LT8708-1
28
Rev 0
For more information
also be used to understand RDCM and RHCM operation
by multiplying the V
(CSPOUT–CSNOUT)M
and the V
(CSPOUT–
CSNOUT)S
axis values by –1.
As mentioned previously, 17.4k resistors must be con-
nected from the ICP and ICN pins to ground. Proper resis-
tor connections are required to produce the correct ICP
and ICN voltages, and result in the correct I
OUT(SLAVE)
currents.
I
OUT(SLAVE)
: Configuration
I
OUT(SLAVE)
regulation is the main regulation loop for
the LT8708-1 and should always be enabled. Therefore,
always connect a 17.4k resistor in parallel with a compen-
sation network from the IMON_OP pin to ground. Note
that the IMON_OP pin cannot be used for monitoring the
I
OUT(SLAVE)
current.
Figure 5 and Figure 11 show that increasing the mas-
ter’s average current sense voltage V
(CSPOUT–CSNOUT)M
above ±60mV results in no additional current from the
slave LT8708-1. As such, the target average of V
(CSPOUT–
CSNOUT)M
should be limited to ±50mV by connecting
appropriate resistors from the IMON_OP and IMON_ON
pins of the LT8708 to ground (see the I
IN
and I
OUT
Current
Monitoring and Limiting section of the LT8708 data sheet).
In addition, the instantaneous differential volt-
age V
(CSPOUT–CSNOUT)S
should remain between
–100mV and 100mV due to the limited current that
can be driven out of IMON_OP. If the instantaneous
V
(CSPOUT–CSNOUT)S
exceeds these limits but the aver-
age V
(CSPOUT–CSNOUT)S
is between –50mV and 50mV,
consider including the current sense filter described in
the I
IN
and I
OUT
Current Monitoring and Limiting sec-
tion of the LT8708 data sheet. The filter can reduce the
instantaneous voltage while preserving the average. In
general, use the same value current sense filter for all
the synchronized LT8708 and LT8708-1(s).
Finally, IMON_OP should be compensated and filtered
with capacitor C
IMON_OP
. At least a few nF of capacitance
is usually necessary.
LOOP COMPENSATION
To compensate a multiphase system of the LT8708 and
LT8708-1(s), most of the initial compensation component
selection can be done by analyzing the individual voltage
regulator and/or current regulator(s) independently of
each other. Use the total input and output bulk capacitance
of the multiphase system in the stability analysis for each
of the following steps.
1. Analyze the stability of the LT8708 as a single phase
without any additional LT8708-1 phases included.
This includes all the regulation loops that will be used
by the master LT8708, such as voltage regulation
(FBOUT, FBIN) and/or current regulation (IMON_INP,
IMON_INN, IMON_OP, IMON_ON). Determine the ini-
tial values for the V
C
pin compensation network, and
the relevant IMON_XX pin capacitors for the master
LT8708. Further adjustment of these values will be
done in Step 4. Adjustment to C
IN
and C
OUT
may also
be necessary as part of this analysis. See the Loop
Compensation section of the LT8708 data sheet for
more details. LTspice
®
transient simulation can be
helpful for this step.
2. Analyze the stability of the I
OUT(SLAVE)
current regu-
lation loop of a standalone LT8708-1 phase. Adjust
the V
C
and IMON_OP compensation networks of the
LT8708-1 to achieve stability and maximum band-
width. Bench stability evaluation of a standalone
LT8708-1 can be carried out by driving the ICP and
ICN pins with external voltage sources.
A similar approach to that used for analyzing the
LT8708 in constant-current regulation can be
employed in compensating the standalone LT8708-1
current regulator. An IMON_OP capacitor of at least a
few nF is necessary to maintain I
OUT(SLAVE)
regulation
loop stability. In addition, adding a resistor of a few
hundred Ohms in series with this capacitor can often
provide additional phase margin.
If any of the I
IN(SLAVE)
regulation loops, i.e. IMON_INP
and IMON_INN, is used for secondary or redundant
current limiting, carry out the corresponding stability
analysis on the standalone LT8708-1. Use the same
APPLICATIONS INFORMATION
