A Simple Circuit For Driving Microcontroller Friendly Pwm Generator--91085A.pdf

TB085

A Simple Circuit for Driving Microcontroller

Friendly PWM Generators

Because the MCP1630 does not contain an on-chip

oscillator, its application in non-intelligent or dumb

power supply designs, is limited. However, marrying

the MCP1630 with a small pin-count microcontroller

solves the oscillator problem and opens up possibilities

for other features such as:

• Variable pulse frequency soft-start

• External shutdown control

• Under-voltage lockout

• Over temperature shutdown

Author:

Keith Curtis

Microchip Technology Inc.

INTRODUCTION

The recent interest in intelligent power supplies has

driven the development of a new class of microcontrol-

ler friendly PWM generators. These PWM generators

are the mixed signal control blocks for Switch mode

power supplies. One such device is the MCP1630. The

MCP1630 contains the 3 main elements for designing

a switching power supply as a peripheral to a microcon-

troller: a set/reset flip-flop, a high-speed voltage

comparator and an op amp to implement the error

amplifier (see Figure 1).

The microcontroller controls the MCP1630 through its

clock input. The frequency of the clock determines the

pulse frequency of the PWM output, and the duty cycle

of the clock limits the maximum PWM duty cycle of the

output. Control of the duty cycle between 0 and the

maximum set by the clock input is determined by the

current feedback to the comparator and the output of

the error amplifier (see Figure 2).

THEORY OF OPERATION

This technical brief will examine a design which

combines the PIC10F206, a 6-pin SOT-23 microcon-

troller, with the MCP1630. The power supply design

presented is a full proportional-feedback continuous

inductor current, current-mode, boost power supply

generating 15V out at .25 amps from a 9 V DC input. The

PIC10F206 generates the clock for the MCP1630 and

through that control, implements the previous list of

features.

FIGURE 1:

TYPICAL SWITCH MODE POWER SUPPLY BLOCK DIAGRAM

Vsupply

MCP1630

Flip-Flo p

Voutput

Clock

Input

–

Op Amp

Reference

–

Comparator

M icrocontrolle r

 2004 Microchip Technology Inc.

DS91085A-page 1

TB085

FIGURE 2:

MCP1630 HIGH-SPEED PWM

V I N

Overtemperature

V IN

0.1 µ A

UVLO

V EXT

OSC IN

V IN

G N D

100 k

COMP

Comp

V IN

Latch Truth Table

V REF

E A

2.7V Clamp

HARDWARE

The temperature sensor is implemented using a digital

output device that pulls its output low when the thresh-

old temperature is exceeded. The software monitors

the input to detect an over-temperature condition and

shuts down the pulse output if the temperature goes to

high. When the temperature falls back below the

threshold temperature, the sensor output returns high

and the software soft-starts the pulse output. Hystere-

sis built in the temperature sensor prevents chattering

and the sensor’s trip temperature is preset when the

sensor is manufactured.

The shutdown input, GP1, is tied to whatever remote

start-up logic is desired. The software polls the input to

determine if a shutdown is requested and terminates

the pulse output if the input is low. Raising the input

restarts the supply.

The PIC10F206 is well suited for this function. It has an

on-chip voltage comparator for the under-voltage

detect and it has sufficient I/O to control the MCP1630

and monitor the external inputs.

The microcontroller monitors the inputs and generates

the 250 kHz clock, all in software. Because the controls

are simple, the control circuit only needs the microcon-

troller and a few components to implement all the con-

trol functions. Figure 3 shows the resulting schematic.

Microcontroller inputs are connected to a divided sup-

ply voltage, a digital temperature sensor and the shut-

down input. The remaining output is the output driving

the MCP1630 clock input.

For under-voltage detection, the divided supply voltage

is routed to the non-inverting input of the comparator.

The inverting input is tied internally to the on-chip 0.6V

reference. The software then monitors the comparator

output to detect an under-voltage condition.

DS91085A-page 2

 2004 Microchip Technology Inc.

+9V

+5V

LM3480-5

Out

Com

0.1

+9V

+C 17

C 13

+9V

+5V

10K

0.1

0.1 µ F

Input

PIC10F206

U MCP1630

B230

C 14

1.0 µ F

C 15

C 16

1000 pF

8.2K

V DD

IRLML2502

V DD

1.0 µ F

GP0/C+

GP1/C-

GP2/T0C KI/CO

GP3/MCLR

OSC

V REF

COMP

DRVR

CFB

VFB

100

V SS

15 V DC

Output

1000 pF

C 12

1500 pF

0.56

2.4K

220

+5V

100 p F

C10

C11

R10

TC6501

510 Ω

100K

V DD

.033 µ F

Tovr

Hyst

G ND

+5V

0.1

V IN

V OUT

V SS

MCP1525

TB085

SOFTWARE

FIGURE 6:

CODE LISTING 2

Soft_Start

MOVLW

The software monitors the inputs and generates the

output pulse using a simple bit-set/bit-clear loop,

expanded to interleave all the input testing. By keeping

the bit-set to bit-set time to 4 cycles, the output duty

cycle is locked to 25% for a 250 kHz clock. The latency

time for a shutdown is 16 cycles. Figure 4 shows the

code listing.

.32

;table of 32

MOVWF

counter

MOVLW

Last-Table

;set to last

MOVWF

pointer

Loop

MOVF

counter,w

;reload delay

MOVWF

count

Delay

;generate delay

NOP

DECFSZ count,f

FIGURE 4:

CODE LISTING 1

;decrement count

loop

BSF

GOTO

Delay

;repeat til done

PWM

MOVF

pointer,w

;get pntr 4 jump

BCF

PWM

;generate a pulse

ADDWF

PCL,f

;jump

BTFSS

CMPOUT

;test 4 low Vin

Table

BSF PWM

BCF PWM ;32 pulse

GOTO $+1 ;2 cycle delay

|-----------;29 copies of pulse + delay

BSF

GOTO

Low_volt

;if low shutdown

BSF

PWM

BCF

PWM

;generate a pulse

BTFSS

GP1

;test 4 hi temp

GOTO

High_temmp ;if hi shutdown

PWM

BSF

PWM

BCF

PWM

;2nd pulse

BCF

PWM

;generate a pulse

GOTO

$+1

BTFSS

GP3

;test 4 shutdown

Last

BSF

GOTO

shtdwn

;if shutdown

PWM

BSF

PWM

BCF

PWM

BCF

PWM

;generate a pulse

GOTO

$+1

GOTO

loop

;infinate loop

DECF

pointer,f

;add a pulse

DECF pointer,f

DECFSZ counter,f

The soft-start function is generated by ramping up the

number of output pulses. At start-up, a single pulse is

followed by a long delay. Next, 2 pulses are followed by

a shorter delay, then 3, 4 and so on until the pulse chain

is continuous.

The soft-start code is implemented as a table of bit-set/

bit-clear/delay instructions, similar to code listing 1 with

a delay and control section. Figure 5 shows the timing

of soft-start and Figure 6 is an excerpt from the actual

code.

;decrease delay

GOTO

Loop

;if 10, continue

loop_forever

;if 0, goto main

CONCLUSION

Using a combination of software and simple hardware,

an efficient control for a PWM generator is imple-

mented with many of the features found in more com-

plex controllers. The result is a modular building-block

style design with many advanced features that can be

easily customized for a customer’s needs.

FIGURE 5:

PWM CLOCK

PWM Clock

TABLE 1:

MEMORY USAGE

GPR

3 bytes

Program

153 words

 2004 Microchip Technology Inc.

DS91085A-page 4

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.

•

Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not

mean that we are guaranteeing the product as “unbreakable.”

Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our

products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts

allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.

Information contained in this publication regarding device

applications and the like is provided only for your convenience

and may be superseded by updates. It is your responsibility to

ensure that your application meets with your specifications.

MICROCHIP MAKES NO REPRESENTATIONS OR WAR-

RANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED,

WRITTEN OR ORAL, STATUTORY OR OTHERWISE,

RELATED TO THE INFORMATION, INCLUDING BUT NOT

LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE,

MERCHANTABILITY OR FITNESS FOR PURPOSE .

Microchip disclaims all liability arising from this information and

its use. Use of Microchip’s products as critical components in

life support systems is not authorized except with express

written approval by Microchip. No licenses are conveyed,

implicitly or otherwise, under any Microchip intellectual property

rights.

Trademarks

The Microchip name and logo, the Microchip logo, Accuron,

dsPIC, K EE L OQ , micro ID , MPLAB, PIC, PICmicro, PICSTART,

PRO MATE, PowerSmart, rfPIC, and SmartShunt are

registered trademarks of Microchip Technology Incorporated

in the U.S.A. and other countries.

AmpLab, FilterLab, Migratable Memory, MXDEV, MXLAB,

PICMASTER, SEEVAL, SmartSensor and The Embedded

Control Solutions Company are registered trademarks of

Microchip Technology Incorporated in the U.S.A.

Analog-for-the-Digital Age, Application Maestro, dsPICDEM,

dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR,

FanSense, FlexROM, fuzzyLAB, In-Circuit Serial

Programming, ICSP, ICEPIC, MPASM, MPLIB, MPLINK,

MPSIM, PICkit, PICDEM, PICDEM.net, PICLAB, PICtail,

PowerCal, PowerInfo, PowerMate, PowerTool, rfLAB,

rfPICDEM, Select Mode, Smart Serial, SmartTel and Total

Endurance are trademarks of Microchip Technology

Incorporated in the U.S.A. and other countries.

SQTP is a service mark of Microchip Technology Incorporated

in the U.S.A.

All other trademarks mentioned herein are property of their

respective companies.

Printed on recycled paper.

Microchip received ISO/TS-16949:2002 quality system certification for

its worldwide headquarters, design and wafer fabrication facilities in

Chandler and Tempe, Arizona and Mountain View, California in

October 2003. The Company’s quality system processes and

procedures are for its PICmicro ® 8-bit MCUs, K EE L OQ ® code hopping

devices, Serial EEPROMs, microperipherals, nonvolatile memory and

analog products. In addition, Microchip’s quality system for the design

and manufacture of development systems is ISO 9001:2000 certified.

 2004 Microchip Technology Inc.

DS91085A-page 5

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: