OM5428T Hendon Semiconductors, OM5428T Datasheet - Page 4

• Manufacturer Part Number: OM5428T
• Description: Triacs 300mA Triac Phase Controller
• Manufacturer: Hendon Semiconductors

Specifications of OM5428T

• Product Category: Triacs
• Rohs: yes
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: SO-16
• Minimum Operating Temperature: - 40 C

7
The OM5428 comprises the following sections:
• supply derived from mains via dropping resistor (Rs);
• reset to ensure correct start-up;
• gate sense for reduction in the number of pulses
• zero-crossing detector synchronizes triac gate pulses;
• difference amplifier passing a signal from a sensor, or
• ramp function generator operating as the saw-tooth
• output amplifier driving the triac gate.
7.1
The OM5428 has been designed so that it is supplied
directly from mains voltage via a dropping resistor. For this
purpose a regulator circuit is included to limit the DC
supply voltage. The external supply dropping resistor Rs
(mains voltage rated) is connected between the mains
active and pin RMNS; V
common line. A smoothing capacitor C1 is connected
between V
supply voltage at V
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 V
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
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 V
reached its minimum value and C1 is fully charged. The
2008 Jul 29, Revision 6.0
produced when firing the triac;
indication of a potentiometer setting or switch position.
oscillator in time proportional or phase control;
FUNCTIONAL DESCRIPTION
Supply
Reset
CC
and V
EE
EE
, which may be used to supply an
. The circuit produces a negative
CC
is connected to the neutral or
EE
by recharging the
EE
Phase Controller and Triac Triggering IC
has
3
input SAW (pin14) to the saw-tooth 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
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.
In a static switch application (see figure 5) where 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
triac conduction is maintained through the mains voltage
zero-crossing, reducing radio interference 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 saw-tooth 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.
7.3.1
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
9µA 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 V
The pulse width can be determined using the following
formula:
Zero Crossing Detector
PW
P W
=
2
⎛
⎜
⎜
⎜
⎝
-------------------------------------------------------------- -
asin
⎛
⎝
(
----------------------------------- rad
Vmains pk
9
×10
100π
–
6
⋅
(
Product Specification
Rz
OM5428
)
)
EE
⎞
⎠
.
⎞
⎟
⎟
⎟
⎠
s