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STCC02-BD5
October 2003 - Ed: 1A
CONTROL CIRCUIT FOR HOME APPLIANCE
MCU BASED APPLICATION
s
Higher module compactness with reduced com-
ponent count
s
Drastic reduction of soldered pins on the board
for faster module assembly time and lower use
of lead
s
High ESD robustness and transient burst immu-
nity compliant with IEC61000-4 standards
s
Enhanced functional reliability
s
Accurate MCU supply for better Analog to Digital
Conversion
s
Enhanced circuit parametric quality
s
Easy to design for short time to market
BENEFITS
s
Wide range input supply voltage operation:
7 to 27 V
s
5 V 10% full tolerance Voltage Regulator
s
MCU reset circuit with activation delay timer and
45s digital noise filter
s
Highly immune and 30 s filtered Zero Voltage
Synchronization
s
Door Closed detection adaptation
s
One 100 mA fan relay coil driver with demagne-
tizing diode
s
One 100 mA magnetron relay coil driver with de-
magnetizing diode including down lock circuit
based on fan drive output state
s
One 10 mA buzzer driver
s
Ambient temperature: - 10 to 85 C
FEATURES
s
Microwaves oven analog and power driver control
s
Home Appliance digital control
APPLICATIONS
DIP-16
V
IN
DL
C
SYN
DS
MAG
2
/RST
CDD
IN
3
BUZ
3
IN
1
IN
2
ZVS
V
DD
1
2
3
4
5
6
7
8
9
12
11
10
13
14
15
16
COM
V
CC
FAN
1
PIN-OUT CONNECTIONS
MCU
C
DD
C
DD
V
DD
V
IN
C
UP
V
IN
V
CC
V
CC
JP
V
CC
R
ZV
MAGNETRON RELAY
FAN RELAY
DOOR SWITCH
MAINS
Neutral
Line
BUZZER
/RST
DL
C
ZVS
SYN
C
DD
DS
IN
1
MAG
2
IN
2
FAN
1
IN
2
IN
2
IN
3
V
CC
BUZ
3
COM
/RST
NMI
P
03
P
02
P
01
P
04
V
SS
5V Regulator
Reset with delay
Zero volts sync.
Door closed detection
Magnetron driver
Fan driver
Buzzer driver
STCC02 BASED APPLICATION DIAGRAM
STCC02-BD5
2/12
V
DD
V
IN
/RST
DL
C
ZVS
SYN
C
DD
DS
IN
2
MAG
2
IN
1
FAN
1
IN
3
V
CC
BUZ
3
COM
5V Regulator
Reset with delay
Zero volts sync.
Door closed detection
Magnetron driver
Fan driver
Buzzer driver
CIRCUIT BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
The STCC02 is a control circuit embedding most of
the analog & power circuitry of a microwaves oven
control module. It interfaces the micro-controller
with the power and process sections of the oven.
s
The voltage supply
The
5V
voltage
regulator
supplies
the
micro-controller MCU: especially functions such
as the timer, the Analog-Digital Converter ADC,
and the low current outputs. Since all the
high-current outputs sink their current from a
different voltage supply, this regulator does not
need to be oversized. Its average output current
can vary from 5 to 20 mA.
Its output voltage accuracy, that contributes to the ADC accuracy of the MCU, is better than 10 % in the
whole operating range of the temperature T
AMB
, the load current I
DD
and the input voltage V
IN
. The
STCC02 input voltage range from 7 to 27 V; and its DC output current is less than 20 mA to keep the
internal dissipation compatible with thermal package capability.
The regulator includes also an over current limiter to prevent high current conditions during the power up
inrush or the output short circuit. This limiter is made of a serial shunt resistance as current sensor and a
circuit that regulates the input over current.
s
The reset circuit
This circuit ensures a Low Voltage Detection (LVD) of the output voltage of the regulator. Most
micro-controllers have an active RESET pin in the low state: so, the /RST pin will be active at low state.
T
DW
~ 40 s
V
DD
circuit output
internal latch output
T
UP
= 6 ms
C
UP
= 47 nF
RST\
T
DW
~ 40 s
V
DD
circuit output
internal latch output
T
UP
= 6 ms
C
UP
= 47 nF
RST\
VIN
Over current
limiter
1.25V
Reference
VDD
R1
-
+
R2
RSENSE
DL
C
If C
= 47 nF, T
= 6 ms
UP
UP
External
Capacitor
C
UP
/RST
PROGRAMMABLE
DELAY
NOISE FILTER
V
DD
V
DD
V
H
V = 4.25 V
H
500
V
L
V = 3.75 V
L
STCC02-BD5
3/12
The reset circuit senses the regulator voltage V
DD
. Its comparator with hysteresis achieves this task.
The /RST pin is high when V
DD
is higher than the high threshold V
H
= 4.25 V; and is low when the V
DD
decreases below the low threshold V
L
= 3.75 V.
The comparator output changes are filtered for a high immunity. When the reset is disabling (V
DD
>V
H
), the
/RST signal rises after the delay time T
UP
. This delay is set by an external capacitor C
UP
connected to the
DL
C
pin: T
UP
= 6 ms for C
UP
= 47 nF.
When the reset is enabling (V
DD
<V
L
), the /RST signal is falling after a delay time T
DW
that is internally set at
40 s when C
UP
= 47 nF.
s
The Zero Voltage Synchronization ZVS circuit
The Zero Voltage Synchronization ZVS circuit generates a low frequency clock using the AC line cycles
(20 ms on 50 Hz or 16.7 ms on 60 Hz). This clock allows the MCU to generate the cooking timings and to
reduce the magnetron inrush current by powering it on at the AC line peak voltage.
50s
115s
50s
50s
115s
115s
R
ZV
CC
CC
= 10 k ; V
= 15 V; I
= 20 mA
V
ZVS
FALLING EDGE
RISING EDGE
V
TF
2V / div
40s / div
The input pin SYN is an image of the mains voltage and is usually connected to the supply transformer
through a resistor R
ZV
.
The circuit is protected against fast line transients because its state change will act on the whole MCU
routines: a 30 s filter is implemented giving a higher immunity to the MCU circuit.
Since the ZVS pin is connected to the Non Maskable Interrupt NMI or INT\ of the MCU, its falling edge is the
active counting event. The delay between the real Zero Crossing event and this ZVS falling edge depends
on the internal filtering time, the resistance R
ZV
, the transformer, the rectifier drop voltage V
F
, the V
CC
supply load and the temperature. The STCC02 contribution to this delay can be evaluated by measuring
the delay between its input voltage V
TF
and its output voltage V
ZVS
. When using V
F
= 0.8V, R
ZV
= 10 k
,
V
CC
= 15V, I
CC
= 20 mA, it is about 50 s on rising voltage V
TF
and 115 s on falling voltage V
TF
.
V
CC
V
DD
ZVS
V
TF
COM
100 k
500 k
25 k
SYN
20 s Filter
R
ZV
AC
LINE
Q
S
2
S
1
STCC02-BD5
4/5
s
The Door closed detection circuit
The magnetron of the oven can be powered only if the door is closed in order to protect the oven user. This
safety feature is ensured mechanically by putting the door switch in series with the magnetron relay coil
supply.
For redundancy purpose, the Door Closed Detection CDD signal is also transmitted to the MCU. Since the
DS input detects the door state from an electromechanical switch, a spike suppressor is added to increase
its robustness. Its EMI immunity in off state (open door) is increased thanks to a 50k
pull down resistor
that maintains the DS signal in low state.
s
The magnetron relay coil driver
This robust driver interfaces a DC relay coil and an MCU output. The relay coil power is rated up to 1.2 W
for V
CC
= 12V.
Its output stage is made of a transistor and a demagnetization diode. The transistor's reference is to the
power ground COM and has a DC current rating of 100 mA. Its collector is connected to the outputs MAG
2
.
The diode is connected between the output pin MAG
2
and the Door Switch pin DS.
To enhance safety rules and to prevent any unventilated operation of the magnetron, the relay coil
magnetization is enabled by the fan conduction state that becomes a logic signal FAN
1
\.
Furthermore, its demagnetization node is connected to the door switch pin DS: when the oven door is
open, the coil of the magnetron relay is immediately disconnected from the relay supply V
CC
to switch off
these heating loads.
The boolean rule of the magnetron relay operation becomes:
(Magnetron relay ON) = DS.IN
2
.FAN
1
\.
V
CC
V
DD
C
DD
DS
Door Switch
EMI Filter
500
V
DD
100 k
50 k
V
DD
MAG
2
IN
3
FAN
1
DS
Demagnetizing Diode
Relay
Transistor
15 k
STCC02-BD5
5/12
s
Fan relay coil driver
This robust driver interfaces a DC relay coil and an MCU output. The relay coil power is rated up to 1.2W for
V
CC
= 12 V.
Its output stage is made of a transistor and a demagnetization diode. The transistor is referred to the
ground COM, has a DC current rating of 100 mA; and its collector is connected to the output FAN
1
. The
diode is connected between the output pin FAN
1
and the supply pin V
CC
.
s
Buzzer driver
The MCU can drive a warning buzzer with a 50% PWM signal. The buzzer driver amplifies this signal in
current and translates it from the 5V MCU output to the VCC supply to produce the right sound level from
the buzzer.
The output stage is made of a transistor and a 5 k
resistor. The transistor is referred to the power ground
COM and is connected by its collector to the output BUZ
3
. It has a DC current rating of 10 mA and runs up
to 5 kHz. Finally, the resistor is connected between the BUZ
3
and V
CC
pins to discharge the capacitance of
the buzzer at turn off and in off state.
V
CC
V
CC
V
DD
V
DD
BUZ
3
FAN
1
IN
3
IN
1
Relay
Transistor
Buzzer
Transistor
Demagnetizing Diode
50 k
15 k
5 k
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