LT8708-1
19
Rev 0
Table 3. Error Amplifiers (EA1–EA6)
AMPLIFIER
NAME
PIN NAME
USED TO LIMIT OR REGULATE
EA1
IMON_INN
Negative I
IN(SLAVE)
EA3
FBIN
V
IN
Voltage
EA4
FBOUT
V
OUT
Voltage
EA5
IMON_INP
Positive I
IN(SLAVE)
EA6
IMON_OP
I
OUT(SLAVE)
Note that the current and power flow can also be restricted
to one direction, as needed, by the selected conduction
mode discussed in the Unidirectional and Bidirectional
Conduction section.
POWER SWITCH CONTROL
The LT8708-1 employs the same power switch control
as the LT8708 (see Power Switch Control section of the
LT8708 data sheet). Figure 4 shows a simplified diagram
of how the four power switches are connected to the
inductor, V
IN
, V
OUT
and ground.
pins together. In addition, the
RVSOFF
pins of all phases
should be connected together.
Note that when operating in the forward hybrid conduction
mode (FHCM), the LT8708-1 operation differs slightly from
the LT8708. Instead of measuring the ICN pin voltage for
light load detection, the LT8708-1 measures the IMON_ON
pin. Light load is detected when IMON_ON is above 245mV
(typical). Therefore, FHCM operation requires a 17.4k resis-
tor, and a parallel filter capacitor, from ground to the IMON_
ON pin of the LT8708-1. Reverse hybrid conduction mode
(RHCM) operates identically in the LT8708 and LT8708-1.
See the Unidirectional and Bidirectional Conduction: HCM
section of the LT8708 data sheet for details.
ERROR AMPLIFIERS
Five internal error amplifiers combine to drive V
C
accord-
ing to Table 4, with the highest priority being at the top.
Table 4. Error Amp Priorities
TYPICAL CONDITION
PURPOSE
if
IMON_INN > 1.21V
then V
C
Rises
to Reduce Negative I
IN(SLAVE)
else
if
FBIN < 1.205V or
then V
C
Falls
to Reduce Positive I
IN(SLAVE)
or
Increase Negative I
IN(SLAVE)
FBOUT > 1.207V or
to Reduce Positive I
OUT(SLAVE)
or Increase Negative I
IN(SLAVE)
IMON_INP > 1.209V or
to Reduce Positive I
IN(SLAVE)
IMON_OP > 1.209V
to Reduce Positive I
OUT(SLAVE)
else
V
C
Rises
Default
Note that certain error amplifiers are disabled under the
conditions shown in Table 5. A disabled error amplifier is
unable to affect V
C
and can be treated as if its associated
row is removed from Table 4.
Table 5. Automatically Disabled Error Amp Conditions
ERROR
AMP
PIN NAME
VOUTLOMON
ASSERTED
VINHIMON
ASSERTED
RDCM or
RHCM
EA1
IMON_INN
EA3
FBIN
2*
EA4
FBOUT
1*
3*
EA5
IMON_INP
EA6
IMON_OP
OPERATION
Figure 4. Simplified Diagram of the Buck-Boost Switches
TG1
BG1
TG2
BG2
R
SENSE
87081 F03
M1
M2
M4
M3
L
SW1
SW2
V
IN
V
OUT
UNIDIRECTIONAL AND BIDIRECTIONAL CONDUCTION
As with the LT8708, the LT8708-1 has one bidirectional
and three unidirectional current conduction modes (CCM,
HCM, DCM and Burst Mode operation, respectively). The
LT8708-1’s MODE, DIR and
RVSOFF
pins operate in the
same way as in the LT8708 to select the desired con-
duction modes. (See the Unidirectional and Bidirectional
Conduction section of the LT8708 data sheet for details).
In general, it is highly recommended to keep all phases
of an LT8708 system in the same conduction mode. This
is done by setting all MODE and DIR pins to the same
states, or shorting all MODE pins together and all DIR
