Power Switch Control; Unidirectional And Bidirectional Conduction; Error Amplifiers - Linear Technology Analog Devices LT8708-1 Datasheet

OPERATION
Table 3. Error Amplifiers (EA1–EA6)
AMPLIFIER
NAME PIN NAME
EA1 IMON_INN
EA3 FBIN
EA4 FBOUT
EA5 IMON_INP
EA6 IMON_OP
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 , V and ground.
IN OUT
V
IN
TG1 M1
SW1
BG1 M2
Figure 4. Simplified Diagram of the Buck-Boost Switches

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
USED TO LIMIT OR REGULATE
Negative I
IN(SLAVE)
V Voltage
IN
V Voltage
OUT
Positive I
IN(SLAVE)
I
OUT(SLAVE)
V
OUT
M4 TG2
L
SW2
M3 BG2
R
SENSE
87081 F03
For more information
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
ing to Table 4, with the highest priority being at the top.
Table 4. Error Amp Priorities
TYPICAL CONDITION
if IMON_INN > 1.21V
else
FBIN < 1.205V or
if
FBOUT > 1.207V or
IMON_INP > 1.209V or
IMON_OP > 1.209V
else
Note that certain error amplifiers are disabled under the
conditions shown in Table 5. A disabled error amplifier is
unable to affect V and can be treated as if its associated
C
row is removed from Table 4.
Table 5. Automatically Disabled Error Amp Conditions
ERROR VOUTLOMON
AMP PIN NAME ASSERTED
EA1 IMON_INN
EA3 FBIN
EA4 FBOUT
EA5 IMON_INP
EA6 IMON_OP
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LT8708-1
accord-
C
PURPOSE
then V
C
to Reduce Negative I
IN(SLAVE)
Rises
to Reduce Positive I
IN(SLAVE)
or
Increase Negative I
IN(SLAVE)
then V
to Reduce Positive I
C
OUT(SLAVE)
Falls
or Increase Negative I
IN(SLAVE)
to Reduce Positive I
IN(SLAVE)
to Reduce Positive I
OUT(SLAVE)
V
C
Default
Rises
VINHIMON RDCM or
ASSERTED RHCM
2*
1*
19
3*
Rev 0