Si85xx
2.7. FAULT Output
The FAULT output (Si8517/8/9) guards against Si85xx
output signal errors caused by missing reset cycles.
FAULT is asserted when a measurement cycle exceeds
the internal watchdog timer times limit of t
WD
. FAULT
can be used to alert a local microcontroller or digital
power controller of a current sense failure, or initiate a
system shutdown. To detect faults, tie a 200 kΩ resistor
from TRST/FAULT to VDD.
2-wire Ping-Pong mode is useful mainly in non-
overlapping two-phase buck converters, but may also
be used in full-bridge applications. In this output mode,
reset inputs R1 and R2 are used, and input R3 is
grounded. Measured current appears on OUT1 when
R2 is high, and appears on OUT2 when R1 is high as
shown in the full-bridge timing example of Figure 9.
3. Application Information
3.1. Board Layout
The Si85xx is connected in the series path of the current
to be measured. The Si85xx must be located as far
away from away from transformer and other magnetic
field sources as possible. Like other analog
components, the Si85xx should be powered from a low-
noise DC source, and preferably connected to a low
noise analog ground plane. Recommended bypass
capacitors are 1 µF in parallel with a 0.1 µF, positioned
as close to the Si85xx as possible.
When using the
Si850x (single output versions), all 3 ground pins MUST
be connected to the same ground point, and both VDD
and VDD2 pins MUST be tied to VDD.
R1
R2
Si85xx State
MEASURE
RESET
tR
MEASURE
RESET
tR
OUT1
OUT2
TIME
Figure 9. Full-Bridge Timing Example A
4-Wire Ping-Pong mode is recommended for full-bridge
applications over 2-wire because it uses all four inputs
making the reset function tolerant to single-point signal
failures. In 4-Wire Ping-Pong mode, current appears on
OUT2 when R1 is high and R2 is low; and appears on
OUT1 when R3 is high and R4 is low as shown in the
full-bridge timing example of Figure 10. Table 3 shows
the states of the Mode and R4 inputs that select each
output, and the resulting reset logic functions and truth
tables.
3.2. Device Configuration
Configuring the Si85xx involves the following steps:
1. Selecting an output mode
2. Configuring integrator reset timing
3. Setting integrator reset time t
R
3.2.1. Device Selection
The Si85xx family offers three output modes: Single
output (Si850x), and 2 and 4-Wire Ping Pong (Si851x).
The Si851x products can be configured to operate in all
three of these output modes.
The Si850x products operate ONLY in Single output
mode. Most half-wave and single-phase applications
require only Single output mode, and will typically use
the Si850x. In Single output mode, output current
always appears on the OUT pin (Si850x) or the OUT1
pin (Si851x). A single integrator reset signal is typically
sufficient when operating in this mode.
Ping-Pong mode routes the current waveform to two
different output pins on alternate measurement cycles.
It is useful in full-wave and push-pull topologies where
external circuitry can be used to monitor and/or control
transformer flux balance.
(Note: The Applications
section of this data sheet (Section 3) shows design
examples using both output modes in various power
topologies.)
10
Preliminary Rev. 0.1
SILABS [ SILICON LABORATORIES ]
