Question: How do I increase the output power of the Switch Mode Pump (SMP) in PSoC 1?
PSoC 1 has a SMP that can boost the voltage of a single cell to 3.0 to 5.0 V. Figure 1 shows the SMP implementation in PSoC 1.
Figure 1. PSoC 1 SMP
PSoC 1 has an internal MOSFET switch and the feedback circuit to implement the boost converter. An external inductor, diode and capacitor are the only components required to implement the boost converter. However, due to the limitations of the internal MOSFET, the maximum load current that can be expected from the SMP is only 50 to 70 mA.
In many applications like LED drivers, wireless transmitters, etc., there may be a requirement for higher current from the boost converter. This can be achieved by using an external MOSFET switch with higher drain current. Figure 2 shows the block diagram of this setup.
Figure 2. Modified SMP Block Diagram
As the output of the internal MOSFET switch is active low, an external inverter is used. The output of the inverter drives the external MOSFET switch. Figure 3 below shows the complete schematic diagram.
Figure 3. Schematic Diagram
Q1 is the external MOSFET switch which has a drain current rating of 1.2 A and RDSON of 0.25Ω. Any other MOSFET with a similar rating can be used as well. L1 is a 10 uH inductor with a current rating of 1.75 A. D1 can be any Schottky diode with a forward current rating of 1 A.
The PSoC SMP operates at 1.3 MHz, 50 percent duty cycle. Regulation of the output voltage is achieved by turning on or off the gate drive (not by pulse width modulation). For a continuous mode-switching converter, the following equation determines the output voltage.
From this equation, it is clear that for a 50 percent duty cycle, the maximum output voltage that can be achieved is twice the input voltage, which is not enough. A higher step up ratio can be achieved by the following technique. R3 in Figure 3 is chosen such that the rising edge of the SMP pin is slowed down due to the input capacitance of the inverter U1 and the output capacitance of the SMP pin. This in turn results in a higher duty cycle. Figure 4 shows the waveforms of the SMP pin and the drain of the external MOSFET switch.
Figure 4. Waveforms at SMP Pin and Drain of External MOSFET
Due to the slow rising edge of the SMP pin, the effective duty cycle now is about 70 percent. With 70 percent duty cycle, the step up ratio now can be as high as 3.33.
With the circuit shown in Figure 3, following performance numbers were achieved.
- SMP operation was stable for input voltages above 1.7 V and load currents upto 500 mA
- When output voltage is set to 5.0 V and the load current is 500 mA, the input voltage has to be at 2.0 V for reliable startup.
- For load currents between 50 mA and 500 mA, efficiency numbers of 70 to 80 percent were observed. Higher output currents can be achieved with an appropriately rated MOSFET and inductor.
Thus, with a higher rated external MOSFET switch, an inverter and higher rated inductor and diode, the PSoC 1 Switch Mode Pump can be modified to provide a higher current output of up to 0.5 A.