LT3965/LT3965-1
19
39651fa
For more information www.linear.com/LT3965
APPLICATIONS INFORMATION
If the LEDREF pin is set to a voltage other than 0V, the
channel shorted LED threshold V
STH
can be programmed
to one of these four values: 1V, 1V + V
LEDREF
, 1V + 2 •
V
LEDREF
and 1V + 3 • V
LEDREF
. The POR default V
STH
of
the LT3965/LT3965-1 is 1V. By using this feature, each
channel is able to detect 1, 2, 3, or 4 shorted LEDs.
This feature can be turned off by grounding the LEDREF
pin. When the LEDREF pin is set to 0V, the V
STH
will be set
to 1V, no matter how the V
STH
[1:0] bits are programmed.
The SCMODE read command (see Figure 10 and Figure 11)
is used to read back the addressed channel SCMREG
register bits and to get the channel LED fault conditions.
The SCMODE read command is three bytes long. The first
byte is the SCMODE device read address. The second byte
comprises (from MSB to LSB) one addressed channel bit
from the OLFREG register, one addressed channel bit from
the SLFREG register, and 6 bits (V
OTH
[1:0], V
STH
[1:0] and
MC[1:0]) from the addressed SCMREG register. The third
byte is the dimming value DV[7:0] from the addressed
SCMREG register.
Unlike the SCMODE write command, the SCMODE read
command
does not contain the channel address. Actu-
ally the channel address received from the last SCMODE
write
command is stored and will be used as the channel
address for incoming SCMODE read operations. In other
words, a SCMODE read command always reads the chan
-
nel SCMREG register addressed by the last SCMODE write
command.
If no SCMODE write command has ever been
received, the default channel address 000 (CH1) is used.
The LT3965/LT3965-1 SCMODE device address is
101A
4
A
3
A
2
A
1
followed by an eighth bit which is a data
direction bit (R/W)— a 0 indicates a write transmission
(the master writes to the addressed LT3965), a 1 indicates
a read transmission (the master reads from the addressed
LT3965/LT3965-1). A
4
A
3
A
2
A
1
is an input logic value from
the programmable address select pins ADDR4, ADDR3,
ADDR2 and ADDR1.
3) Broadcast Mode (BCMODE) Command
The BCMODE write command (see Figure 12) is used
to synchronize the dimming cycles among the multiple
LT3965/LT3965-1 slaves on the I
2
C bus. The LT3965/
LT3965-1 slaves must be operating with a common
external clock in order to be synchronized. The BCMODE
write command is only one byte
long: 00011000. The
command does not modify any register bits. It only resets
each channel counter to synchronize the dimming cycles.
The BCMODE read command (see Figure 13) is used to
inquire about which LT3965/LT3965-1 slave on the bus is
sending the alert (see LT3965/LT3965-1 Alert Response
Protocol section for detail). This command is two bytes
long. The first byte is the broadcast read address 00011001.
The second byte 010A
4
A
3
A
2
A
1
1 is sent by the alerting slave
to indicate its ACMODE device read address to the master.
A
4
A
3
A
2
A
1
is an input logic value from the programmable
address select pins ADDR4, ADDR3, ADDR2 and ADDR1.
If the BCMODE read command is issued when no LT3965/
LT3965-1 slave on the bus is sending alert, the master
receives no acknowledgement.
LT3965/LT3965-1 ALERT RESPONSE PROTOCOL
USING ALERT RESPONSE ADDRESS (ARA)
In a system where several slaves share a common inter
-
rupt line
, the master can use the alert response address
(ARA)
to determine which device initiated the interrupt.
The master initiates the ARA procedure with a START
condition and the special 7-bit ARA bus address (0001100)
followed by the read bit (R) = 1. If the LT3965
/LT3965-1
is
asserting the ALERT pin, it acknowledges and responds
by sending its 7-bit bus address (010A
4
A
3
A
2
A
1
) and a 1.
While it is sending its address, it monitors the SDA pin to
see if another device is sending an address at the same
time using standard I
2
C bus arbitration. If the LT3965/
LT3965-1 is sending a 1 and reads a 0 on the SDA pin on
the rising edge of SCL, it assumes another device with a
