Transfer Function: Ccm; Transfer Function: Dcm, Hcm And Burst Mode Operation; Current Monitoring And Limiting; Monitoring: Iout(Slave) - Linear Technology Analog Devices LT8708-1 Datasheet

OPERATION

Transfer Function: CCM

Figure 5 shows the transfer function of the slave's regu-
lated current sense voltage (V
master's current sense voltage (V
when the MODE pin is selecting CCM operation.
At light current levels (|V
I is regulated slightly lower than I
OUT(SLAVE)
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
At high current levels (|V
I is regulated to be the same as I
OUT(SLAVE)
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
master's current sense voltage (V
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 < 10mV: In this region, where the
(CSPOUT–CSNOUT)M
master's current is relatively small, the slave phases
deliver zero current.
2. 10mV < V
(CSPOUT–CSNOUT)M
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
was most recently below 10mV or above
CSNOUT)M
20.5mV, respectively.
) vs the
(CSPOUT–CSNOUT)S
(CSPOUT–CSNOUT)M
|<20mV),
(CSPOUT–CSNOUT)M
OUT(MASTER)
.
OUT
| > 20mV),
(CSPOUT–CSNOUT)M
OUT(MASTER)
) vs the
(CSPOUT–CSNOUT)S
– ) when
(CSPOUT CSNOUT)M
< 20.5mV: In this region,
(CSPOUT–
For more information
3. V
(CSPOUT–CSNOUT)M
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
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
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
the LT8708's IMON_ON pin, the LT8708-1's IMON_ON
pin does not regulate or limit I
tive direction. See the I
and Limiting section of the LT8708 data sheet for how to
configure the IMON_ON current monitoring.
Monitoring and Limiting: I
The LT8708-1 can monitor V
the positive and negative directions by measuring the volt-
age across a current sense resistor R
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
limiting the command current to the slave. However, since
the slave has its own independent input current sensing
www.analog.com
LT8708-1
> 20.5mV: In this region of moder-
< 0mV. Simply multiply the values
current (I
OUT OUT(SLAVE)
OUT(SLAVE)
in the nega-
OUT(SLAVE)
and I Current Monitoring
IN OUT
IN(SLAVE)
current (I
IN IN(SLAVE)
SENSE1
IN(MASTER)
) in
. Unlike
) in both
using the
) thus
Rev 0
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