI emissions by reducing layout sensitivity. Selectable spread spectrum mode further improves EMI performance, making it perfect solution to the noise sensitive applications.
Peak current mode control with minimum on-time of as small as 25ns allows high step-down conversion even at high frequency. The LT8638S switching frequency can be programmed either via oscillator resistor or external clock over a 200kHz to 3MHz range. The default frequency of demo circuit 2929A is 2MHz.
The SYNC/MODE pin on the demo board DC2929A is grounded (JP1 at BURST position) by default for low ripple Burst Mode® operation. It can be configured into different operation modes through JP1 and SYNC terminal (Table 1).
Figure 1 in the demo manual shows the efficiency of the circuit at 12V input and 24V input in BURST mode. To get accurate efficiency measurement, measure the input voltage at the VIN SENSE terminal and measure the output voltage at the VO SENSE terminal.
Figure 2 shows the LT8638S temperature rise on DC2929A demo board at 12V input and 24V input in BURST mode, with continuous load current. LT8638S can deliver 12A of peak output current for short periods of time. Figure 3 shows its case temperature rise vs the duty cycle of a 1kHz pulsed 12A load. The LT8638S is in a 5mmx4mm LQFN package with exposed pads for low thermal resistance. The rated maximum continuous load current is 10A, while derating may be necessary for certain input voltage and thermal conditions.
|JP1 POSITION||SYNC (E5) INPUT||OPERATION MODE|
|SPREAD-SPECTRUM||N/A||Forced Continuous Mode w/Spread-Spectrum|
|BURST||N/A||Low Ripple Burst Mode®|
|FCM/SYNC||Floating||Forced Continuous Mode w/o Spread-Spectrum|
|FCM/SYNC||External Clock||Forced Continuous Mode w/Synchronization|
The demo board has an EMI filter installed. The EMI performance of the board is shown in Figure 4 in the demo manual. The red lines in EMI Performance are CISPR25 Class 5 limits. 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 to VIN_EMI terminal, and the test setup can be referred to the CISPR25 standards. If the input is applied to VIN terminal, the EMI filter is bypassed.
The LT8638S datasheet gives a complete description of the part, the operation and application information. The datasheet must be read in conjunction with the demo manual for demo circuit 2929A. 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. Contact ADI applications engineer for support.
Tools & Simulations
LTspice® is a powerful, fast and free simulation software, schematic capture and waveform viewer with enhancements and models for improving the simulation of analog circuits.
Models for the following parts are available in LTspice:
LTpowerCAD® is a power supply design program which selects power stage components, provides detailed power efficiency, shows quick loop Bode plot stability and load transient analysis, and can export to LTspice for simulation.
To open the project file for this circuit:
Step 1: Download and install LTpowerCAD on your computer.
Step 2: Click on the link(s) in the section below to download part-specific project files.
Step 3: If the project file doesn't run, right-click the link and select "Save Target As." Then start the LTpowerCAD tool and open the project file by selecing "Open Project" from the "File" menu.
LTpowerCAD Project Files
Design tools for the following parts are available in LTpowerCAD:
ADI has always placed the highest emphasis on delivering products that meet the maximum levels of quality and reliability. We achieve this by incorporating quality and reliability checks in every scope of product and process design, and in the manufacturing process as well. "Zero defects" for shipped products is always our goal.
Sample & Buy
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