NCN5120 - Driving Relays in an Efficient Way

AND9149/D
NCN5120 - Driving Relays
in an Efficient Way
http://onsemi.com
APPLICATION NOTE
INTRODUCTION
because these relays are not operated at a very high rate this
approach works perfectly.
NCN5120 is a receiver−transmitter IC suitable for use in
KNX® twisted pair networks (KNX TP1−256). It supports
the connection of actuators, sensors, microcontrollers,
switches or other applications in a building network.
NCN5120 handles the transmission and reception of data
on the bus. It generates from the unregulated bus voltage
stabilized voltages for its own power needs as well as to
power external devices, for example, a microcontroller.
NCN5120 assures safe coupling to and decoupling from
the bus. Bus monitoring warns the external microcontroller
for loss of power so that critical data can be stored in time.
NCN5120 has several voltage regulators to generate
different voltages. One of these voltage regulators is a 20 V
low drop linear regulator typically used for driving relays.
Because NCN5120 has a high efficient adjustable DC−DC
converter, driving of relays can be done in a more efficient
way. This application note describes how to use the
adjustable DC−DC converter for driving relays in an
efficient way under all operating conditions.
NCN5120 RELAY APPLICATION
The 20 V LDO is not an efficient regulator to be used.
NCN5120 has an adjustable DC−DC converter which is
high efficient (DC2). One could decide to use DC2 to drive
the relays. However, due to some limitations of NCN5120
this DC2 voltage could drop too much under certain
situations (mainly when the capacitor needs to be charged).
This would result in a too low voltage to drive the relays.
Also, to store as much as possible energy in the big storage
capacitor it’s advised to have an as high as possible voltage.
Figure 2 gives the schematic for driving relays with DC2
and assures a stable voltage under all situations. Figure 3
displays only the DC2 part (which gives a more clear view
on the operation).
With exception of DC2, the schematic as given in Figure 2
is very basic (see also NCN5120 datasheet). The connection
between the KNX bus (see connector A and B) and
NCN5120 makes use of all the standard components as
given in the datasheet. A standard 16 MHz crystal is used
and a clock signal is supplied back to the microcontroller
(uC CLK). It’s not mandatory to work in this way. The relay
driving principle will also work when the microcontroller
supplies the clock to NCN5120. RESETB and SAVEB are
supplied to the microcontroller for monitoring of the status.
A standard UART interface is used as communication
between microcontroller and NCN5120 but any of the other
interfaces will also do. DC1 is used as normal.
FANIN/WAKE−pin is pulled to ground. Although this is not
necessary, it’s advised to do this. When FANIN/WAKE−pin
is pulled to ground, more power can be taken from the KNX
bus to recharge the big buffer capacitor faster (C13 in
Figure 2). V20V is not used in Figure 2 because we use DC2
to drive the relays.
Table 1 gives the components list for all required DC2
components. All components from Figure 2 which cannot
be found back in Table 1 can be found back in the NCN5120
datasheet.
STANDARD RELAY APPLICATION
Figure 1 gives the block diagram of the relay driving
principle as used in KNX relay actuators today. A 20 V LDO
is used to power the relays. Because 12 V relays are
generally used, this 20 V needs to be converted down to
12 V. To minimize the required power to drive the relays,
bistable relays are used which are controlled by H−bridges.
The H−bridges itself are controlled by a microcontroller
(microcontroller not displayed in Figure 1). Although
bistable relays are used still a relatively high current is
required to change the state of the relays. This current cannot
be requested from the 20 V LDO because this would result
in a too high current taken from the KNX bus. To make sure
not too much current is sourced from the KNX bus a current
limit circuit is added. A big storage capacitor (10 mF or
higher) is used to store the energy required to drive the
relays. Every time the relays are activated the voltage on the
big storage capacitor will drop. Recharging of this capacitor
will be relatively slow (due to the current limiter) but
© Semiconductor Components Industries, LLC, 2013
August, 2013 − Rev. 0
1
Publication Order Number:
AND9149/D
AND9149/D
Optional
Current
Limiter
20 V LDO
H−bridge
Relay 1
20 V
to
H−bridge
Relay 2
12 V
H−bridge
Relay 3
H−bridge
Relay n
Figure 1. Relay Block Diagram
RESETb
SAVEb
uC CLK
C8
C9
3.3
3.3
VSSD
31
XCLK
32
XTAL2
XSEL
33
SAVEB
XTAL1
35
36
RESETB
37
38
TESTOUT
39
34
25
10
21
VDD1M
VDD1
VSS1
B
VSW1
C4
SCK/UC2
SDO/TXD
CSB/UC1
TREQ
MODE2
MODE1
NC
NC
3.3
C 10
R8
R6
R7
C 12
L2
T1
R5
D3
V2
R4
C 13
Figure 2. Application Example for Driving Relays
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2
TxD
SDI/RXD
R2
L1
GND
VDDD
20
22
19
23
9
18
8
17
24
16
7
VDD2MV
C3
6
11
V20V
26
NCN5120
15
VFILT
5
VIN
D2
CEQ2
27
VDD2MC
C2
28
4
VSW2
CEQ1
3
14
CAV
VBUS1
29
13
C1
VCC2
C6
30
VSS2
CCP
VCC
3.3
2
VDD2
A
39
VDDA
TXO
V2
1
12
VSSA
VBUS2
R1
D1
FANIN/WAKE
X1
C5
RxD
AND9149/D
R6
VFILT
VDD2MC
VDD2
VIN
V2
D3
L2
VSW2
NCN5120
R7
C12
C 13
T1
R4
VDD2MV
R8
R5
Figure 3. DC2 Schematic for Driving Relays
Table 1. DC2 BOM
1.
2.
3.
4.
Comp.
Value
Tolerance
C12
56 pF
±10%
Remarks
Notes
1
C13
12 mF
±20%
2, 4
D3
MBR1H100SFT3G
L2
220 mF
±10%
Coils Electronic DA54NP−221K
R4
39 kW
±5%
0.0625 W
R5
10 kW
±5%
0.0625 W
4
R6
2.7 W
±1%
0.0625 W
3, 4
R7, R8
100 kW
±1%
0.0625 W
T1
2N7002LT1G
ON Semiconductor
4
ON Semiconductor
It’s advised to take this value as close as possible to the input capacitance of T1.
The value of this capacitor will depend on the required energy for driving the relays.
R2 (see Figure 2) should be lower than R6!
See below for defining the value.
R4 and R5 can be calculated as next:
R4 +
R5
R VDD2M
− V2 = output voltage when relays are active (advised
value is 13 V or 14 V)
− tdel = 0.15 (0.25 worst case)
The capacitor C13 can be calculated as next:
V 2 * 3.3
3.3
R 5 ) R VDD2M
The current limiting resistor R6 can be calculated as next:
R6 +
Ǹǒ
2
L
2 V
V OC
I
FILT
1
T
1
)
V
FILT
Ǔ
C 13 +
*
V
FILT
L
t
del
n
i relay
t act
V drop
where:
− n = number of relays that are switched
simultaneously
− irelay = current required by one relay (V2 / Rrelay
gives a good estimation)
− tact = activation time of the relays
− Vdrop = the allowed drop on VOUT
A calculator is provided on the ON Semiconductor
website as a guide for defining the optimum values
(http://www.onsemi.com/PowerSolutions/supportDoc.do?t
ype=tools&rpn=NCN5120).
2
2
where:
− VOC = 0.15 (0.165 worst case)
− IFILT = the maximum VFILT current
− VFILT = VFILT voltage when relays are active
(advised value is 12 V)
− T = 4 (5 worst case)
− L2 = inductance of DC2 (in mH)
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AND9149/D
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