Novel soft-start circuit design and its application in Bluetooth amplifier

Bluetooth is an open, short-range wireless communication technology standard. Its power amplifier is the module with the highest power consumption in the Bluetooth wireless transmitter. In order to reduce the power consumption of the Bluetooth system and extend the battery life, it is required to achieve an output power step of 2 to 8 dBm. There are two main types of power control circuits for commercial power amplifiers: direct closed-loop control and indirect closed-loop control. The indirect closed-loop control method is widely used due to its high integration and low cost. Considering factors such as integration and cost, the literature does not detect the output power, but predicts the power supply voltage corresponding to the output power level, and then uses the structure of a voltage regulator to control the power supply voltage to indirectly achieve power control.

When the circuit is just started, if the voltage regulator generates a large capacitance when it starts or the VMP tube has a small resistance when it is turned on, the surge current will be very large, sometimes close to the current limit of the voltage regulator, so it must be limited

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The indirect closed-loop control loop is shown in Figure 1. The control loop includes an error amplifier, a feedback loop and an adjustment tube (VMP tube). The working principle is that the error amplifier compares the output feedback voltage VFB with the reference voltage Vref and amplifies the difference (Vref-VFB) to control the conduction state of the adjustment tube (VMP) to achieve negative feedback, thereby obtaining a stable output Vcon. When the circuit is just started, the feedback voltage VFB is very low. If a large capacitance is generated when the regulator starts (or the load capacitance is large) or there is a small resistance when the VMP tube is turned on, the surge current (i.e., the instantaneous current flowing through the VMP tube) will be very large, sometimes close to the current limit of the regulator. Therefore, a soft start circuit must be designed in the error amplifier to ensure that the entire circuit enters a normal DC bias state after power-on and does not generate surge current.

The main circuit of the closed-loop control system is shown in Figure 2. VM1~VM8 form a folded error amplifier, and VM9~VM10 form a single-stage current mirror as a load. The advantage of this is that the system can be stabilized by using a compensation capacitor of about 10 pF. VM5~VM14 and VQ form an interstage buffer stage. R1 and R2 form a feedback network, where

Since the output impedance of the N2 node (VMP tube gate) is very small, the parasitic capacitance of the N2 node is discharged in a very short time at the moment of conduction, and the voltage of the N2 node changes from high to low. This will cause a large surge current to flow through the PMOS adjustment tube, up to 1.45 A. This current has a great impact on the performance of the system. For example, if there is a parasitic resistance at the input end, the transient current will cause a large voltage drop difference, causing the system to not work properly, and will also increase the system noise, so it is necessary to limit it.

2 Soft start circuit design

From the above analysis, it can be seen that due to the rapid change of the voltage at the N2 node, a large surge current flows through the P power tube (VMP tube). Therefore, the current can be controlled by simply controlling the change of the voltage at the N2 node. Controlling the change of the surge current is nothing more than controlling the slope and peak value of the change. The specific circuit schematic includes two parts, namely the slope control part and the peak value control part of the surge current. The specific implementation circuit is shown in Figure 3.

2.1 Control of surge current slope

Transistors VM22 and VM23 form a differential pair, VM16 and VM17, VM18, VM19, VM20, VM28 and R2 form a negative feedback loop, so the voltage of the B node will increase as the voltage of the C node increases, and eventually the voltage values ​​of the B and C nodes will be equal.

When VM25 is off and VM24 is on, that is, the input voltage of the control terminal Vcontrol is high, the voltage of capacitor C1 is 0 V; when Vm25 is on and Vm24 is off, that is, the input voltage of the control terminal Vcontrol is low, the current source I3 will charge the capacitor. At this time, the voltage of node C is Vc=I3t/C. Assuming that the current flowing through the VMP1 tube is I, the voltage of node B is VB=IR2, and because VC=VB, I=I3×t/R2C. The power tube VMP tube and the VMP1 tube form a current mirror, thereby indirectly realizing the linear control of the surge current.

2.2 Control of surge current peak value

Because VA=IR2+VGS(M28), when the voltage of VA is greater than the sum of the threshold voltage of VM18 and VGS(M26), VM18 will be turned on; note that VM16, VM17, VM18, and VMP1 form a negative feedback loop, so the loop eventually reaches stability and the current I will be controlled at a certain value, which also indirectly realizes the control of the peak current. The maximum value is determined by R2, VM28, VM26, and VM18.

3 Simulation results and discussion

Figure 4 shows the current flowing through the power tube VMP when the system is just started. Figure 4 (a) shows the transient current flowing through the power tube P when there is no soft start circuit, and its peak current is as high as 1.45 A. Figure 4 (b) shows that the soft start circuit successfully controls the transient current from the slope and peak value respectively. Its peak value is only 520 mA, and the rising slope is almost a constant value, but this will increase the response time of the system. Therefore, the slope and peak value of the surge current can be determined according to other requirements of the application system.

4 Conclusion

Aiming at the surge current phenomenon generated by the power control circuit of the Bluetooth power amplifier at startup, a novel surge current control circuit is designed from the two aspects of the surge current peak value and the rising slope. It has the advantages of simple structure. At the same time, the control principles of the surge current slope and peak value are analyzed respectively, and finally the surge current is controlled and soft start is successfully realized.