EVM Theory and Operation
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When none of the buttons are pressed, the TPL7407L inputs will be open circuit and the internal resistors
in the TPL7407Lwill ensure zero volts on the inputs. With the inputs low, the TPL7407Loutput pins will set
to a high impedance state; therefore, no current will flow through the relay coils. The relay contact will not
be engaged and the voltage on the J3-Contact pins 1 to 7 will be pulled up to the relay supply voltage by a
10k
Ω
resistor on the PCB.
Pressing one of the input buttons, labeled IN1 to IN7, will apply the input voltage supply on TB1-1 to the
corresponding input pin on the TPL7407L. The internal resistor on the TPL7407L input pin will draw a
small current proportional to the input voltage. The nominal current is input voltage divided by 1M
Ω
, it can
also be expressed as the ratio, 1µA/V. The NMOS switch inside the TPL7407Lturns on providing a low
resistance path from output to ground. This completes the circuit and current flows from the relay supply
through the G5NB relay coil and through the TPL7407Loutput switch to ground and finally back through
the relay supply return lead. The relay coil current will engage the relay contacts. The relay contacts will
short the corresponding J3 “contact” pin to ground. It will also complete the corresponding LED circuit and
the LED will illuminate.
Releasing one of the input buttons, labeled IN1 to IN7, will remove the input voltage from the
corresponding input pin on the TPL7407L. The internal resistor on the TPL7407L input pin will decrease
the input voltage to zero. The NMOS switch inside the TPL7407Lturns off breaking the current path for the
relay coil. Since the coil is an inductor, the current cannot change in zero time. The coil voltage will
change polarity resulting in a TPL7407Loutput voltage that is greater than the relay supply voltage. This
will forward bias the diode inside the TPL7407Lpassing current back to the relay supply voltage. This
current will continue until the stored coil energy is depleted. The relay contacts will disengage and the
short on the J3-Contact pin will be removed and the pin voltage will increase back to the relay supply
voltage. The LED circuit will be open, thus extinguishing the LED.
The voltage on the output pins is always available on the J2-Output connector pins 1-7. Pin 8 is connected
to the COM pin on the TPL7407Land the relay supply voltage on TB1 pin 4. The J2-Output connector can
be used measure the output voltage. It can also be used to add additional loads to the TPL7407Loutput
pins. The series resistance between the J2-output connector and the TPL7407L is approximately 20 m
Ω
.
The onboard AGQ2003 relay coils can be removed from the TPL7407L by removing the seven 0
Ω
resistors at locations R41 to R47.
The voltage on the input pins is always available on the J1-Input connector pins 1-7. Pin 8 is connected to
the GND pin on the TPL7407Land the ground voltage on TB1 pins 2 & 3. The J1-Input connector can be
used measure the input voltage. It can also be used to inject external signals onto the TPL7407Linput
pins.
Three user supplied components, per channel, can be added if needed. All three circuit board footprints
are SMD 0603 sized. The first location, R1 to R7, allows adding a resistor from each input to ground. The
second, R21 to R27, and third location, C21 to C27, are in series with each other and parallel with the
G5NB relay coils.
The terminal block, TB1, provides power for the input pushbuttons and relay coils. When directly
controlling the inputs using the J1-Input connector, the input source on TB1 pin1 may be disconnected.
The TPL7407L EVM board has seven identical channels. The single channel schematic is easier to read
then the complete schematic. The TPL7407Lsingle model functional diagram is enclosed by dotted lines.
The input pin has a 1M
Ω
resistor that keeps the driver off when no input is disconnected or put in to a high
impedance state. The NMOS transistor sinks to a shared ground connection when the input voltage is
applied. When the load is inductive and the NMOS turns off, the output voltage will increase beyond the
relay supply voltage and inductor current will continue to flow though the free wheeling diode to the COM
pin until the inductor is discharged.
Resistors R41 through R47 can be removed to isolate the output from the relay coils when external load
or automated test equipment is provided through the J2-Output connector.
The relay is an G5NB relay with a 2,880
Ω
, 24V, 8.3mA nominal coil. The pull-in voltage is less than 2.25V
(3V × 75%) and the drop out voltage is greater than 0.3V. The maximum coil voltage is 43.2V (24V ×
180%).
The relay contact when open will allow the J3-Contact pin to rise to the Relay supply voltage. The voltage
on J3-Contact connector can be measured by any high impedance (>100k
Ω
) measuring device. When the
contacts close the J2-Contact pin will be pulled down to ground potential.
4
TPL7407L7-Channel Relay and Inductive Load Sink Driver EVM
SLRU005 – February 2013
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