Hi Team,
Do you have detailed description and block diagram of KB20 DS 6.3.7? What is the difference between D-CAP4 and other D-cap versions? Thanks!
-A
This is not the correct forum for sharing a detailed block diagram for D-CAP4, however I can walk you through the theory of operation for each of the various forms of D-CAP control.
D-CAP
is Texas Instruments trademarked Constant On-Time Control which employs an "Adaptive On-time" generator, where the on-time is set by VIN, VOUT, the desired switching frequency, and some additional loss factors to stabilize the switching frequency compared to conventional constant on-time.
D-CAP uses the ESR of the output capacitors as a sense element in order to detect the AC current in the output capacitors, and attempts to regulate the average current into the capacitors to 0A. However, since D-CAP relies on the ESR of the output capacitors, it places constraints on the cycle by cycle ripple of the output as well as the ESR zero of the output capacitors. These limitations make it difficult to use with ultra-low ESR polymer capacitors and multi-layer ceramic capacitors (MLCCs) which do not have enough ESR or a low enough ESR-Zero for D-CAP control.
To use D-CAP control devices with these lower ESR capacitors, TI introduced the application solution of adding an R-C observer across the inductor to emulate an ESR, and then capacitively coupling that into the feedback, adding this emulated ESR ripple to the actual output ripple. Because of the components used, this is often referred to as RCC ripple injection.
D-CAP2 replaced this externally generated ripple with an internally generated, fixed time-constant ripple, that was capacitively coupled to the feedback. This eliminated the external components and simplified the design process while allowing D-CAP2 to support ultra-low ESR polymer capacitors and MLCCs.
The problem, however, was Constant On-time Control is a valley regulated control architecture, and the capacitively coupled ramp introduced an offset between the valley voltage of regulation and the average sensed output voltage. This made selection of feedback resistor more complicated since the ripple contributed to the effective reference voltage.
D-CAP3 improved on D-CAP2 with a combination of ramp offset cancelation to provide regulation accuracy and simply the feedback divider selection, along with selectable ramp amplitudes to allow users to select a ramp that provided the best trade-off between pulse frequency jitter of the switching frequency, and fast response to a transient. However, D-CAP3 continued to use fixed time-constant ramps.
The disadvantage of a fixed-time constant ramp is that both the Feedback Divider and the Inductor value contribute to the transient performance, and both increase with rising VOUT, this results in higher output voltages having slower transient responses than lower output voltages.
D-CAP4 improves on D-CAP3 by updating the ramp generation circuit to replace the fixed-time constant ramp with a variable time-constant ramp which is automatically adjusted for the output voltage or duty cycle in order to maintain the same ramp amplitude across a wide range of output voltages and a programmable forward gain-stage between the sensed output voltage at the FB pin, and the ramp comparator.
In the TPS54KB20, this produces a constant effective ramp amplitude during the inductor OFF time independent of the output voltage. This provides more consistent transient performance at low output voltage and high output voltage, without the need to modify the external circuit.
The low-frequency error integrator to cancel the ramp-offset is retained from earlier D-CAP3 in order to provide the most precise output voltage control possible.
Like all prior versions of D-CAP, D-CAP4 performs as a frequency stabilized, pulse-frequency modulation emulated current mode control scheme, providing the benefits of variable frequency, direct capacitor voltage regulation and the wide-band stable operation of current mode control.
If you need more specific details about the implementation of D-CAP4 in the TPS54KB20, please reach out to me through messages and we can discuss separately.
