LT8640S-8643S.PDF

Part: Analog Devices LT8640S/LT8643S

Type: Synchronous Step-Down Silent Switcher 2

Key Specs:

• Input Voltage Range: 3.4V to 42V
• Maximum Continuous Output Current: 6A
• Peak Output Current: 7A
• Quiescent Current (LT8640S): 2.5μA
• Efficiency (1MHz, 12VIN to 5VOUT): Up to 96%
• Efficiency (2MHz, 12VIN to 5VOUT): Up to 95%
• Minimum Switch On-Time: 30ns
• Dropout: 100mV at 1A
• Adjustable and Synchronizable Frequency: 200kHz to 3MHz

Features:

• Silent Switcher 2 Architecture (Ultralow EMI Emissions, eliminates PCB layout sensitivity, internal bypass capacitors, optional spread spectrum modulation)
• High Efficiency at High Frequency
• Ultralow Quiescent Current Burst Mode Operation (Output Ripple < 10mVP-P)
• External Compensation (LT8643S)
• Fast Minimum Switch On-Time
• Low Dropout Under All Conditions
• Forced Continuous Mode
• Adjustable and Synchronizable Frequency
• Output Soft-Start and Tracking
• AEC-Q100 Qualified for Automotive Applications

Applications:

• Automotive and Industrial Supplies
• General Purpose Step-Down
• Noise-sensitive applications and environments

Package:

• 24-Lead 4mm × 4mm LQFN Package

The LT®8640S/LT8643S 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 bypass 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 LT8640S/LT8643S ideal for noise-sensitive applications and environments.

Peak current mode control with a 30ns minimum on-time allows high step-down ratios even at high switching frequencies. The LT8643S has external compensation to enable current sharing and fast transient response at high switching frequencies.

Burst Mode operation enables ultralow 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.

All registered trademarks and trademarks are the property of their respective owners. Protected by U.S. patents, including 8823345.

• ⁿ Automotive and Industrial Supplies
• ⁿ General Purpose Step-Down

The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. ELECTRICAL CHARACTERISTICS

Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.

Note 2: The LT8640SE/LT8643SE is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. The LT8640SI/LT8643SI is guaranteed over the full –40°C to 125°C operating junction temperature range. The junction temperature (TJ, in °C) is calculated from the ambient temperature (TA in °C) and power dissipation (PD, in Watts) according to the formula:

$T_J = T_A + (PD bullet θ_JA)where θJA (in °C/W) is the package thermal impedance.

Note 3: θ values determined per JEDEC 51-7, 51-12. See the Applications Information section for information on improving the thermal resistance and for actual temperature measurements of a demo board in typical operating conditions.

Note 4: This IC includes overtemperature protection that is intended to protect the device during overload conditions. Junction temperature will exceed 150°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature will reduce lifetime.





























For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT8640S/LT8643S. The ground pins on the bottom of the package should be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread heat dissipated by the LT8640S/LT8643S. Placing additional vias can reduce thermal resistance further. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. Power dissipation within the LT8640S/LT8643S can be estimated by calculating the total power loss from an efficiency measurement and subtracting the inductor loss. The die temperature is calculated by multiplying the LT8640S/LT8643S power dissipation by the thermal resistance from junction to ambient.

The internal overtemperature protection monitors the junction temperature of the LT8640S/LT8643S. If the junction temperature reaches approximately 180°C, the LT8640S/LT8643S will stop switching and indicate a fault condition until the temperature drops about 10°C cooler.

Temperature rise of the LT8640S/LT8643S is worst when operating at high load, highV_{IN}$ , and high switching frequency. If the case temperature is too high for a given application, then either $V_{IN}$ , switching frequency, or load current can be decreased to reduce the temperature to an acceptable level. Figure 8 shows examples of how case

temperature rise can be managed by reducing $V_{IN}, switching frequency, or load.

The LT8640S/LT8643S's internal power switches are capable of safely delivering up to 7A of peak output current. However, due to thermal limits, the package can only handle 7A loads for short periods of time. This time is determined by how quickly the case temperature approaches the maximum junction rating. Figure 9 shows an example of how case temperature rise changes with the duty cycle of a 1kHz pulsed 7A load.

The LT8640S/LT8643S's top switch current limit decreases with higher duty cycle operation for slope compensation. This also limits the peak output current the LT8640S/LT8643S can deliver for a given application. See curve in Typical Performance Characteristics.


Figure 8. Case Temperature Rise


Figure 9. Case Temperature Rise vs 7A Pulsed Load

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