Application Note 64

VISHAY SEMICONDUCTORS
www.vishay.com
Optocouplers and Solid-State Relays
Application Note 64
Using the LH1525 and LH1526 (Dual) Solid State Relay
DESCRIPTION
The LH1525 solid state relay (SSR) combines the latest
high-voltage integrated circuit technology with an intelligent
circuit design to provide an extremely versatile SSR. The
LH1525 is able to achieve this versatility by combining low
turn-on current with fast switching speeds. With standard
SSRs, low turn-on current typically results in slow switching
speeds. Likewise, high switching speeds usually require a
substantial LED drive current. The LH1525 provides a fast
switching speed at a much lower drive current. For
applications where minimal power dissipation is critical, the
LH1525 provides the low turn-on current in an optically
coupled solid state relay.
The LH1525 can be used to minimize power dissipation in
battery-powered applications or in applications where
power management is a concern. It can be used in
instrumentation applications where fast switching speed is
critical. Or it can be used in telecom applications where its
robust current-limit circuitry will protect the relay from
lightning and other fault conditions commonly present on
telephone lines.
MINIMIZING POWER DISSIPATION
Figure 1 plots switch load current versus ambient
temperature. The alevel of LED forward current (IF) required
for switch operation is based on these parameters. If power
dissipation is the only concern, select the lowest LED drive
current curve that encompasses your load current and
ambient operating temperature design window. A given LED
forward current will support the operating area below and to
the left of the curves. Extrapolate forward current values
between 300 μA and 1 mA as required. If switching speed is
also a concern, refer to the next section.
CURRENT
VS.
SWITCHING
The LH1525 can provide switching speeds from as slow as
3 ms to as fast as 100 μs. Switching speed performance is
dependent upon the magnitude of LED drive current used.
This application note addresses three areas of operation of
turn-on speed and turn-on current. Turn-on speed is the
time it takes for the contact to close after current is applied
to the LED. Turn-on current is the amount of current
required through the LED to sustain a given load current.
Rev. 1.3, 02-Jul-12
VERY LOW TURN-ON
SWITCHING SPEED
CURRENT,
SLOW
LED drive currents between 0.3 mA to 1.5 mA are required
to keep switching speeds at 1 ms or more. This slow speed
operation is desirable in telecom applications due to the way
the relay’s current limit circuitry responds to a lightning
surge. The LH1525, like many other Vishay form A SSRs,
has integrated current-limiting circuitry. When an SSR is
directly connected to a telephone line (e.g., switch hook or
ring/test access in a PBX or central office) and high current
transients occur from lightning, the current-limit circuit will
operate to protect the relay. For a large transient, as those
specified by various regulatory agencies, the current-limit
circuit will shut down the relay with sub-microsecond
speed. While the relay is off, the power from the lightning
dissipates in a transient voltage suppressor. The relay
remains off for the duration of one turn-on period. It is
important that this off period be long enough to allow the
lightning wave to subside. An off period of 1 ms or greater
provides the most robust solution. To achieve 1 ms or
slower at room temperature, 1.0 mA to 1.5 mA of LED
forward current should be used. Refer to figure 1 to obtain
an adequate LED current value for elevated temperature
operation.
Another advantage that SSRs bring to this solution is
noise-free operation. SSRs will not be a source of acoustical
noise and will not generate transients during closure.
LOW TURN-ON CURRENT,
SWITCHING SPEED
MODERATE
LED drive currents between 3 mA to 5 mA provide nominal
switching speeds better than 500 μs. This fast switching
speed is desirable in many data acquisition or
instrumentation systems where scanning time needs to be
Document Number: 83864
1
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
APPLICATION NOTE
LED DRIVE
SPEED
The graph in figure 2 plots LED drive current versus turn-on
speed. It has been segmented into these three areas of
operation. A combination of low turn-on current and slow
speed is desirable for some battery powered applications
and also telecom applications. Low turn-on current,
moderate-speed performance is suitable for a variety of
applications where both speed and power consumption are
critical. In instrumentation applications where every
microsecond counts, high-speed performance using high
turn-on current would be a logical choice.
Application Note 64
www.vishay.com
Vishay Semiconductors
Using the LH1525 and LH1526 (Dual) Solid State Relay
minimized. Most optically coupled MOSFET relays require
5 mA to 10 mA of LED drive for operation at elevated
temperatures. The lower drive currents required for the
LH1525 minimizes power consumption of the relays. This is
very desirable in battery-powered equipment or in large
multiplexed relay systems where power management is a
concern. Of course another advantage the SSR brings to
these applications is that no extra settling time is required
since there is no contact bounce.
HIGH
TURN-ON
SWITCHING SPEED
CURRENT,
HIGH
LED drive currents above 8 mA provide typical switching
speeds below 200 μs. This current can be supplied as a
steady-state current or as a current pulse from an RC
peaking network. Note that with high LED drive currents the
turn-off time will actually exceed the turn-on time.
Depending on temperature, turn-off time will run between
200 μs to 300 μs with LED drive currents above 8 mA.
current flowing through the LED. For 5 V operation, a
2700  resistor will limit the drive current to about 1.4 mA.
Where high-speed actuation is desirable, use a lower value
resistor for R1.
R2 is an optional pull-up resistor which pulls the logic level
high output (VOH) up towards the VS potential. The pull-up
resistance is set at a high value to minimize the overall
current drawn from VS. The primary purpose of this resistor
is to keep the differential voltage across the LED below its
turn-on threshold. The LED dropout voltage is graphed
versus temperature in the typical performance
characteristics section of the designer’s guide. Many
applications will operate satisfactorily without this pull-up
resistor. In the logic circuit of figure 5, the only path for
current to flow is back into the logic gate. Logic leakage is
usually negligible. Each application should be evaluated,
however, over the full operating temperature range to make
sure that leakage current through the input control LED is
kept to a value less than the minimum LED forward current
for the switch turn-off specification.
TESTING AND TEMPERATURE VARIATION
120
100
Load Current (mA)
The previous discussions referred to typical LH1525 speed
performance and 25 °C ambient operation. The LH1525 is
tested for a maximum turn-on time of 800 μs with 5 mA of
LED drive and a turn-off time of 400 μs. If maximum speed
is critical to a design, these worst-case test limits must be
used. Figures 3 and 4 provide LED drive current versus
speed graphs for the extreme temperatures as well as room
ambient. Use this data to estimate performance at extreme
temperatures.
Figure 5 shows a typical logic circuit for providing LED drive
current. R1 is the input resistor which limits the amount of
Rev. 1.3, 02-Jul-12
40
0
- 40
17359
IF = 0.3 mA
IF = 1.0 mA
IF = 1.5 mA
IF = 2.0 mA
IF = 2.25 mA to
20 mA
- 20
0
20
40
60
80
TA - Ambient Temperature (°C)
Fig. 1 - SSR Recommended Operating Conditions
3.0
Very low turn-on current
slow speed
2.4
Turn-on Time (ms)
APPLICATION NOTE
You will not experience any difficulty in actual use, however,
if your logic circuit provides an adequate pull-up voltage.
The LH1525 is designed with highly sensitive
photo-detection circuits which will detect even the most
minute currents flowing through the LED. The relay typically
turns on with only 120 μA of LED drive at room temperature.
At elevated temperatures only 1 mA or 2 mA of LED drive is
required to turn the relay on. Leakage current must be
considered when designing a circuit to turn these relays on
and off.
60
20
INPUT CONTROL
If you are familiar with our parts and commonly evaluate
their performance using a curve tracer (step generator
sourcing current to LED, relay outputs tied to collector and
emitter) you may notice that the part is difficult to turn off.
Using the 1 mA scale from the step generator may still
source microamps of leakage current even when the dial is
turned to zero. You will need to select a lower range like the
50 μA range to fully turn the relay off.
80
1.8
1.2
Low turn-on current
moderate speed
0.6
High turn-on current
high speed
0.0
0
17360
4
8
12
16
20
LED Forward Current (mA)
Fig. 2 - Turn-on Time vs. LED Current
Document Number: 83864
2
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note 64
www.vishay.com
Vishay Semiconductors
Using the LH1525 and LH1526 (Dual) Solid State Relay
3.0
Turn-on Time (ms)
2.4
1.8
85 °C
25 °C
- 40°C
1.2
0.6
0.0
0
4
8
12
16
20
LED Forward Current (mA)
17361
Fig. 3 - Typical Turn-on Time vs. LED Current
0.32
85 °C
Turn-off Time (ms)
0.29
25 °C
0.26
0.23
- 40 °C
0.20
0.17
0
17362
4
8
12
16
20
LED Forward Current (mA)
Fig. 4 - Typical Turn-off Time vs. LED current
APPLICATION NOTE
VS
R2
R1
2700 Ω
100 kΩ
Any TTL or
buffered CMOS
logic
SSR
17363
Fig. 5 - Input Control Circuit
Rev. 1.3, 02-Jul-12
Document Number: 83864
3
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
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