TB3102

TB3102
Boost Converter Using the PIC12F1501 NCO Peripheral
Author:
PERFORMANCE SPECIFICATIONS
Mihnea Rosu
Microchip Technology Inc.
Electrical specifications over operating range:
3V ≤ VDD ≤ 5V.
INTRODUCTION
TABLE 1:
ELECTRICAL SPECIFICATIONS
This technical brief describes a digitally-controlled
boost power supply based on the PIC12F1501. The
implementation has very good efficiency at light loads,
hardware overvoltage protection and uses a very small
number of components.
Input Voltage Range
The peripherals needed for the application are:
•
•
•
•
•
Two 10-bit ADC channels
One Fixed Voltage Reference (FVR)
One Comparator (COMP)
One Numerically Controlled Oscillator (NCO)
One Complementary Waveform Generator
(CWG)
The peripherals are internally connected through
firmware, reducing the number of external pins to just
three.
FIGURE 1:
3-5
Volts DC
5
Volts DC
Output Current
2
Amperes
Output Power
10
Watts
Code Size
452
Words
Ram Size
31
Bytes
Efficiency
86.8%
Output Voltage
Measured at 2A
Available Code Size
572
Words
Available RAM Size
33
Bytes
BLOCK DIAGRAM
The output voltage and current are regulated using a
proportional control loop. Output values are read using
two ADC channels and the control signal is adjusted
accordingly. The NCO uses fixed-on-time pulses with
variable frequency to adjust the duty cycle Pulse
Frequency Modulation (PFM).
Figure 1 illustrates the block diagram of SMPS.
BLOCK DIAGRAM
VBAT
Load
CWG
CWG1A
PIC12F1501
OV prot
VOUT-div
COMP
NCO
FVR
ADC
Isense
SW Control
Button
 2013 Microchip Technology Inc.
DS90003102A-page 1
TB3102
The NCO control signal is routed through the CWG, so
that the auto-shutdown feature can be used together
with the comparator to provide hardware overvoltage
protection.
The control loop update rate is limited because of the
ADC conversion speed and computational power.
FIGURE 2:
EFFICIENCY
94.00 %
92.00 %
90.00 %
88.00 %
4V Input
86.00 %
3V Input
84.00 %
82.00 %
80.00 %
50 100 250 500 750 1000 1200 1400 1600 1800 2000
mA mA mA mA mA mA mA mA mA mA mA
FUNCTIONAL DESCRIPTION
The power supply uses the PIC12F1501 NCO and
ADC to implement a proportional control loop. For each
of the two ADC channels (voltage and current), four
samples are taken on each measurement. The values
are used to adjust the NCO frequency in order to adjust
the converter output. The NCO functions in Pulse
Frequency mode, having a “fixed-on-time” of 2 us. The
operating frequency is limited in firmware to about 275
kHz, resulting in a maximum duty cycle of 55%. For a
16 MHz clock source and a 2 us pulse length, the NCO
offers 15 bits of duty cycle resolution with a 16 Hz
frequency step.
Since only one control loop is running for both voltage
and current regulation, a special function decides on
each update which of the two needs to be regulated. In
normal operating conditions, the control loop tries to
match the output voltage to the reference value. If the
output current goes over the limit, the loop tries to
match the output current to the maximum allowed value
by reducing the output voltage. A special counter
prevents erratic behavior when transitioning from one
mode to the other.
The input voltage is connected to the microcontroller
using a small diode and is bootstrapped to the output.
This way, when the output voltage rises, it will power
the microcontroller and MOSFET driver. This is more
efficient because a higher VGS improves RDS(ON) and
the interval below 4.5V is problematic for most power
transistors. This also makes the FVR the only stable
reference available, and requires a few changes in the
way the output regulation is achieved. Instead of
reading the output voltage through the divider, the FVR
is read internally with the output voltage as a reference
(connected to VDD). The divider is still used for
hardware overvoltage protection and is calculated to
trip the comparator just above the desired output
voltage. The FVR voltage is used as a reference for the
comparator.
It is important to mention that the boost topology has a
clear DC path from the power source to the output,
through the rectifier diode, even if the switching
transistor is blocked. The current limiting loop can only
prevent overcurrent until the switching frequency
becomes zero. From this point on, catastrophic shortcircuit events can occur without an additional protection
switch. A second transistor can be placed on the output
low side to cut off the load if a short circuit occurs.
DS90003102A-page 2
 2013 Microchip Technology Inc.
TB3102
FIGURE 3:
DUTY CYCLE vs. FREQUENCY USING FIXED ON TIME
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XV
XV
3XOVH)UHTXHQF\PRGH
1&22XWSXW
APPLICATIONS
The proportional control loop regulating the output has
an update rate of only 4 kHz, which makes it slow to
respond to sudden load variations and input voltage
changes. For this reason, the output voltage is clamped
by a very fast comparator-based, overvoltage
protection mechanism.
On the other hand, this is a digitally-controlled power
supply which offers huge advantages. Output voltage
and current can be modified during run-time by the
application and complex algorithms, like multi-step
battery charging, it can be easily implemented. The
NCO peripheral allows 15 bits of duty cycle resolution
for the transistor control signal, which in turn allows
very fine control of output voltage and current.
The main application areas of this type of power supply
are battery chargers, LED drivers with current mode
dimming, thermoelectric cell drivers, programmable
bias generators and others. With an accurate voltage
reference, the circuit is more than adequate for
charging sensitive Li-Ion batteries.
 2013 Microchip Technology Inc.
DS90003102A-page 3
TB3102
MCU PERIPHERAL CONFIGURATION
DRAWING
The application needs only two analog channels. One
is for the output current and one is for the hardware
overvoltage protection. The output voltage ADC
channel is read internally from the FVR.
The input-only digital pin, MCLR, can be used for a
button or a similar purpose. During run-time, the
programming clock and data I/O pins (PGC and PGD)
are free for user-specific functionality (see Table 2):
TABLE 2:
Pin No.
PIC12F1501 PERIPHERAL
CONFIGURATION
Name
Function
1
VDD
Supply voltage
2
RA5
CWG output – transistor control
signal
3
AN3
Analog input (COMP) used for
clamping output voltage
4
RA3
Digital input (BUTTON) – can be
used to turn output on or off
5
AN2
Analog input (ADC) used for
reading the current value on the
shunt
6
PGC
Programming clock
7
PGD
Program data
8
VSS
Ground reference
FIGURE 4:
PERIPHERAL CONFIGURATION
CWG
RA5
CWG1A
PFM
AUTO
SHUTDOWN
NCO
COMP
(PFM)
RA4
2.048V
OV PROTECTION
FVR
FIRMWARE
FVR
2.048V
ADC
AN2
DS90003102A-page 4
RA2
ISENSE
 2013 Microchip Technology Inc.
TB3102
SCOPE PLOTS OF KEY
PARAMETERS
FIGURE 5:
Table 3 contains some characteristics of the charger
obtained with an input of 3V and an output of 5V.
OPERATING FREQUENCY
vs. LOAD CURRENT
300 kHz
250 kHz
200 kHz
TABLE 3:
POWER SUPPLY
CHARACTERISTICS
150 kHz
4V Input
3V Input
100 kHz
Output
Current
(mA)(2)
Sw. Frequency
(kHz)(3)
0
0.1-0.5
50
80.7
85.7
100
157.4
88.1
250
229.1
90.7
500
233.0
91
750
236.5
90.4
1000
239.3
89.5
1200
241.1
89.1
1400
244.5
88.5
1600
247.3
87.8
1800
250.6
87.3
2000
243.8
86.8
Efficiency
(%)(1)
50 kHz
0 kHz
50
mA
100
mA
250
mA
500
mA
750 1000 1200 1400 1600 1800 2000
mA mA mA mA mA mA mA
Figure 5 shows the converter operating frequency for
3V and 4V input at different output currents. Once the
inductor current becomes continuous, the frequency/
duty cycle changes very little, only to compensate for
component power losses. The power loss becomes
very easy to see on the efficiency graph (see Figure 2).
Note 1: Efficiency is calculated including power
loss on current shunt.
2: At 2A, the converter is running in Current
Limiting mode.
3: PWM is fixed-on-time (2 µs) with variable
frequency.
 2013 Microchip Technology Inc.
DS90003102A-page 5
TB3102
This algorithm is run 3906 times per second (TIMER0
overflow period) to regulate the power supply output
voltage and current. For each step, a decision is made
to regulate either current or voltage, since the values
cannot be over the set point, at the same time. In this
way, only one proportional control loop is used,
reducing the computational requirements. For
seamless transition between voltage limiting and
current limiting, a debouncing counter has to go down
FIGURE 6:
to zero in order to change from voltage to current
limiting. Overvoltage changes the working mode to
voltage limiting immediately.
For a better understanding of the implemented
algorithm, please see the following flowchart diagram
(Figure 6).
REGULATION ALGORITHM
START
Read ADC
iout
vout
iout > iref
cmode = 1
iout > iref
(de-bouncing)
OR
vout > vref
Regulate Current
pid(iout, iref)
vout > vref
cmode = 0
Regulate Voltage
pid(vout, vref)
STOP
GLOSSARY
TABLE 4:
PWM
ACRONYMS
Pulse-Width Modulation
ADC
Analog-to-Digital Converter
DAC
Digital-to-Analog Converter
NCO
Numerically Controlled Oscillator
PID
Proportional Integral Derivative
CWG
Complementary Waveform Generator
FVR
Fixed Voltage Reference
DS90003102A-page 6
 2013 Microchip Technology Inc.
TB3102
APPENDIX A:
FIGURE A-1:
SCHEMATIC
 2013 Microchip Technology Inc.
DS90003102A-page 7
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
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Printed on recycled paper.
ISBN: 9781620777527
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DS90003102A-page 8
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DS90003102A-page 9