The main control loop of the two bucks is adaptive on-time pulse width modulation (PWM) controller that supports a proprietary DCAP3 mode control. The DCAP3 mode control combines adaptive on-time control with an internal compensation circuit for pseudo-fixed frequency and low external component count configuration with both low-ESR and ceramic output capacitors. It is stable even with virtually no ripple at the output. The TPS65296 also includes an error amplifier that makes the output voltage very accurate.

At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after internal one-shot timer expires. This one-shot duration is set proportional to the converter input voltage, VIN, and is inversely proportional to the output voltage, VO, to maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below the reference voltage. An internal ripple generation circuit is added to reference voltage for emulating the output ripple, this enables the use of very low-ESR output capacitors such as multi-layered ceramic caps (MLCC). No external current sense network or loop compensation is required for DCAP3 control topology.

Both VDD1 buck and VDD2 buck include an error amplifier that makes the output voltage very accurate. For any control topology that is compensated internally, there is a range of the output filter it can support. The output filter used with the TPS65296 is a low-pass L-C circuit. This L-C filter has a double-pole frequency described in Equation 1.

Equation 1.

At low frequencies, the overall loop gain is set by the internal output set-point resistor divider network and the internal gain of the TPS65296. The low-frequency L-C double pole has a 180 degree in-phase. At the output filter frequency, the gain rolls off at a –40 dB per decade rate and the phase drops rapidly. The internal ripple generation network introduces a high-frequency zero that reduces the gain roll off from –40 dB to –20 dB per decade and increases the phase to 90 degree one decade above the zero frequency. The internal ripple injection high-frequency zero is related to the switching frequency. The inductor and capacitor selected for the output filter must be such that the double pole is placed close enough to the high-frequency zero, so that the phase boost provided by this high-frequency zero provides adequate phase margin for the stability requirement. The crossover frequency of the overall system should usually be targeted to be less than one-fifth of the switching frequency (F_SW).