D -CAP2 mode of TPS54XXX

Dear comunity,

Can anyone give me an idea what is difference between adaptive on-time D-CAP2 mode and Adaptive Constant-On-Time (COT) control mode? I am not sure it is same operation or not. I looking for DC-DC with hight transient response and high efficiency over a wide output current load range.

Thanks,

NT

  • DCAP is adaptive on time , semi hysteretic off time.  The on time is determined pulse by pulse by comparing VOUT to VIN (VOUT / VIN = D and Ton = VOUT /( VIN *Fsw)).  The ontime is calculated so that the switching frequency remains relatively costant.  The OFF time is semi hysteretic, so OFF time ends when the output voltage falls below the reference voltage.  For DCAP mode control, the "ramp' is provided by the output ripple voltage and requires higher ESR output capacitor.  For DCAP2, there is an intenal ripple injection circuit that allows use of low ESR MLCC capacitors.

    Constant on time (COT) is by definition not adaptive on time.  It sets a fixed on time with semi hysteretic off time, so the effective switching frequency will vary with line and load conditions.  See this document for details:  www.ti.com/lit/an/slva281b/slva281b.pdf

  • In reply to JohnTucker:

    John,

    Seem D CAP2 is more attrative than COT if the converter requires high transient response?

    NT

  • In reply to Knighty Nimp:

    Yes, DCAP2 is optimized for fast transient response with relatively small amounts of ceramic output capacitors.  2 x 22 uF is usually sufficient.

  • Hello NT,

    Adaptive constant-on-time (COT) is an improved constant on time control scheme that keeps the switching frequency nearly constant over VIN range. The switching frequency is also nearly independent of load current. At light load the non-synchronous COT converters (LM5007/8/8A/9/9A/10, LM34923, LM5085) enter discontinuous mode (DCM) operation which results in reduced frequency operation at light load. This automatically results in high efficiency over a wide output current range.

    The synchronous COT converters (LM5006, LM5017) maintain fixed frequency down to zero load.

    COTs have really fast transient response as there is no bandwidth limiting error amplifier in the control loop.

    Let me know your VIN, IOUT range and I can suggest matching COT parts.

    Thanks and regards,

    Vijay