LT8708-1
27
Rev 0
results in higher I
OUT(SLAVE)
current and vice versa.
V
C
is driven by error amplifier EA6 during I
OUT(SLAVE)
regulation.
• During regulation, the IMON_OP voltage is very
close to the EA6 reference of 1.209V. Small changes
in IMON_OP voltage make large adjustments to V
C
,
and thus the I
OUT(SLAVE)
current.
• Resistor R
SENSE2
converts the I
OUT(SLAVE)
current into
a voltage that can be measured by amplifier A1. This
voltage is denoted as V
(CSPOUT–CSNOUT)S
in Figure 10.
• Transconductance amplifier A1 makes sure that
I
OUT(SLAVE)
is equal to the current set by the ICP
and ICN signals. If I
OUT(SLAVE)
becomes higher than
requested by ICP and ICN, additional current is deliv-
ered out of A1. This raises IMON_OP which reduces
V
C
and reduces I
OUT(SLAVE)
. Conversely, if I
OUT(SLAVE)
becomes lower than requested by ICP and ICN, the
current out of A1 is reduced. This lowers IMON_OP
which raises V
C
and increases I
OUT(SLAVE)
.
Figure 11 illustrates, in CCM mode, the typical relation-
ship between the master’s output current I
OUT(MASTER)
,
the resulting ICP and ICN control voltages, and the fur-
ther resulting I
OUT(SLAVE)
current. Figure 11 can best be
explained with a few examples.
In these examples, the output current sense resistors are
R
SENSE2
= 10mΩ for the master and the slave devices.
First, assume the master’s output current I
OUT(MASTER)
is 4A. This results in the master LT8708 measuring a
current sense voltage of V
(CSPOUT–VCSNOUT)M
= 4A • 10mΩ
= 40mV. Locate 40mV along the X-axis of Figure 11. The
corresponding ICP and ICN voltages are ~1V and 0V,
respectively. These ICP and ICN voltages are sent from
the LT8708 to the LT8708-1. As a result, the LT8708-1
regulates I
OUT(SLAVE)
to:
I
OUT(SLAVE)
=
V
(CSPOUT– CSNOUT)S
R
SENSE2
=
40mV (from Figure 10)
10mΩ
= 4A
Alternatively, if the master’s output current I
OUT(MASTER)
is –2A. Then the master LT8708 will measure a current
sense voltage of V
(CSPOUT–VCSNOUT)M
= –2A • 10mΩ =
–20mV. Locate –20mV along the X-axis of Figure 11. The
corresponding ICP and ICN voltages are 0V and ~0.7V,
respectively. These ICP and ICN voltages are sent from
the LT8708 to the LT8708-1. As a result, the LT8708-1
regulates I
OUT(SLAVE)
to:
I
OUT(SLAVE)
=
V
(CSPOUT– CSNOUT)S
R
SENSE2
=
–20mV (from Figure 10)
10mΩ
= – 2A
Figure 12 illustrates the relationship between I
OUT(MASTER)
,
ICP, ICN and I
OUT(SLAVE)
in FDCM, FHCM and Burst Mode
operation. Use Figure 12, instead of Figure 11, to under-
stand the control voltage relationships when operating
in FDCM, FHCM or Burst Mode Operation. Figure 12 can
TYPICAL APPLICATIONS
Figure 11. I
OUT(SLAVE)
Control Voltage Relationships (CCM)
V
(CSPOUT–CSNOUT)M
(mV)
–80 –60 –40 –20
0
20
40
60
80
–1.6
–1.2
–0.8
–0.4
0.0
0.4
0.8
1.2
1.6
2.0
–80
–60
–40
–20
0
20
40
60
80
100
ICP
, ICN VOL
TAGE (V)
V
(CSPOUT–CSNOUT)S
(mV)
87081 F11
V
(CSPOUT–CSNOUT)S
ICN
ICP
Figure 12. I
OUT(SLAVE)
Control Voltage Relationships
(FDCM, FHCM and Burst Mode Operation)
V
(CSPOUT–CSNOUT)S
ICN
ICP
V
(CSPOUT–CSNOUT)M
(mV)
0
10
20
30
40
50
60
70
80
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0
10
20
30
40
50
60
70
80
90
100
ICP
, ICN VOL
TAGE (V)
V
(CSPOUT–CSNOUT)S
(mV)
87081 F12
