LTC6820 Ver la hoja de datos (PDF) - Linear Technology
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LTC6820 Datasheet PDF : 28 Pages
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LTC6820
Operation
MASTER
SDO
SDI
SCK
CS
MSTR IP
LTC6820
MOSI
MISO
SCK
CS
IM
IBIAS
ICMP
ISOLATION BARRIER
RM
TWISTED-PAIR CABLE
RB1 WITH CHARACTERISTIC IMPEDANCE RM
RB2
IP MSTR
RM LTC6820
IM MOSI
IBIAS MISO
RB1
SCK
CS
ICMP
RB2
SLAVE
SDI
SDO
SCK
CS
6820 F01
Figure 1. Typical System Using Two LTC6820 Devices
The transmitter drive current and comparator voltage
threshold are set by a resistor divider (RBIAS = RB1 + RB2)
between the IBIAS pin and GND, with the divided voltage
tied to the ICMP pin. When the LTC6820 is enabled (not
IDLE), IBIAS is held at 2V, causing a current, IB, to flow
out of the IBIAS pin. The IP and IM pin drive currents are
20 • IB. The comparator threshold is half the voltage on
the ICMP pin (VICMP).
As an example, if divider resistor RB1 is 1.21k and resistor
RB2 is 787Ω (so that RBIAS = 2k), then:
IB
=
2V
RB1 +RB2
=
1mA
IDRV = IIP = IIM = 20 • IB = 20mA
VICMP
= 2V
• RB2
RB1 +RB2
= IB
• RB2
=
788mV
VTCMP = 0.5 • VICMP = 394mV
In this example, the pulse drive current IDRV will be 20mA,
and the receiver comparators will detect pulses with IP-IM
amplitudes greater than ±394mV.
If the isolation barrier uses 1:1 transformers connected
by a twisted pair and terminated with 100Ω resistors on
each end, then the transmitted differential signal amplitude
(±) will be:
VA
= IDRV
• RM
2
= 1V
(This result ignores transformer and cable losses, which
will reduce the amplitude).
isoSPI Pulse Detail
The isoSPI transmitter can generate three voltage levels:
+VA, 0V, and –VA. To eliminate the DC signal component
and enhance reliability, isoSPI pulses are defined as
symmetric pulse pairs. A +1 pulse pair is defined as a
+VA pulse followed by a –VA pulse. A –1 pulse pair is –VA
followed by +VA.
The duration of each pulse is defined as t1/2PW. (The total
isoSPI pulse duration is 2 • t1/2PW). The LTC6820 allows
for two different t1/2PW values so that four types of pulses
can be transmitted, as listed in Table 1.
Table 1. isoSPI Pulse Types
PULSE TYPE FIRST LEVEL
Long +1
Long –1
+VA (150ns)
–VA (150ns)
Short +1
+VA (50ns)
Short –1
–VA (50ns)
SECOND LEVEL
–VA (150ns)
+VA (150ns)
–VA (50ns)
+VA (50ns)
ENDING LEVEL
0V
0V
0V
0V
Long pulses are used to transmit CS changes. Short pulses
transmit data (MOSI or MISO). An LTC6820 detects four
types of communication events from the SPI master: CS
falling, CS rising, SCK latching MOSI = 0, and SCK latch-
ing MOSI = 1. It converts each event into one of the four
pulse types, as shown in Table 2.
Table 2. Master Communication Events
SPI MASTER EVENT
TRANSMITTED PULSE
CS Rising
Long +1
CS Falling
Long –1
SCK Latching Edge, MOSI = 1
Short +1
SCK Latching Edge, MOSI = 0
Short –1
6820f
10
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