LT8708
21
Rev 0
modes (DCM: discontinuous conduction mode, HCM: hy-
brid current mode and Burst Mode operation) only allow
current and power to flow in one direction. Unidirectional
settings override the V
C
pin’s attempt to direct current and
power opposite to the selected direction.
The DIR pin selects the allowed power direction when
using the DCM and HCM unidirectional modes. The Burst
Mode operation only operates in the forward direction and
is not affected by the DIR pin. In DCM and HCM modes,
driving DIR > 1.6V (typical) selects forward operation
which only allows power flow from V
IN
to V
OUT
. Driving
DIR < 1.2V (typical) selects reverse operation which only
allows power flow from V
OUT
to V
IN
.
Next, a low state on the
RVSOFF
pin inhibits reverse current
and power flow.
RVSOFF
is an open-drain pin that requires
a pull-up resistor. LDO33 or INTV
CC
are convenient nodes
to pull
RVSOFF
up to. Normally,
RVSOFF
is only pulled low
in response to a low V
OUT
voltage (via the VOUTLOMON
comparator) or a high V
IN
voltage (via the VINHIMON
comparator). However, external devices are permitted to
pull
RVSOFF
low as needed. More information is available
in the VINHIMON, VOUTLOMON and
RVSOFF
section.
Table 2 summarizes selection of the various conduction
modes. See the Electrical Characteristics for the voltage
thresholds of the DIR, VINHIMON, VOUTLOMON and
RVSOFF
pins.
Table 2. Conduction Configurations
MODE PIN
DIR PIN
STATE
RVSOFF
PIN STATE
CONDUCTION
MODE
POSSIBLE
DIRECTION
<0.4V
–
Hi
CCM
Forward and
Reverse
Lo
DCM
Forward
0.8V to
1.2V
Hi
–
HCM
Forward
Lo
Hi
Reverse
Lo
–
None
1.6V to
2.0V
Hi
–
DCM
Forward
Lo
Hi
Reverse
Lo
–
None
>2.4V
–
Hi
Burst Mode
Operation
Forward
Lo
–
None
The conduction configuration can be changed during
operation, as needed, with the following restrictions:
1. Before transitioning from MODE = Burst Mode opera-
tion to MODE = CCM, the DIR pin must be driven to
the Hi (Forward) state.
2. Avoid control pulses on the MODE and DIR pins nar-
rower than 15 LT8708 clock cycles.
Note: The V
C
pin may be railed at the moment the DIR pin
or MODE pin changes state. The railed V
C
voltage cor-
responds to zero current in one direction and maximum
current in the other. Therefore, if a small value R
SENSE
resistor is used, the chip may momentarily command high
inductor current immediately after the DIR or MODE pin
change. An undersized inductor may become saturated
in this case. An edge detector on the DIR and/or MODE
pin can be used to reset the chip, forcing a soft-start and
limiting the initial current. See the 48V to 14V Bidirectional
Dual Battery System with FHCM & RHCM in the Typical
Applications section as an example.
More details about each of the four conduction modes are
provided in the following sub-sections.
Bidirectional Conduction: CCM
The continuous conduction mode allows the inductor cur-
rent to flow in the forward or reverse direction, depending
on the V
C
voltage. When CCM is selected, high V
C
voltage
causes current and power to flow from V
IN
to V
OUT
and low
V
C
voltage causes current and power to flow from V
OUT
to
V
IN
. At very light load currents the inductor current may
ripple positive and negative as the appropriate average
current is delivered to the appropriate output.
Unidirectional Conduction: DCM
The discontinuous conduction mode restricts the inductor
current so that it can only flow in one direction, positive
towards V
OUT
(Forward DCM) or negative towards V
IN
(Reverse DCM). The forward/reverse selection is made
by driving the DIR pin as desired.
When FDCM is selected, higher V
C
voltage increases the
power flowing from V
IN
to V
OUT
. Lower V
C
voltage reduces
or stops the flow. When RDCM is selected, lower V
C
voltage
OPERATION
