LT8708-1
26
Rev 0
For more information
more chips’ VINHIMON or VOUTLOMON comparator is
triggered, the
RVSOFF
pin is pulled low to prevent the
entire multiphase system from delivering reverse current
and power. The multiphase system will exit the
RVSOFF
operation when all the VINHIMON and VOUTLOMON com-
parators are de-asserted.
CONFIGURING THE I
IN(SLAVE)
CURRENT LIMITS
As discussed in the Monitoring and Limiting: I
section, the LT8708-1 can monitor and limit the input
current independently of the master. The current limiting
discussed in this section is intended to be secondary, or
redundant, since the master is primarily in control of the
amount of current commanded from the slave.
As shown in Figure 9, the LT8708-1 measures I
IN(SLAVE)
with the CSPIN and CSNIN pins and can independently
monitor and limit the current in both positive and nega-
tive directions. The operation of the input current moni-
tor circuits is identical to the LT8708. More information
about configuring these circuits can be found in the I
IN
and I
OUT
Current Monitoring and Limiting section of the
LT8708 data sheet.
When setting the I
IN(SLAVE)
current limits, it is recom-
mended to set them equal to or higher than the magni-
tudes of the I
IN(MASTER)
limits. Consider that if the slave
reaches input current limiting before the master, the slave
can no longer deliver additional current as requested by
the master. With equal I
IN(SLAVE)
and I
IN(MASTER)
limits,
slight output current mismatch, and hence slight thermal
imbalance can still happen due to device tolerance. Bench
evaluation should be carried out to ensure the selected
I
IN(SLAVE)
limits meet the application’s thermal and stabil-
ity requirements.
REGULATING I
OUT(SLAVE)
I
OUT(SLAVE)
: Circuit Description
This section describes the control circuitry in the
LT8708-1 that regulates the output current I
OUT(SLAVE)
.
The master LT8708 sends the ICP and ICN control signals
to the slave LT8708-1 to set I
OUT(SLAVE)
. See the Transfer
section for related
information.
Figure 10 shows the primary LT8708-1 circuits involved in
the regulation of I
OUT(SLAVE)
. Additional circuitry is shown
in Figure 1. I
OUT(SLAVE)
is regulated by a feedback loop
with ICP and ICN setting the desired current. The feedback
loop involves the following sections:
• The V
C
pin controls the inductor current, thus indi-
rectly controlling I
OUT(SLAVE)
. Higher V
C
voltage
APPLICATIONS INFORMATION
Figure 9. I
IN(SLAVE)
Current Monitor and Limit
–
+
EA1
IMON_INP
IMON_INN
87081 F08
C
IMON_INP
R
IMON_INP
CSPIN
R
SENSE1
FROM
SYSTEM
V
IN
TO
CONTROLLER
V
IN
CSNIN
LT8708-1
I
IN(SLAVE)
V
C
1.21V
–
+
EA5
1.209V
–
+
g
m
= 1m
Ω
A3
C
IMON_INN
R
IMON_INN
+
–
20μA
20μA
TO INDUCTOR
CURRENT
CONTROL
Figure 10. I
OUT(SLAVE)
Current Regulation and Monitor
IMON_OP
87081 F09
CSNOUT
R
SENSE2
FROM
CONTROLLER
V
OUT
TO SYSTEM
V
OUT
CSPOUT
LT8708-1
V
C
–
+
EA6
1.209V
–
+
A1
C
IMON_OP
R
IMON_OP
17.4k
+
–
TO INDUCTOR
CURRENT
CONTROL
70μA
ICP
ICN
ICP
FROM
MASTER
ICN
FROM
MASTER
I
OUT(SLAVE)
