LT8708
23
Rev 0
Table 4. Automatically Disabled Error Amp Conditions
ERROR
AMP
PIN NAME
VOUTLOMON
ASSERTED
VINHIMON
ASSERTED
RDCM or RHCM
–
RVSOFF
<1.207V
EA1
IMON_INN
4*
EA2
IMON_ON
4*
EA3
FBIN
2*
4*
EA4
FBOUT
1*
3*
4*
EA5
IMON_INP
4*
EA6
IMON_OP
4*
A 1* – 4* indicates that the error amplifier listed for that row is disabled
under that column’s condition. The purposes of disabling the respective
amplifiers are listed below.
1* This improves transient response when
VOUTLOMON deasserts.
2* This improves transient response when
VINHIMON deasserts.
3* Since power can only transfer from V
OUT
to V
IN
, this
prevents higher FBOUT/V
OUT
voltages from interfering
with the FBIN/V
IN
voltage regulation.
4* No switching occurs in this condition. Disabling the error
amplifiers improves transient response when resuming
switching operation.
Some applications don’t require the use of all six error
amplifiers. When unused, the respective input pin(s) should
be driven so that they don’t interfere with the operation of
the remaining amplifiers. Use Table 5 as a guide.
Table 5. Disabling Unused Amplifiers
AMPLIFIER
NAME
PIN NAME
TIE TO DISABLE
EXAMPLE DISABLED
PIN CONNECTION
EA1
IMON_INN
< 0.9V
GND
EA2
IMON_ON
EA3
FBIN
> 1.5V
LDO33
EA4
FBOUT
< 0.9V
GND
EA5
IMON_INP
EA6
IMON_OP
V
OUT
REGULATION AND SENSING
Two pins, FBOUT and VOUTLOMON, are provided to sense
the V
OUT
voltage and issue the appropriate response to
the switching regulator.
V
OUT
: Regulation
V
OUT
is regulated, subject to the priorities in Table 3, us-
ing a resistor divider between V
OUT
, FBOUT and ground.
FBOUT connects to the EA4 amplifier to drive V
C
. When
FBOUT rises near or above the EA4 reference (1.207V
typical), V
C
typically falls, commanding less current into
V
OUT
. The V
OUT
regulation voltage is given by the equation:
V
OUT
=
1.207V • 1+
R
FBOUT1
R
FBOUT2
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
where:
R
FBOUT1
and R
FBOUT2
are shown in Figure 1.
V
OUT
: Above Regulation
When the FBOUT pin and EA4 detect that V
OUT
is signifi-
cantly above regulation, V
C
typically falls to its minimum
voltage. The LT8708 responds to the minimum V
C
voltage
according to the conduction mode enabled by MODE, DIR
and
RVSOFF
. If reverse conduction is not allowed (FDCM,
FHCM and Burst Mode operation) then switching will stop
and current won’t be delivered to V
IN
. If reverse conduc-
tion is allowed (CCM, RDCM and RHCM), then current
and power will flow from V
OUT
to V
IN
.
V
OUT
: Below Regulation and Undervoltage
When the FBOUT pin and EA4 detect V
OUT
is below regula-
tion, V
C
typically rises. If forward conduction is enabled
(CCM, FDCM, FHCM and Burst mode), then current and
power will flow from V
IN
to V
OUT
.
A resistor divider between V
OUT,
VOUTLOMON and ground
is used to detect V
OUT
undervoltage. This function prevents
reverse conduction, from V
OUT
to V
IN
, from drawing V
OUT
down lower than desired. When undervoltage is detected
by VOUTLOMON,
RVSOFF
is pulled low to disable reverse
current and power. This function can be used as a UVLO
(undervoltage lockout), for example, when a battery or
supercapacitor, connected to V
OUT
, is supplying power
to V
IN
. See the VINHIMON, VOUTLOMON and
RVSOFF
section for more detailed information.
OPERATION
