Design of PCM-COT Buck-boost Converter with Dynamic Adjustable Output-Voltage

Article By : Sam Hu, Bo Yang Ph.D., and Yang Yang, M3 Technology Inc. (M3Tek)

To achieve best performance while keeping power consumption low, many systems need to adjust their supply voltage dynamically during operation.

What’s happening in the power management space amid the never-ending drive to lower power consumption in more and more complex technologies and applications? What about in applications dealing with higher and higher voltages? This month’s In Focus highlights the various design developments and manufacturing strategies happening in the power management segment.


To achieve best performance while keeping power consumption low, many systems need to adjust their supply voltage dynamically during operation. High supply voltage during high performance operation with best speed. Low supply voltage when system is in idle state to reduce leakage and switching current. This method is widely used in power management for memory such as DDRx/SSD, smartphones and computer system for CPU/GPU.

As mobile devices become ubiquitous, charging of mobile devices is a headache felt by all users. To solve this problem, the industry moved from low current USB BC1.2 to other proprietary charging protocols. With recent adoption of USB Type C PD, it is expected that we will converge to an universal charging interface for mobile devices, big and small.

In USB Type C PD 2.0, the bus voltage can be programed with communication channel (CC) between power source and power sink. This enables wide range of devices to be powered by USB Type C port. For example, a mobile phone can use 5V or 9V to supply power through USB Type C port. A notebook can program the bus voltage to 20V to get enough power too.

With the introduction of USB Type C PD 3.0, one important feature is the PPS(programmable power supply). This feature enables adjustable bus voltage (VBUS) with very fine step (20mV/step) through communication link between power souece and power sink. This feature could greatly reduce the thermal stress of battery charging system in mobile device by dynamically adjust the bus voltage to track the battery voltage.

The wide input and output voltage range and dynamic adjustable output voltage created a technical challenge for power management in USB Type C PD applications. The article will introduce a wide input range fully integrated 4-switch Buck-boost converter with dynamic adjustable the output-voltage. This converter is based on M3tek patented peak current mode constant off-time control (PCM-COT). This control method can enable a simple system design with minimal external components. The control method provides automatic and smooth transition between buck, buck-boost, or boost mode of operation based on input and output voltage to achieve excellent efficiency and output
regulation.

Application Schematic

Figure 1: The voltage source is injected to FB network to achieve the precise output voltage control.

Buck-Boost with Peak Current Mode Constant Off-Time Control

The MT5629 is a wide input range fully integrated 4-switch Buck-boost converter with up to 5A output current. Specifically, it operates in Buck mode when the input is higher than the output, and in Boost mode when the input is lower than the output. When the input and output are close to each other, the MT5629 operates in a proprietary Buck-boost mode that controls all four switches to modulate inductor ripple current hence achieves minimum output voltage ripple.

Next, a buck-boost converter with adjustable output voltage based on MT5629 Buck-Boost IC will be discussed in detail.

Figure 2: Functional block diagram for MT5629.

Figure 3: Buck-boost switch control.

During normal operation when each switching cycle starts, the peak current mode control circuit compares the sensed inductor current VCS with the control voltage VCTRL from the error amplifier.

Once the VCS reaches the VCTRL level, the control circuit terminates the main power switches, i.e. the HS of Buck mode (HS1) or the LS of Boost mode (LS2), and then immediately turns on the synchronous power switches, i.e. the LS of Buck mode (LS1) or the HS of Boost mode (HS2). The same time when the synchronous power switches are turned on, the off-time timers for Buck mode and Boost mode respectively start to count. When either timer times out, the main power switches, either HS1 or LS2 depending on operation mode, are turned on to initiate the next switching cycle.

When VIN is higher than VOUT by typ. 10% or more, the MT5629 operates in Buck mode. In this mode, during normal switching cycles, the LS2 switch remains off and the HS2 switch is kept fully on, while the HS1 and LS1 switches alternatively the same way as normal Buck converters. To keep HS2 fully on for the Buck mode operation, its floating supply BST2 voltage needs to be periodically recharged. MT5629 has built-in BST refresh function, which turns off HS2 and turn on LS2 for about 150ns in roughly every 200us for the BST2 voltage to be recharged.

Figure 4: Buck mode.

Figure 5: Actual waveforms in buck mode.

When VIN and VOUT are close to each other within typ. 10% range, the MT5629 operates in Buck-boost mode where all four switches are involved in switching in a specific order. First, HS1 is turned on to start the Buck mode where the HS1 and HS2 are on. The buck mode occupies about 90% of the switching cycle, then the Boost mode starts which turns off HS2 and turns on LS2. When the Boost mode is terminated by the peak current control circuit, the HS1 and LS2 are turned off and LS1 and HS2 are turned on which starts the off-timers. Once the Buck off-timer times out, the LS1 is turned off and the HS1 is turned on to start the new switching cycle.

Figure 6: Buck-boost mode.

Figure 7: Actual waveforms in buck-boost mode.

When VIN is lower than VOUT by typ. 10% or more, the MT5629 operates in Boost mode. In this mode, during normal switching cycles, the HS1 switch is kept fully on and the LS1 switch remains off, while the LS2 and HS2 switches alternatively the same way as normal Boost converters. To keep HS1 fully on for the Buck mode operation, its floating supply BST1 voltage needs to be periodically recharged. MT5629 has built-in BST refresh function, which turns off HS1 and turn on LS1 for about 150ns in roughly every 200us for the BST1 voltage to be recharged.

Figure 8: Boost mode.

Figure 9: Actual waveforms in boost mode.

VOUT Dynamic Adjust Circuit

A dynamically adjustable output can use a digital-to-analog converter (DAC) or other input voltage source. A typical VOUT dynamic adjust circuit is with a resistor RSUB connected between input voltage VS to VFB. The following analysis is carried out assuming that the EA (OTA) is an ideal one.

Figure 10: A subtractor with RSUB between VS and VFB.

From the Equation (2) it acts as a Subtractor by selecting proper values of RSUB, Rtop and Vs, we can achieve the output voltage dymanic adjustment provided with the value of VS if fixing the Rtop and RSUB ratio. The output voltage will increase when the VS<VFB. Whereas the VS >VFB (1V) can decrease the output voltage. Below Table 1 show the different configurations of output voltage and VS with Rtop=80.6K, RSUB=5K.

Table 1: USB PD-Type-C 2.0 recommend fix output voltage.

Example:

The circuit was tested in the laboratory under the following conditions:

  • VIN=12V;
  • VOUT=5V/12V/15V;
  • VS=0V to 1V;
  • RSUB=5k, Rtop=80.6k, Rbot=20k
  • Standard value 1% resistors were used

The trend of “calculated output data” and “test data” curve is close shown in following Figure 11.

Figure 11: Output voltage versus VS.

The comparison of calculated output values and test values are shown in Table 2.

Table 2: Output voltage calculated and test data.

Through this circuit, very accurate and linear output voltage adjustment can be achieved. In above test, input voltage and output voltage relationship also change from Vin> Vout to Vin=Vout and Vin<Vout.

The MT5629 with patented PCM-COT control adds a proprietary four-switch Buck-boost mode, thus provides a natural and smooth transition between the Buck mode and Boost mode operations.

Figure 12: Buck mode and buck-boost mode interconversion.

Figure 13: Buck-boost mode and boost mode interconversion.

Figure 14: Buck mode and boost mode interconversion.

Inductor DCR Current Sense and Designing Loop Compensation

To achieve the three modes flexible conversion and appropriate compensation, MT5629 requires sensing the inductor current for processing.

The MT5629 uses a unique design to sense the inductor DCR current that only requires three external capacitors, eliminating the need for external sense resistor and improves system power efficiency. The equivalent circuit of the current sense block is shown in fig. 15. For different choices on the inductor and its DCR value, the external capacitors can be calculated following below guidance:

Figure 15: The equivalent circuit of the current sense block.

First, calculated the required equivalent capacitance needed between CSP and CSN pin:

where RIND is the DCR of the inductor.

Then the recommended choice for the three external capacitors are:

The MT5629 employs Peak Current Mode Constant Off-Time (PCM-COT) control, thus it has similar loop transfer function as traditional peak current mode (PCM) controlled converters. Specifically, the power train of the inductor and output capacitor can be modeled as a first-order system consists of a voltage controlled current source and the output capacitor, thanks to the peak current regulation loop of the PCM control. For the MT5629, the transconductance of the voltage controlled current source is determined by the inductor DCR and the current sense gain. The MT5629 has three modes of operations: buck mode, boost mode, and buck-boost mode, which should all be considered when designing the loop compensation network (Figure 16).

Figure 16: Compensation network.

Due to the Right Half Plane (RHP) zero in boost mode operation, boost mode usually limits the achievable loop bandwidth. With this in mind, the typical loop bandwidth is recommended to about one fourth of RHP zero frequency.

In a boost converter, the RHP zero is given by:

Where RL is the equivalent load impedance, i.e. RL = VOUT/ILOAD; Dmax is the maximum duty cycle of the converter for its applicable applications. If we set the loop bandwidth to one fourth of the RHP:

The loop bandwidth of the MT5629 boost mode can also be obtained by:

To set loop bandwidth at one fourth of the RHP zero, we can calculate RZ as:

Once RZ is obtained, we can quickly calculate the CC1 and CC2:

Place the zero formed by RZ and CC1 to ~1/5 of the bandwidth to bring back the phase shift from the output pole:

Place the pole formed by RZ and CC2 to ~5x of the bandwidth to suppress high frequency noise:

Example: The below is calculated by above compensation formula.

C1=22nF, C2=C3=2.2nF, L=2.2µH (DCR=10mΩ)
RZ=5.1k, CC1=2.2nF, CC2=100pF, VIN=12V, VOUT=9V/12V/15V, P=30W

Figure 17: Bode waveform in buck mode.

Figure 18: Bode waveform in buck-boost mode.

VIN=12V, VOUT=15V, IOUT=2A

Figure 19: Bode waveform in boost mode.

Following the above guideline of the compensation formula to design, the MT5629 phase margin is over 45deg in three modes with bode plot measurement, and the stability of MT5629 are pass in the three modes.

Conclusion

For many USB Type C PD applications, there are two very challenge technical requirements: wide input/output voltage range with input and output voltage cross over in operation and dynamic adjustable output voltage. MT5629 is designed to tackle these two challenges with simple circuit and robust operation.

MT5629 is based on M3Tek’s patented constant toff time control method for 4-switch Buck-boost converter. It greatly simplified system circuit, provide smooth transition between different operation modes to deal with all Vin/Vout conditions with excellent efficiency and output regulation.

A Vout dynamic adjust circuit is also discussed. It receives a control voltage from USB Type C PD Controller and dynamically adjust the FB voltage of MT5629. From laboratory test, excellent linearity and accuracy are achieved. Together with MT5629, a power converter can satisfy the most demanding USB Type C PD application is realized with simple system circuit, reliable operation and excellent efficiency and output regulation.

 

Reference

  1. “MT5629 28VIN 6A Synchronous 4-Switch Buck-Boost Converter” M3tek Datasheet

 

About the Author

Sam Hu is Director, AE Department, for M3 Technology Inc. (M3Tek). 

Bo Yang Ph.D. is the director for R&D at M3Tek.

Yang Yang is an associate manager for the AE Department at M3Tek.


 

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