Linear Technology Analog Devices LT8708-1 Datasheet page 27

TYPICAL APPLICATIONS
results in higher I
OUT(SLAVE)
V is driven by error amplifier EA6 during I
C
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
and thus the I
OUT(SLAVE)
• Resistor R converts the I
SENSE2
a voltage that can be measured by amplifier A1. This
voltage is denoted as V
• Transconductance amplifier A1 makes sure that
I is equal to the current set by the ICP
OUT(SLAVE)
and ICN signals. If I
OUT(SLAVE)
requested by ICP and ICN, additional current is deliv-
ered out of A1. This raises IMON_OP which reduces
V and reduces I
C OUT(SLAVE)
becomes lower than requested by ICP and ICN, the
current out of A1 is reduced. This lowers IMON_OP
which raises V and increases I
C
Figure 11 illustrates, in CCM mode, the typical relation-
ship between the master's output current I
the resulting ICP and ICN control voltages, and the fur-
ther resulting I
OUT(SLAVE)
explained with a few examples.
In these examples, the output current sense resistors are
R = 10mΩ for the master and the slave devices.
SENSE2
First, assume the master's output current I
is 4A. This results in the master LT8708 measuring a
2.0
1.6
1.2
0.8
0.4
0.0
–0.4
–0.8
–1.2
–1.6
–80 –60 –40 –20
V
(CSPOUT–CSNOUT)M
Figure 11. I Control Voltage Relationships (CCM)
OUT(SLAVE)
current and vice versa.
OUT(SLAVE)
current.
current into
OUT(SLAVE)
in Figure 10.
(CSPOUT–CSNOUT)S
becomes higher than
. Conversely, if I
OUT(SLAVE)
.
OUT(SLAVE)
OUT(MASTER)
current. Figure 11 can best be
OUT(MASTER)
100
80
60
40
20
0
–20
–40
V
(CSPOUT–CSNOUT)S
ICN
–60
ICP
–80
0 20 40 60 80
(mV)
87081 F11
For more information
current sense voltage of V
= 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)
,
C
V
(CSPOUT – CSNOUT)S
I =
OUT(SLAVE)
40mV (from Figure 10)
10mΩ
Alternatively, if the master's output current I
is –2A. Then the master LT8708 will measure a current
sense voltage of V
–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)
V
(CSPOUT – CSNOUT)S
I =
OUT(SLAVE)
,
–20mV (from Figure 10)
10mΩ
Figure 12 illustrates the relationship between I
ICP , ICN and I
OUT(SLAVE)
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
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 10
Figure 12. I
OUT(SLAVE)
(FDCM, FHCM and Burst Mode Operation)
www.analog.com
LT8708-1
= 4A • 10mΩ
(CSPOUT–VCSNOUT)M
to:
=
R
SENSE2
= 4A
= –2A • 10mΩ =
(CSPOUT–VCSNOUT)M
to:
=
R
SENSE2
= – 2A
in FDCM, FHCM and Burst Mode
V
(CSPOUT–CSNOUT)S
ICN
ICP
20 30 40 50 60 70 80
V (mV)
(CSPOUT–CSNOUT)M
87081 F12
Control Voltage Relationships
OUT(MASTER)
,
OUT(MASTER)
100
90
80
70
60
50
40
30
20
10
0
Rev 0
27