Microchip MIC24066 Buck Power Supply Design: How to Implement Adaptive On-Time Control?

Semiconductor Integrated Circuits (ICs)
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A Buck power supply converts the high input voltage (VIN) into a lower, stable output voltage (VOUT) by turning the high-frequency switching transistor (MOSFET) on and off. The output voltage is determined by the on-time (the duration the switching transistor is on) and off-time (the duration the switching transistor is off) of the switching transistor.

Adaptive on-time control offers the following advantages:

  • The output voltage remains stable even when the input voltage changes.
  • Higher efficiency over a wide input voltage range.
  • Fast transient response.

Core Working Principle of Adaptive On-Time Control

We will take the Microchip MIC24066T switching buck regulator as an example.

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(Image source: Microchip)

1. Determination of On-Time (TON): Fixed Estimation

The internal On-Time generator produces an approximately fixed TON based on VIN and VOUT. The equivalent formula is: TON(EST) = VOUT / (VIN × fSW)

2. Adjustment of Off-Time (TOFF): Adaptive Feedback

The off-time is dynamically controlled by the feedback voltage (FB), following this logic:

  • The feedback voltage (FB) is collected via resistor voltage division (R1/R2) at the output terminal and compared with the internal 0.6V reference voltage (VREF).
  • When VFB < VREF (indicating the output voltage is too low), the error comparator triggers the next on-cycle, and the off-time ends early.
  • When VFB ≥ VREF, the low-side MOSFET remains on, and the off-time continues.

Key Limitation: If the TOFF calculated based on feedback is shorter than the minimum off-time (TOFF (MIN), typical value 300ns), TOFF (MIN) will be used compulsorily. This ensures the Bootstrap capacitor (CBST) has sufficient time to charge (to provide drive voltage for the next turn-on of the high-side MOSFET, avoiding MOSFET damage due to insufficient drive).

3. Maximum Duty Cycle (DMAX): Determined by Minimum Off-Time

Duty cycle（D）= TON / (TON + TOFF). Since TOFF cannot be shorter than 300ns, the maximum duty cycle is limited by this constraint. The formula is as follows:D_{MAX} = \dfrac{T_{S} \ - \ T_{off(MIN)}}{T_{S}} = 1 - \dfrac {300ns}{T_{S}}

(Where T_{S} = \dfrac{1} {f_{SW}}, the switching period)

Note: At high frequencies, the limitation of minimum off-time is more likely to become a bottleneck, narrowing the output voltage adjustment range.

Final Summary: Advantages of Adaptive On-Time Control

  1. The output voltage remains stable when the input voltage changes.
  2. Higher efficiency over a wide input voltage range.
  3. Fast transient response.

Although adaptive on-time control can maintain a stable output when the input voltage changes, the FB ripple must meet the amplitude and phase requirements; otherwise, the stability of the power supply will be affected.

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