
Si3232
44
Preliminary Rev. 0.96
Not
Recommended
fo
r N
ew
D
esi
gn
s
4.12. Relay Driver Considerations
The Si3232 includes a general-purpose driver output for
each channel (GPOa, GPOb) to drive external test
relays. In most applications, the relay can be driven
directly from the Si3232 with no external relay drive
circuitry required.
Figure 21 illustrates the internal relay
driver circuitry using a 3 V relay.
Figure 21. Si3232 Internal Relay Drive Circuitry
The internal driver logic and drive circuitry is powered
from the same 3.3 V supply as the chip’s main supply
(VDD1–VDD4 pins). When operating external relays
from a VCC supply that is equal to the chip’s VDD supply,
an internal diode network provides protection against
overvoltage conditions caused by flyback spikes when
the relay is opened. Only 3 V relays may be used in the
configuration shown in
Figure 21, and either polarized
or non-polarized relays are acceptable provided both
VCC and VDD are powered by a 3.3 V supply. The input
impedance, RIN, of the relay driver pins is a constant
11
while sinking less than the maximum rated 85 mA
into the pin.
If the desired operating voltage of the relay, VCC, is
higher than the Si3232’s VDD supply voltage, an
external drive circuit is required to eliminate leakage
from VCC to VDD through the internal protection diode.
In this configuration, a polarized relay is recommended
to provide optimal overvoltage protection with minimal
external components.
Figure 22 illustrates the required
external
drive
circuit,
and
provides
recommended
values
for
RDRV for typical relay
characteristics
and
VCC
supplies.
The
output
impedance, ROUT, of the relay driver pins is a constant
63
while sourcing less than the maximum rated
28 mA out of the pin.
Figure 22. Driving Relays with VCC > VDD
The maximum allowable RDRV value can be calculated
using the following equation:
where
Q1,MIN ~ 30 for a 2N2222
Si3232
Relay
Driver
Logic
V
DD
GND
GPOa/b
V
CC
3 V/5 V Relay
(polarized or
non-polarized)
Table 26. Recommended RDRV Values
ProSLIC
VDD
Relay
VCC
Relay
RCOIL
Maximum
RDRV
Recom-
mended
5% Value
3.3 V
±5%
3.3 V
±5%
64
Not
Required
—
3.3 V
±5%
5V
±5%
178
2718
2.7 k
3.3 V
±5%
12 V
±10%
1028
6037
5.6 k
3.3 V
±5%
24 V
±10%
2880
8364
8.2 k
3.3 V
±5%
48 V
±10%
7680
11092
11 k
Si3232
V
DD
GPOa
GPOb
V
CC
Polarized
relay
R
DRV
I
DRV
Q1
MaxR
DRV
V
DD,MIN
0.6 V
–
R
RELAY
Q1,MIN
V
CC,MAX
0.3 V
–
-------------------------------------------------------------------------------------------------
R
SOURCE
–
=