LT8708-1
21
Rev 0
3. V
(CSPOUT–CSNOUT)M
> 20.5mV: In this region of moder-
ate to high current from the master, the slave delivers the
same current as the master.
The transfer function is a mirror image of Figure 6 when
operating in the RDCM and RHCM conduction modes for
V
(CSPOUT–CSNOUT)M
< 0mV. Simply multiply the values
on the X and Y axes of Figure 6 by –1 to illustrate the
transfer function.
CURRENT MONITORING AND LIMITING
Monitoring: I
OUT(SLAVE)
The LT8708-1 can monitor V
OUT
current (I
OUT(SLAVE)
) in
the negative direction. An external resistor is connected
from the IMON_ON pin to ground, and the resulting volt-
age is linearly proportional to negative I
OUT(SLAVE)
. Unlike
the LT8708’s IMON_ON pin, the LT8708-1’s IMON_ON
pin does not regulate or limit I
OUT(SLAVE)
in the nega-
tive direction. See the I
IN
and I
OUT
Current Monitoring
and Limiting section of the LT8708 data sheet for how to
configure the IMON_ON current monitoring.
Monitoring and Limiting: I
IN(SLAVE)
The LT8708-1 can monitor V
IN
current (I
IN(SLAVE)
) in both
the positive and negative directions by measuring the volt-
age across a current sense resistor R
SENSE1
using the
CSPIN and CSNIN pins. The voltage is amplified and a
proportional current is forced out of the IMON_INP and
IMON_INN pins to allow for monitoring and limiting. This
function is identical to the LT8708 and more information
can be found in the Current Monitoring and Limiting sec-
tion of the LT8708 data sheet.
As described above, the LT8708-1 has circuitry allowing
for independent input current limiting of each phase. This
per-phase current limiting is intended to be secondary to
the limits imposed by the master. Typically, the master is
configured to limit its own input current (I
IN(MASTER)
) thus
limiting the command current to the slave. However, since
the slave has its own independent input current sensing
OPERATION
Transfer Function: CCM
Figure 5 shows the transfer function of the slave’s regu-
lated current sense voltage (V
(CSPOUT–CSNOUT)S
) vs the
master’s current sense voltage (V
(CSPOUT–CSNOUT)M
)
when the MODE pin is selecting CCM operation.
At light current levels (|V
(CSPOUT–CSNOUT)M
|<20mV),
I
OUT(SLAVE)
is regulated slightly lower than I
OUT(MASTER)
.
This ensures that the LT8708-1 delivers zero current when
the master is delivering zero current and also ensures a
smooth transition from positive to negative I
OUT
.
At high current levels (|V
(CSPOUT–CSNOUT)M
| > 20mV),
I
OUT(SLAVE)
is regulated to be the same as I
OUT(MASTER)
,
offering good current sharing and thermal balance
between the phases.
Note: If the LT8708-1 is configured to be in CCM while
RVSOFF
is being pulled low, use the FDCM transfer func-
tion in the next section.
Transfer Function: DCM, HCM and Burst Mode
Operation
Figure 6 shows the transfer function of the slave’s regu-
lated current sense voltage (V
(CSPOUT–CSNOUT)S
) vs the
master’s current sense voltage (V
(CSPOUT
–
CSNOUT)M
) when
the MODE pin is selecting FDCM, FHCM or Burst Mode
operation.
The transfer function, in the non-CCM modes, is shown
in Figure 6 and has three distinct regions:
1. V
(CSPOUT–CSNOUT)M
< 10mV: In this region, where the
master’s current is relatively small, the slave phases
deliver zero current.
2. 10mV < V
(CSPOUT–CSNOUT)M
< 20.5mV: In this region,
where the master’s current is moderate, the slave
phases deliver less current than the master. The trans-
fer function is hysteretic in this region. Therefore, the
slave current will operate from 0mV to 13.5mV or
from 6.7mV to 20.5mV if the master’s V
(CSPOUT–
CSNOUT)M
was most recently below 10mV or above
20.5mV, respectively.
