Tiny microcontroller hosts dual dc/dc-boost converters

Batteries are the typical power sources for portable-system applications, and it is not unusual these days to find microcontroller-based portable systems. A variety of microcontrollers operates at low power-supply voltages, such as 1.8V. Thus you can employ two AA or AAA cells to power the circuit. However, if the circuit requires higher voltage—for LED backlighting for an LCD, for example, which requires approximately 7.5V dc—you must employ a suitable dc/dc converter to boost the power-supply voltage from, for example, 3V to the required voltage. However, you can also employ a microcontroller to develop a suitable dc/dc-boost-voltage converter (Reference 1) with the help of a few additional discrete components.

This Design Idea shows how to create not just one, but two dc/dc converters with just a tiny eight-pin microcontroller and a few discrete components. The design is scalable, and you can adapt it for a wide range of output-voltage requirements just by changing the control software for the microcontroller. You can even program the microcontroller to generate any required output-voltage start-up rate. Figure 1 shows the basic topology of a boost switching regulator. The output voltage in such a regulator is more than the input voltage. The boost switching regulator operates 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 the inductor current falls to 0A for some time during each PWM period in DCM; in CCM, the inductor current is never 0A. The maximum current passes through the inductor at the end of high period of the PWM output (when the switch is on) and is:

The total period of the PWM wave is T and is a system constant. D is the duty cycle of the PWM wave, and T_R is the time during which the diode conducts. At the end of T_R, the diode current falls to 0A. The period of the wave is T>D×T+T_R for DCM. The difference of the PWM period, T, and (D×T+T_R) is the dead time.

The switch that operates the inductor is usually a BJT (bipolar-junction transistor) or a MOSFET. A MOSFET is preferable because of its ability to handle large current, better efficiency, and higher switching speed. However, at low voltages, a suitable MOSFET with low enough gate-to-source threshold voltage is hard to find and can be expensive. So, this design uses a BJT (Figure 2).

Microcontrollers offer PWM frequencies of 10 kHz to more than 200 kHz. A high PWM frequency is desirable because it leads to a lower inductor value, which translates to a small inductor. The Tiny13 AVR microcontroller from Atmel has a “fast” PWM mode with a frequency of approximately 37.5 kHz and a resolution of 8 bits. A higher PWM resolution offers the ability to more closely track the desired output voltage. The maximum inductor current from Equation 1 is 0.81A for a 20-µH inductor. The transistor that switches the inductor should have a maximum collector current greater than this value. A 2SD789 NPN transistor has a 1A collector-current limit, so it is suitable for this dc/dc converter. The maximum load current achievable with these values, from Equation 4, is 54 mA and thus meets the requirement of maximum required load current for an output voltage of 7.5V.

The Tiny13 microcontroller boasts two high-speed PWM channels and four 10-bit ADC channels. Another PWM channel and an ADC channel create the second dc/dc converter for an output voltage of 15V and a maximum load current of 15 mA. The inductor for this converter has a value of 100 µH. To calculate the output-capacitor value, use Equation 6. For a 5-mV ripple, the value of the capacitor for 7.5V output voltage is 270 µF, because the output current is 50 mA and the PWM-time period is 27 µsec, so this circuit uses the nearest larger value of 330 µF. Similarly, for the 15V output voltage, the required capacitor value is 81 µF, so the design uses a 100-µF capacitor.

The programs for the microcontroller are in C and use the open-source AVR GCC compiler. The AVR Tiny13 microcontroller operates at an internal clock frequency of 9.6 MHz without an internal-clock-frequency divider, so the PWM frequency is 9.6 MHz/256=37.5 kHz. The internal reference voltage is 1.1V. The main program alternately reads two channels of ADCs that monitor the output voltages in an interrupt subroutine. The main program executes an endless loop, monitoring the output voltage by reading the ADC values and adjusting the PWM values accordingly.

 #include<avr/interrupt.h>
 #include<avr/io.h>

 #define vref 931 // 931 is 1V for a 1.1 internal reference
 #define MAX_A 70
 #define MAX_B 128

 volatile unsigned int adc_val[];

 SIGNAL (SIG_ADC)
 {
 unsigned int temp, temp1;
 unsigned char ch;

 ADCSRA &= ~(<<ADEN);//ADC disabled
 temp1=ADCL;
 temp = ADCH;
 temp = temp << ;
 temp =temp | temp1;

 ch = ADMUX & 0x01;//channel number
 //ch = ch ^0x01;
 //ADMUX = ch | 0x40;

 if(ch==)
 adc_val[]=temp;
 else adc_val[]=temp;

 ADMUX = ADMUX ^ 0x01; // toggle last bit of MUX to change ADC channel

 ADCSRA = 0b11101100; //enable ADC
 ADCSRA = 0b11101100;

 ch = PORTB ^ 0b00000100;
 PORTB = ch;

 }

 int main(void)
 {

 unsigned int ad_temp, temp;

     DDRB = 0b00000111;

     TCCR0A = 0b10100011;
 //Fast PWM on OC0A and OC0B
 //    TCCR0A - Timer/Counter Control Register A
 //    ---------------------------------------------------------
 //    |COM0A1|COM0A0|COM0B1|COM0B0|  ?  |  ?  | WGM01| WGM00|
 //    |  7   |  6   |  5   |  4   |  3   |  2   |  1   |  0   |
 //    ---------------------------------------------------------

     TCCR0B = 0b00000001;
 //Fed by System Clock of 9 600 000 divided by 1 = 9.6 MHz
 //    TCCR0B - Timer/Counter Control Register B
 //    -------------------------------------------------
 //    |FOC0A|FOC0B|  ? |  ? |WGM02| CS02| CS01| CS00|
 //    |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
 //    -------------------------------------------------

     ADMUX = 0b01000010; //ADC2 is input to ADC
 //    ADMUX - ADC Multiplexer Selection Register
 //    -------------------------------------------------
 //    |  -  |REFS0|ADLAR|  -  |  -  |  -  | MUX1| MUX0|
 //    |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
 //    -------------------------------------------------

     ADCSRA = 0b11101100;    //Sampling rate is system clock divided by 16
 //    ADCSRA - ADC Control and Status Register A
 //    -------------------------------------------------
 //    |ADEN |ADSC |ADATE| ADIF| ADIE|ADPS2|ADPS1|ADPS0|
 //    |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
 //    -------------------------------------------------

     ADCSRB=;

     DIDR0 = 0b00011000;    //Disable Digital input buffer on PB3 and PB4
 //    DIDR0 - Digital Input Disable Register 0
 //    -------------------------------------------------
 //    |  ? |  ? |ADC0D|ADC2D|ADC3D|ADC1D|AIN1D|AIN0D|
 //    |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
 //    -------------------------------------------------
 adc_val[]=;
 adc_val[]=;
 sei();

 OCR0A =MAX_A;
 OCR0B = MAX_B;

 while()
     {
         ad_temp = adc_val[];

         if(ad_temp < vref)
             {

                     OCR0A++;

                 if( OCR0A > MAX_A)
                     OCR0A = MAX_A;

             }
         ad_temp = adc_val[];
         if(ad_temp > vref)
             {
                     OCR0A--;

                 if( OCR0A > MAX_A)
                     OCR0A = ;

             }

         ad_temp = adc_val[];

         if(ad_temp < vref)
             {

                     OCR0B++;

                 if( OCR0B > MAX_B)
                     OCR0B = MAX_B;

             }
         ad_temp = adc_val[];
         if(ad_temp > vref)
             {
                     OCR0B--;

                 if( OCR0B > MAX_B)
                     OCR0B = ;

             }

     for(temp=; temp <; temp++)
         {    temp = temp +;
             temp = temp -;
         }

     }
 }

 #include<avr/interrupt.h>
 #include<avr/io.h>

 #define voltref 931 // 931 is 1V for a 1.1 internal reference
 #define MAX_A 70
 #define MAX_B 128

 volatile unsigned int adc_val[];

 SIGNAL (SIG_ADC)
 {
 unsigned int temp, temp1;
 unsigned char ch;

 ADCSRA &= ~(<<ADEN);//ADC disabled
 temp1=ADCL;
 temp = ADCH;
 temp = temp << ;
 temp =temp | temp1;

 ch = ADMUX & 0x01;//channel number
 //ch = ch ^0x01;
 //ADMUX = ch | 0x40;

 if(ch==)
 adc_val[]=temp;
 else adc_val[]=temp;

 ADMUX = ADMUX ^ 0x01; // toggle last bit of MUX to change ADC channel

 ADCSRA = 0b11101100; //enable ADC
 ADCSRA = 0b11101100;

 ch = PORTB ^ 0b00000100;
 PORTB = ch;

 }

 int main(void)
 {

 unsigned int ad_temp, temp, vref;

     DDRB = 0b00000111;

     TCCR0A = 0b10100011;
 //Fast PWM on OC0A and OC0B
 //    TCCR0A - Timer/Counter Control Register A
 //    ---------------------------------------------------------
 //    |COM0A1|COM0A0|COM0B1|COM0B0|  ?  |  ?  | WGM01| WGM00|
 //    |  7   |  6   |  5   |  4   |  3   |  2   |  1   |  0   |
 //    ---------------------------------------------------------

     TCCR0B = 0b00000001;
 //Fed by System Clock of 9 600 000 divided by 1 = 9.6 MHz
 //    TCCR0B - Timer/Counter Control Register B
 //    -------------------------------------------------
 //    |FOC0A|FOC0B|  ? |  ? |WGM02| CS02| CS01| CS00|
 //    |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
 //    -------------------------------------------------

     ADMUX = 0b01000010; //ADC2 is input to ADC
 //    ADMUX - ADC Multiplexer Selection Register
 //    -------------------------------------------------
 //    |  -  |REFS0|ADLAR|  -  |  -  |  -  | MUX1| MUX0|
 //    |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
 //    -------------------------------------------------

     ADCSRA = 0b11101100;    //Sampling rate is system clock divided by 16
 //    ADCSRA - ADC Control and Status Register A
 //    -------------------------------------------------
 //    |ADEN |ADSC |ADATE| ADIF| ADIE|ADPS2|ADPS1|ADPS0|
 //    |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
 //    -------------------------------------------------

     ADCSRB=;

     DIDR0 = 0b00011000;    //Disable Digital input buffer on PB3 and PB4
 //    DIDR0 - Digital Input Disable Register 0
 //    -------------------------------------------------
 //    |  ? |  ? |ADC0D|ADC2D|ADC3D|ADC1D|AIN1D|AIN0D|
 //    |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
 //    -------------------------------------------------
 adc_val[]=;
 adc_val[]=;
 sei();

 OCR0A = ;
 OCR0B = ;

 vref =;

 while()
     {

         ad_temp = adc_val[];

         if(ad_temp < vref)
             {

                     OCR0A++;

                 if( OCR0A > MAX_A)
                     OCR0A = MAX_A;

             }
         ad_temp = adc_val[];
         if(ad_temp > vref)
             {
                     OCR0A--;

                 if( OCR0A > MAX_A)
                     OCR0A = ;

             }

         ad_temp = adc_val[];

         if(ad_temp < vref)
             {

                     OCR0B++;

                 if( OCR0B > MAX_B)
                     OCR0B = MAX_B;

             }
         ad_temp = adc_val[];
         if(ad_temp > vref)
             {
                     OCR0B--;

                 if( OCR0B > MAX_B)
                     OCR0B = ;

             }

     vref=vref +;
     if (vref > voltref)
         vref = voltref;

     for(temp=; temp <; temp++)
         {    temp = temp +;
             temp = temp -;
         }

     }
 }