LT864XS

Part: LT8640S/LT8643S — Analog Devices Type: Synchronous Step-Down Silent Switcher 2 Description: 3.4–42 V input, 6 A continuous (7 A peak) synchronous step-down regulator with ultralow EMI emissions, 2.5 μA quiescent current, and 30 ns minimum on-time.

Operating Conditions:

• Supply voltage: 3.4–42 V
• Operating junction temperature: -40°C to 125°C
• Switching frequency: 200 kHz to 3 MHz (adjustable and synchronizable)

Absolute Maximum Ratings:

• Max supply voltage (V_IN, EN/UV, PG): 42 V
• Max BIAS voltage: 25 V
• Max storage temperature: -65°C to 150°C

Key Specs:

• Minimum Input Voltage: 3.4 V (max)
• LT8640S V_IN Quiescent Current in Sleep: 1.7 μA (typ) at V_EN/UV = 2V, V_FB > 0.97V, V_SYNC = 0V
• Feedback Reference Voltage: 0.970 V (typ) at V_IN = 6V
• Minimum On-Time: 30 ns (typ) at I_LOAD = 1.5A, SYNC = 0V
• Top Power NMOS On-Resistance: 66 mΩ (typ) at I_SW = 1A
• Top Power NMOS Current Limit: 10 A (typ)
• Bottom Power NMOS On-Resistance: 27 mΩ (typ) at V_INTVCC = 3.4V, I_SW = 1A
• SW Leakage Current: ±1.5 μA (max) at V_IN = 42V, V_SW = 0V, 42V

Features:

• Silent Switcher 2 Architecture for ultralow EMI
• Internal bypass capacitors reduce radiated EMI
• Optional Spread Spectrum Modulation
• Ultralow Quiescent Current Burst Mode Operation
• Fast Minimum Switch On-Time: 30ns
• Low Dropout Under All Conditions: 100mV at 1A
• Forced Continuous Mode
• Output Soft-Start and Tracking
• AEC-Q100 Qualified for Automotive Applications

Applications:

• Automotive and Industrial Supplies
• General Purpose Step-Down

Package:

• 24-Lead 4mm × 4mm LQFN

• n Silent Switcher ® 2 Architecture
• n Ultralow EMI Emissions on Any PCB
• n Eliminates PCB Layout Sensitivity
• n Internal Bypass Capacitors Reduce Radiated EMI
• n Optional Spread Spectrum Modulation
• n High Efficiency at High Frequency
• n Up to 96% Efficiency at 1MHz, 12V IN to 5V OUT
• n Up to 95% Efficiency at 2MHz, 12V IN to 5V OUT
• n Wide Input Voltage Range: 3.4V to 42V
• n 6A Maximum Continuous, 7A Peak Output
• n Ultralow Quiescent Current Burst Mode ® Operation
• n 2.5μA I Q Regulating 12V IN to 3.3V OUT (LT8640S)
• n Output Ripple < 10mV P-P
• n External Compensation: Fast T ransient Response and Current Sharing (LT8643S)
• n Fast Minimum Switch On-Time: 30ns
• n Low Dropout Under All Conditions: 100mV at 1A
• n Forced Continuous Mode
• n Adjustable and Synchronizable: 200kHz to 3MHz
• n Output Soft-Start and T racking
• n Small 24-Lead 4mm × 4mm LQFN Package
• n AEC-Q100 Qualified for Automotive Applications

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

Rev. C

• BIAS..........................................................................25V
• FB, TR/SS .
• SYNC/MODE Voltage .

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, high V 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.

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8640S F08

Figure 8. Case Temperature Rise

Pulsed Load

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8640S F09

Figure 9. Case Temperature Rise vs 7A Pulsed Load

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Figure 10. 5V 6A Step-Down Converter with Soft-Start and Power Good

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Figure 11. 3.3V, 6A Step-Down Converter with Soft-Start and Power Good

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Figure 12. Ultralow EMI 5V, 6A Step-Down Converter with Spread Spectrum

• V C pin and components only apply to LT8643S.

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