SN6505BDBVT Integrated Circuits (ICs)

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1. Description

The SN6505BDBVT is a low-noise, low-EMI push-pull transformer driver, specifically designed for small form factor, isolated power supplies. It drives lowprofile, center-tapped transformers from a 2.25 V to 5 V DC power supply. Ultra-low noise and EMI are achieved by slew rate control of the output switch voltage and through Spread Spectrum Clocking (SSC). The SN6505x consists of an oscillator followed by a gate drive circuit that provides the complementary output signals to drive groundreferenced N-channel power switches. The device includes two 1-A Power-MOSFET switches to ensure start-up under heavy loads. The switching clock can also be provided externally for accurate placement of switcher harmonics, or when operating with multiple transformer drivers. The internal protection features include a 1.7 A current limiting, under-voltage lockout, thermal shutdown, and break-before-make circuitry. SN6505x includes a soft-start feature that prevents high inrush current during power up with large load capacitors. SN6505A has a 160 kHz internal oscillator for applications that need to minimize emissions whereas SN6505B has a 420 kHz internal oscillators for applications that require higher efficiency and smaller transformer size. The SN6505x is available in a small 6-pin SOT23/DBV package. The device operation is characterized for a temperature range from –55°C to 125°C.

2. Features

    1. Push-pull driver for transformers

    2. Wide input voltage range: 2.25 V to 5.5 V

    3. High output drive: 1 A at 5 V supply

    4. Low RON 0.25 Ω max at 4.5 V supply

    5. Ultra-low EMI

    6. Spread spectrum clocking

    7. Precision internal oscillator options: 160 kHz (SN6505A) and 420 kHz (SN6505B)

    8. Synchronization of multiple devices with external clock input

    9. Slew-rate control

  10. 1.7 A Current-limit

  11. Low shutdown current: <1 μA

  12. Thermal shutdown

  13. Wide temperature range: –55°C to 125°C

  14. Small 6-Pin SOT23 (DBV) package

  15. Soft-start to reduce In-rush current

3. Applications

    1. Isolated power supply for CAN, RS-485, RS-422, RS-232, SPI, I2C, low-power LAN

    2. Low-noise isolated USB supplies

    3. Process control

    4. Telecom supplies

    5. Radio supplies

    6. Distributed supplies

    7. Medical instruments

    8. Precision instruments

    9. Low-noise filament supplies

4. Feature Description

Push-pull converters require transformers with center-taps to transfer power from the primary to the secondary When Q1 conducts, VIN drives a current through the lower half of the primary to ground, thus creating a negative voltage potential at the lower primary end with regards to the VIN potential at the center-tap. At the same time the voltage across the upper half of the primary is such that the upper primary end is positive with regards to the center-tap in order to maintain the previously established current flow through Q2 , which now has turned high-impedance. The two voltage sources, each of which equaling VIN, appear in series and cause a voltage potential at the open end of the primary of 2×VIN with regards to ground. Per dot convention the same voltage polarities that occur at the primary also occur at the secondary. The positive potential of the upper secondary end therefore forward biases diode CR1. The secondary current starting from the upper secondary end flows through CR1 , charges capacitor C, and returns through the load impedance RL back to the center-tap. When Q2 conducts, Q1 goes high-impedance and the voltage polarities at the primary and secondary reverse. Now the lower end of the primary presents the open end with a 2×VIN potential against ground. In this case CR2 is forward biased while CR1 is reverse biased and current flows from the lower secondary end through CR2 , charging the capacitor and returning through the load to the center-tap.

When Q1 conducts the magnetic flux is pushed from A to A’, and when Q2 conducts the flux is pulled back from A’ to A. The difference in flux and thus in flux density is proportional to the product of the primary voltage, VP, and the time, tON, it is applied to the primary: B ≈ VP × tON. This volt-seconds (V-t) product is important as it determines the core magnetization during each switching cycle. If the V-t products of both phases are not identical, an imbalance in flux density swing results with an offset from the origin of the B-H curve. If balance is not restored, the offset increases with each following cycle and the transformer slowly creeps toward the saturation region.

5. Application Information

The SN6505 is a transformer driver designed for low-cost, small form- factor, isolated DC/DC converters using the push-pull topology. The device includes an oscillator that feeds a gate-drive circuit. The gate-drive, comprising a frequency divider and a break-before-make (BBM) logic, provides two complementary output signals which alternately turn the two output transistors on and off. The output frequency of the oscillator is divided down by an asynchronous divider that provides two complementary output signals, S and S, with a 50%  The output frequency of the oscillator is divided by an asynchronous divider that provides two Complementary output signals S and S with 50% duty cycle. Subsequent break-before-make logic inserts a Dead time between high pulses of two signals. The resulting output signals G1 and G2 , presenting the gate drive signals for the output transistors Q1 and Q2. This short time, called the break-before-make time, is required to avoid shorting the primary across both ends.