2002 Nov 08
5
Integrated Electronic Solutions, Hendon, South Australia
Data Sheet
General purpose triggering circuit
OM5428
7
FUNCTIONAL DESCRIPTION
Fig.2 shows the functional block
diagram of the OM5428. It comprises
the following sections:
• d.c. supply derived from the mains
via a dropping resistor (Rs);
• reset to ensure correct startup;
• gate sense for reduction in the
number of pulses produced when
firing the triac;
• zero-crossing detector for
synchronization of the trigger
pulses;
• difference amplifier passing a
signal from a sensor, or indication
of a potentiometer setting or switch
position, etc.;
• ramp function generator operating
as the sawtooth oscillator in time
proportional or phase control;
• output amplifier amplifying trigger
pulses and driving the triac gate.
7.1
Supply
The OM5428 has been designed so
that it is supplied directly from mains
voltage. For this purpose a regulator
circuit is included to limit the DC
supply voltage. An external resistor
Rs (mains voltage rated) is connected
between the mains active and pin
RMNS; VCC is connected to the
neutral line. A smoothing capacitor
C1 is connected between VCC and
VEE. The circuit produces a negative
supply voltage at VEE, which may be
used to supply an external circuit
such as a temperature sensing
bridge.
During the negative half of mains,
current through the external voltage
dropping resistor Rs charges the
external smoothing capacitor C1 to
the shunt voltage of the regulator. The
value of Rs should be chosen such
that it can supply the current for the
OM5428, plus the charge required to
drive the triac gate and any external
(peripheral) circuits connected to VEE
by recharging the smoothing
capacitor C1 on the mains negative
half cycles. Any excess current is
bypassed through the shunt transistor
of the regulator. The maximum rated
current must not be exceeded.
During the positive half of the mains
cycle the external smoothing
capacitor C1 supplies the circuit. Its
capacitance must be large enough to
maintain the supply voltage above the
minimum specified limit.
A suitable VDR may be connected
across the mains to provide
protection for the OM5428 and the
triac against mains-born transients.
7.2
Reset
A reset circuit providing four reset
functions throughout the OM5428 has
been included.
Initially the reset signal ensures that
trigger pulses are not produced until
VEE has reached its minimum value
and C1 is fully charged. The input
SAW (pin14) to the sawtooth
generator is also held at a low state
until the reset threshold has been
reached.
During start-up the reset is also
responsible for holding the input pins
to the difference amplifier, IC+ (pin 5)
at a high state and IC- (pin 6) at a low
state. As a result, functions such as
soft and hard start while phase firing
can be realised.
7.3
Gate sense
Included in the OM5428 is a function
that is capable of determining the
state of the triac. Used to inhibit the
output amplifier, the gate sense circuit
ensures that multiple gate pulses are
not produced, hence reducing overall
current consumption.
7.4
Zero-crossing detector
The OM5428 contains a zero-
crossing detector to produce pulses
that coincide with the zero crossings
of the mains voltage to minimise RF
interference and transients on the
mains supply.
If the load to be driven is purely
resistive, the synchronization voltage
is obtained direct from the mains via a
resistor. As a result trigger pulses
start shortly before, and end shortly
after, each zero-crossing of the mains
voltage. In this manner radio
interference is reduced to a minimum.
If the load contains an inductive
component, the synchronization will
be produced by the internal gate
sense circuit rather than the
zero-crossing detector. The trigger
pulse is then produced at the earliest
possible moment, i.e. immediately
following zero-crossing of the
phase-shifted load current.
During phase control the zero-
crossing detector is used to generate
a sawtooth voltage synchronous with
the mains. As soon as the d.c. control
voltage corresponding to a preset
trigger angle is exceeded the output is
pulsed.
The pulse width control input PW
(pin 1) allows adjustment of the pulse
width at output XOUT (pin 2), to the
value required for the triac. This is
done by choosing the value of
external synchronization resistor Rz
between PW and the AC mains. The
pulse width is determined by the
amount of current flowing to or from
pin PW. Any current exceeding 9uA
will result in the output of the
zero-crossing detector being
disabled. The zero-crossing detector
output is also inhibited when the XDIS
input (pin 16) is HIGH, and enabled
when LOW, e.g. connected to VEE.
The pulse width can be determined
using the following formula:
PW
2
=
9
6
–
×10
R z
⋅
()
Vmains pk
()
----------------------------------- rad
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100
π
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