TB085
A Simple Circuit for Driving Microcontroller
Friendly PWM Generators
Because the MCP1630 does not contain an on-chip
Author: Keith Curtis
oscillator, its application in non-intelligent or dumb
Microchip Technology Inc.
power supply designs, is limited. However, marrying
the MCP1630 with a small pin-count microcontroller
solves the oscillator problem and opens up possibilities
INTRODUCTION
for other features such as:
The recent interest in intelligent power supplies has
" Variable pulse frequency soft-start
driven the development of a new class of microcontrol-
" External shutdown control
ler friendly PWM generators. These PWM generators
" Under-voltage lockout
are the mixed signal control blocks for Switch mode
power supplies. One such device is the MCP1630. The
" Over temperature shutdown
MCP1630 contains the 3 main elements for designing
a switching power supply as a peripheral to a microcon-
THEORY OF OPERATION
troller: a set/reset flip-flop, a high-speed voltage
comparator and an op amp to implement the error
This technical brief will examine a design which
amplifier (see Figure 1).
combines the PIC10F206, a 6-pin SOT-23 microcon-
troller, with the MCP1630. The power supply design
The microcontroller controls the MCP1630 through its
presented is a full proportional-feedback continuous
clock input. The frequency of the clock determines the
inductor current, current-mode, boost power supply
pulse frequency of the PWM output, and the duty cycle
generating 15V out at .25 amps from a 9 VDC input. The
of the clock limits the maximum PWM duty cycle of the
PIC10F206 generates the clock for the MCP1630 and
output. Control of the duty cycle between 0 and the
through that control, implements the previous list of
maximum set by the clock input is determined by the
features.
current feedback to the comparator and the output of
the error amplifier (see Figure 2).
FIGURE 1: TYPICAL SWITCH MODE POWER SUPPLY BLOCK DIAGRAM
Vsupply
MCP1630
Flip-Flop
R Q
Voutput
Clock
Input
S Q
+
Op Amp
Reference

+
Comparator

Microcontroller
© 2004 Microchip Technology Inc. DS91085A-page 1
TB085
FIGURE 2: MCP1630 HIGH-SPEED PWM
VIN
Overtemperature
VIN
0.1 µA
UVLO
VEXT
OSC IN
VIN
S
Q
GND
100 k&!
CS
R
Q
+
COMP
Comp
-
VIN
Latch Truth Table
FB
-
SR Q
EA
VREF
+
2R
0 0 Qn
R
0 1 1
2.7V Clamp
1 0 0
1 1 1
The temperature sensor is implemented using a digital
HARDWARE
output device that pulls its output low when the thresh-
The PIC10F206 is well suited for this function. It has an
old temperature is exceeded. The software monitors
on-chip voltage comparator for the under-voltage
the input to detect an over-temperature condition and
detect and it has sufficient I/O to control the MCP1630
shuts down the pulse output if the temperature goes to
and monitor the external inputs.
high. When the temperature falls back below the
threshold temperature, the sensor output returns high
The microcontroller monitors the inputs and generates
and the software soft-starts the pulse output. Hystere-
the 250 kHz clock, all in software. Because the controls
sis built in the temperature sensor prevents chattering
are simple, the control circuit only needs the microcon-
and the sensor s trip temperature is preset when the
troller and a few components to implement all the con-
sensor is manufactured.
trol functions. Figure 3 shows the resulting schematic.
The shutdown input, GP1, is tied to whatever remote
Microcontroller inputs are connected to a divided sup-
start-up logic is desired. The software polls the input to
ply voltage, a digital temperature sensor and the shut-
determine if a shutdown is requested and terminates
down input. The remaining output is the output driving
the pulse output if the input is low. Raising the input
the MCP1630 clock input.
restarts the supply.
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.
U5
+9V +5V
LM3480-5
Out In
Com
C5 C6 +9V
3
0.1 µF 0.1 µF
+C17 C13
+9V +5V L1
22 µF 1 µF
+5V
+5V
J1 R1
C1
C7
10K
0.1 µF
0.1 µF
1
J2
2
U1
U2
1
PIC10F206 5
D1
3 MCP1630 7
C14 C15 C16
R2 VDD 3 B230 2
VDD
4 1 4 4 6
8.2K 1000 pF
OSC Q1 1.0 µF 1.0 µF
GP0/C+ GP2/T0CKI/CO DRVR
3
8 3 1
3 6 IRLML2502
Input
GP1/C- GP3/MCLR VREF CFB R9
4
1 2 2
100
VSS COMP VFB
VSS
R3 15 VDC
2
1K 5 Output
C2 C12
R8
1000 pF
0.56
1500 pF
R6
220 µF
2.4K
+5V C3 C10
100 pF
C11 R7
R10
U4
4 510&!
TC6501
100K
.1 µF
VDD
5 3
.033 µF
Tovr Hyst
GND GND +5V C8
0.1 µF
1 2
1
VIN U3
2
MCP1525
VOUT
VSS
C9
1 µF
3
©
2004 Microchip Technology Inc.
FIGURE 3:
SWITCH MODE POWER SUPPLY SCHEMATIC
DS91085A-page 3
TB085
TB085
FIGURE 6: CODE LISTING 2
SOFTWARE
Soft_Start
The software monitors the inputs and generates the
MOVLW .32 ;table of 32
output pulse using a simple bit-set/bit-clear loop,
MOVWF counter
expanded to interleave all the input testing. By keeping MOVLW Last-Table ;set to last
MOVWF pointer
the bit-set to bit-set time to 4 cycles, the output duty
Loop
cycle is locked to 25% for a 250 kHz clock. The latency
MOVF counter,w ;reload delay
time for a shutdown is 16 cycles. Figure 4 shows the
MOVWF count
code listing.
Delay ;generate delay
NOP
FIGURE 4: CODE LISTING 1
DECFSZ count,f ;decrement count
loop
GOTO Delay ;repeat til done
BSF PWM
MOVF pointer,w ;get pntr 4 jump
BCF PWM ;generate a pulse
ADDWF PCL,f ;jump
BTFSS CMPOUT ;test 4 low Vin
Table
GOTO Low_volt ;if low shutdown
BSF PWM
BSF PWM
BCF PWM ;32 pulse
BCF PWM ;generate a pulse
GOTO $+1 ;2 cycle delay
BTFSS GP1 ;test 4 hi temp
|-----------;29 copies of pulse + delay
GOTO High_temmp ;if hi shutdown
BSF PWM
BSF PWM
BCF PWM ;2nd pulse
BCF PWM ;generate a pulse
GOTO $+1
BTFSS GP3 ;test 4 shutdown
Last
GOTO shtdwn ;if shutdown
BSF 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
The soft-start function is generated by ramping up the
DECF pointer,f
number of output pulses. At start-up, a single pulse is DECFSZ counter,f ;decrease delay
GOTO Loop ;if 10, continue
followed by a long delay. Next, 2 pulses are followed by
loop_forever ;if 0, goto main
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/
CONCLUSION
bit-clear/delay instructions, similar to code listing 1 with
Using a combination of software and simple hardware,
a delay and control section. Figure 5 shows the timing
an efficient control for a PWM generator is imple-
of soft-start and Figure 6 is an excerpt from the actual
mented with many of the features found in more com-
code.
plex controllers. The result is a modular building-block
style design with many advanced features that can be
FIGURE 5: PWM CLOCK
easily customized for a customer s needs.
PWM Clock
TABLE 1: MEMORY USAGE
GPR 3 bytes
Program 153 words
© 2004 Microchip Technology Inc. DS91085A-page 4
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DS91085A-page 6 © 2004 Microchip Technology Inc.