LT8708
22
Rev 0
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increases the power flowing from V
OUT
into V
IN
. Higher
V
C
voltage reduces or stops the flow.
Forward (or reverse) DCM affects the power switches
as follows. Under light loading conditions, in FDCM (or
RDCM), synchronous switch M4 (or M1) is turned off
whenever instantaneous reverse (or forward) current in
the inductor is detected. This is to prevent drawing current
from V
OUT
(or V
IN
) and feeding current into V
IN
(or V
OUT
).
Under very light loads, the current comparator may also
remain tripped for several cycles and force switches M1
(or M2) and M3 (or M4) to stay off for the same number
of cycles i.e., skipping pulses. Synchronous switch M2
(or M3) will remain on during the skipped cycles, but
since switch M4 (or M1) is off, the inductor current will
not reverse directions.
Unidirectional Conduction: HCM
Large inductor current ripple can sometimes result in
high power dissipation of the M4 (or M1) junction diode
during the FDCM (or RDCM) operation described above.
This can happen, for example, when V
IN
>> V
OUT
and the
average V
OUT
current is relatively high, but M4 is turned
off to block negative components of the AC inductor cur-
rent. The hybrid current mode (or HCM) is an alternative
to DCM that often reduces the maximum M4 (or M1)
heating in such cases.
The hybrid current mode is a mixture of the light load
DCM operation and CCM operation, but only allows aver-
age current flow in one direction. As such, it is possible
to have the lower portions of the inductor current ripple
flow opposite to the selected direction while the average
current remains in the selected direction. The DIR pin is
used to select the desired forward (or FHCM) or reverse
(or RHCM) direction of average current flow.
HCM works by measuring the average forward V
OUT
cur-
rent and the average reverse V
IN
current indicated on ICN
and IMON_INP, respectively. In FHCM (or RHCM), light
load is detected when ICN (or IMON_INP) is above 255mV
(typical). As a result, M4 (or M1) is turned off to prevent
average current flow opposite to the desired direction.
Heavy load is detected when ICN (or IMON_INP) is below
205mV (typical). As a result, CCM operation is enabled,
allowing M4 (or M1) to turn on and reduce the diode’s
power dissipation.
NOTE: In FHCM operation connect a 17.4k resistor from
ground to the ICN pin, and in RHCM operation, connect
a 17.4k resistor from ground to the IMON_INP pin.
Unidirectional Conduction: Burst Mode
In Burst Mode operation, a V
C
voltage is set, with about
25mV of hysteresis, below which switching activity is
inhibited and above which switching activity is re-enabled.
A typical example is when, at light output currents, V
OUT
rises and forces the V
C
pin below the threshold that tem-
porarily inhibits switching. After V
OUT
drops slightly and
V
C
rises ~25mV, the switching is resumed, initially in the
buck-boost region. Burst Mode operation can increase ef-
ficiency at light load currents by eliminating unnecessary
switching activity and related power losses. In Burst Mode
operation, inductor current is only allowed in the forward
direction, regardless of the voltage on the DIR pin. Burst
Mode operation handles reverse-current detection similar
to forward DCM. The M4 switch is turned off when reverse
inductor current is detected.
ERROR AMPLIFIERS
The six internal error amplifiers combine to drive V
C
ac-
cording to Table 3, with the highest priority being at the top.
Table 3. Error Amp Priorities
TYPICAL CONDITION
PURPOSE
if
IMON_INN > 1.21V or
then V
C
Rises
to Reduce Negative I
IN
IMON_ON > 1.21V
to Reduce Negative I
OUT
else
if
FBIN < 1.205V or
then V
C
Falls
to Reduce Positive I
IN
or
Increase Negative I
IN
FBOUT > 1.207V or
to Reduce Positive I
OUT
or
Increase Negative I
OUT
IMON_INP > 1.209V or
to Reduce Positive I
IN
IMON_OP > 1.209V
to Reduce Positive I
OUT
else
V
C
Rises
Default
Note that certain error amplifiers are disabled under the
conditions shown in Table 4. A disabled error amplifier is
unable to affect V
C
and can be treated as if its associated
row is removed from Table 3.
OPERATION
