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Soft-staring SN6501-Q1 with Load-Switch TPS22965

Thomas Kugelstadt
Mastermind 28265 points
Other Parts Discussed in Thread: TPS22965, SN6501-Q1, SN6501

Soft-staring SN6501-Q1 with Load-Switch TPS22965

The SN6501-Q1 transformer driver is a simple square-wave oscillator with two power FET output stages whose complementary output signals drive the primary of a center-tapped transformer, commonly used in isolated, push-pull, dc-to-dc converter designs.

To implement a soft-start function for the SN6501-Q1, you can use a load switch, such as TPS22965, whose output rise time is adjustable through the application of an external transition capacitor, CT. Figure 1 shows an example for a 5V design where the output of the TPS22965 provides the input voltage for the SN6501-Q1.

Figure 1. Soft-starting the SN6501-Q1 transformer driver with the TPS22965 load-switch

Figures 2 and 3 show the output signals of the load-switch (Channel 1), the transformer driver (Channel 3), and the push-pull converter (Channel 4) during soft-start and shut-down.

Figure 2. Soft-starting the SN6501-Q1 transformer driver with the TPS22965 load-switch

Capacitor CT is chosen with 1nF yielding a ramp-up time at the load-switch output of approximately 2.2ms (Ch1 in yellow). The output of the push-pull converter starts ramping when the SN6501 reaches its start-up voltage at about 2.2V. During the ramp-up the SN6501 output changes from high-impedance to low-impedance drive mode, which creates a sudden increase in supply current. This in turn causes short drops of both, the load-switch and the converter output voltages. While the drop at the load-switch remains minute, the drop at the converter output can reach up to 1.3V under 100mA load current.

Note that during the entire time, from soft-start until normal operation, the oscillator frequency remains constant.

                                   Figure 3. SN6501-Q1 during soft turn-off

During power-down the converter output tracks the load-switch output over a similar ramp-down time of 2.2ms. Again the oscillator frequency remains constant during the entire ramp-down phase.

While there are a wide variety of isolated dc-to-dc converters with integrated soft-start available on the market, the SN6501-Q1 and TPS22965 assure low-noise performance, which is crucial in automotive applications.

We hope this information is useful for your next automotive dc-dc converter design.

Best regards,

Bryce Sinclair and Thomas Kugelstadt