Linear Technology Analog Devices LT8708, Техническое описание

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LT8708
18
Rev 0
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with constant current (EA6) to a maximum voltage (EA4)
and also reversed, at times, to supply power back to V
IN
using the other error amplifiers to regulate V
IN
and limit
the maximum current.
POWER SWITCH CONTROL
The following discussions about the power switch control
assume that the LT8708 is operating in the continuous
conduction mode (see Bidirectional Conduction: CCM).
Other conduction modes have slight differences that are
discussed later in their respective Conduction sections.
Figure 3 shows a simplified diagram of how the four power
switches are connected to the inductor, V
IN
, V
OUT
and
ground. Figure 4 shows the regions of operation for the
LT8708 as a function of V
OUT
– V
IN
or switch duty cycle
(DC). The power switches are properly controlled so the
transfer between modes is continuous.
OPERATION
Figure 3. Simplified Diagram of the Buck-Boost Switches
Figure 4. Operating Regions vs V
OUT
− V
IN
TG1
BG1
TG2
BG2
R
SENSE
8708 F03
M1
M2
M4
M3
L
SW1 SW2
V
IN
V
OUT
M1 ON, M2 OFF
PWM M3, M4 SWITCHES
M4 ON, M3 OFF
PWM M1, M2 SWITCHES
4-SWITCH PWM
V
OU
T
-V
IN
SWITCH
M3 DC
MAX
SWITCH
M2 DC
MAX
SWITCH
M3 DC
MIN
SWITCH
M2 DC
MIN
BOOST REGION
BUCK REGION
0 BUCK/BOOST REGION
8708 F04
can be used to limit or regulate their respective voltages or
currents as shown in Table 1.
Table 1. Error Amplifiers (EA1 − EA6)
AMPLIFIER NAME PIN NAME USED TO LIMIT OR REGULATE
EA1 IMON_INN Negative I
IN
EA2 IMON_ON Negative I
OUT
EA3 FBIN V
IN
Voltage
EA4 FBOUT V
OUT
Voltage
EA5 IMON_INP Positive I
IN
EA6 IMON_OP Positive I
OUT
The V
C
voltage typically has a min-max range of about 1.2V.
The maximum V
C
voltage commands the most positive
inductor current and, thus, commands the most power
flow from V
IN
to V
OUT.
The minimum V
C
voltage commands
the most negative inductor current and, thus, commands
the most power flow from V
OUT
to V
IN
.
In a simple example of V
OUT
regulation, the FBOUT pin
receives the V
OUT
voltage feedback signal which is com-
pared to the internal reference voltage using EA4. Low
V
OUT
voltage raises V
C
and, thus, more current flows into
V
OUT
. Conversely, higher V
OUT
reduces V
C
, thus, reducing
the current into V
OUT
or even drawing current and power
from V
OUT
.
Note that the current and power flow can also be restricted
to one direction, as needed, by the selected conduction
mode discussed in the Uni and Bidirectional Conduction
section.
As mentioned previously, the LT8708 also provides
bidirectional current regulation capabilities at both the
input and the output. The V
OUT
current can be regulated
or limited in the forward and reverse directions (EA6 and
EA2, respectively). The V
IN
current can also be regulated
or limited in the forward direction and reverse directions
(EA5 and EA1, respectively).
In a common application, V
OUT
might be regulated using
EA4, while the remaining error amplifiers are monitoring for
excessive input or output current or an input undervoltage
condition. In other applications, such as a battery backup
system, a battery connected to V
OUT
might be charged