The huge market and development potential of current consumer electronics products has led to the rapid development of low-power, low-power, high-efficiency, small-volume, lightweight DC-DC converters for battery-powered portable products. For many electronic systems used in portable products, such as color LCD displays, cell phone backlights, etc., DC / DC is an ideal power conversion device.
In this paper, a current-controlled PFM Boost DC-DC switching converter chip is designed based on 2μm 15V bipolar process. The loss of external feedback resistor is reduced by using low feedback resistor technology, and the duty cycle of the system is adjusted by load current feedback technology. To reduce the output voltage and current ripple coefficient of the system steady state. The chip adopts the Fixed-On-Time control mode, which is in the discontinuous conduction mode (DCM) of the Boost PFM when the whole system is in steady state, and this working mode has natural stability.
1, circuit system structure design
The system adopts the typical current-controlled PFM Boost DC-DC converter topology as shown in Figure 1. The schematic block diagram of the chip is shown in the dotted line frame, and the peripheral device is connected as a schematic diagram. Among them, STDN is the enable end of the chip, when the low level is off, the whole chip is turned off to reduce the static power consumption; SENSE is the output voltage feedback sampling end; VFB is the load current feedback sampling end; DRIVE is the external power switch control end; the reference voltage is passed The resistor divider generates the reference voltage VRA2 of the A2 comparator; the reference voltage of the A1 comparator is VRA1; A1 and A2 control a transient steady state of 1.7μs through a two-terminal NAND gate; the output stage DRIVE drives the external power. Tube QT.
The system will operate in two states: continuous conduction mode (CCM) and discontinuous conduction mode (DCM). When VIN is powered up, STDN is set high, and the reference source is the comparison reference voltage provided by the A2 comparator to VRA2. Since the system just starts, A1 and A2 output high level, the monostable circuit does not trigger, the output is high level, and the external power tube QT is turned on. When VSENSE>VRA1, A1 outputs low level, the monostable circuit triggers, the DRIVE voltage is quickly pulled low, and the external C2 is started to be charged. The system will repeat the above process before the voltage across RS2 does not reach A2 to compare the reference voltage. Work in continuous conduction mode. When the voltage across RS2 exceeds A2 comparison voltage VRA2, the A2 comparator outputs a low level, the monostable circuit triggers, and the external power transistor is turned off. From then on, L is charged to C2 within 1.7μs, when L is discharged, C2 starts to discharge, causing the voltage across RS2 to still exceed the A2 comparison voltage, A2 output low level, the monostable circuit continues to output low level, the external power tube continues to be in the off state, and the system works in discontinuous conduction mode. The system starts to boost to continuous conduction mode, and the system is in discontinuous conduction mode after entering steady state.
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