Microcontroller housing two DC/DC boost converters

　　Batteries are the typical power source for portable system applications, and portable systems based on microcontrollers are not uncommon today. Various microcontrollers operate from low supply voltages. Therefore, one can use two AA or AAA batteries to power the circuit. However, if the circuit requires a higher voltage - such as the LED backlighting of an LCD, which requires about 7.5VDC, a suitable DC/DC converter must be used to increase the supply voltage from 3V to the required voltage. However, with the help of a few additional discrete components, one can also develop a suitable DC/DC boost converter using a microcontroller.

　　This Design Idea shows how to create not just one but two DC/DC converters using a tiny 8-pin microcontroller and a few discrete components. The design is scalable, and one can adapt it to a variety of output voltage requirements simply by changing the microcontroller's control software. One can even program the microcontroller to generate any necessary output voltage startup rate. Figure 1 depicts the basic topology of a boost switching regulator. In this type of regulator, the output voltage is greater than the input voltage. Boost switching regulators operate in either CCM (continuous conduction mode) or DCM (discontinuous conduction mode). It is easier to set up a circuit for DCM operation (Reference 2). The name comes from the fact that during each PWM period in DCM, the inductor current drops to 0 A for a period of time; in CCM, the inductor current is never 0 A. At the end of the high period at the PWM output (when the switch is on), the maximum current flowing through the inductor is: (

　　1)

　　where VDC is the input voltage, D is the duty cycle, T is the total cycle time, and L is the inductance value. The current through the diode drops to zero during the TR time.

　　  (2)

　　The load current is the average diode current.
　　  (3)

　　According to equations (1) and (2), it is simplified to:
　　 (4)

　　The output voltage VOUT is:
　　 (5)
　　The value of the output capacitor determines the ripple voltage. The capacitance value is:
　　  (6)

　　Where dV/dt represents the output voltage drop during the PWM signal, I is the load current, and C is the required output capacitance.
 
　　The total period of the PWM wave is T and is a system constant. D is the duty cycle of the PWM wave, and TR is the diode conduction time. At the end of TR, the diode current drops to 0A. For DCM, T>D×T+TR. The difference between the PWM period T and (D×T+TR) is the dead time.

　　The switch that operates the inductor is usually a BJT (bipolar junction transistor) or a MOSFET. MOSFET is the first choice because it can handle large currents, is more efficient, and switches faster. However, at low voltages, it is difficult to find a suitable MOSFET with a low enough gate-to-source threshold voltage and it can be expensive. Therefore, this design uses a BJT (Figure 2).

　　The microcontroller provides PWM frequencies from 10kHz to over 200kHz. A higher PWM frequency is desirable because it results in lower inductance values, which enables the use of small inductors. The Atmel Tiny13AVR microcontroller has a "fast" PWM mode with a frequency of approximately 37.5kHz and a resolution of 8 bits. The higher PWM resolution allows one to more closely track the desired output voltage. For a 20mH inductor, the maximum inductor current from equation (1) is 0.81A. The maximum collector current of the transistor switching this inductor should be greater than this value. The collector current limit of the 2SD789NPN transistor is 1A, making it suitable for this DC/DC converter. According to equation (4), the maximum load current that can be achieved with these values ​​is 54mA, thus meeting the maximum required load current requirement for a 7.5V output voltage. The

　　Tiny13 microcontroller has two high-speed PWM channels and four 10-bit ADC channels. Another PWM channel and one ADC channel create a second DC/DC converter for a 15V output voltage and a 15mA maximum load current. The value of the inductor for this converter is 100mH. To calculate the output capacitor value, equation (6) should be used. For 5mV ripple, the value of the capacitor for 7.5V output voltage is 270mF, since the output current is 50mA and the PWM time period is 27ms, the circuit uses the nearest larger value of 330mF. Similarly, for 15V output voltage, the required capacitor value is 81mF, so the design uses a 100mF capacitor. The

　　microcontroller is programmed in C language using the open source AVRGCC compiler (www.avRFreaks.net). The AVRTiny13 microcontroller operates at an internal clock frequency of 9.6MHz without an internal clock divider, so the PWM frequency is 9.6MHz/256=37.5kHz. The internal reference voltage is 1.1V. The main program alternately reads the two channels of the ADC, which monitors the output voltage in the interrupt routine. The main program executes an infinite loop, monitoring the output voltage by reading the ADC value and adjusting the PWM value accordingly.