LT8708
40
Rev 0
For more information
The purpose of the VINHIMON and VOUTLOMON func-
tions becomes clearer when considering the priorities of
the error amplifiers (see Table 3). A few important cases
should be considered.
1. V
IN
and V
OUT
are both above regulation: In this case
FBIN is greater than 1.205V while FBOUT is greater
than 1.207V. Normally this condition causes V
C
to
fall due to FBOUT being above 1.207V. The LT8708
responds by increasing the reverse current and power
being fed into V
IN
.
This can be an undesirable response, for example,
if V
IN
is connected to a battery being charged from
V
OUT
. The solution is to use VINHIMON to detect the
maximum V
IN
and disable reverse conduction by pull-
ing
RVSOFF
low.
2. V
IN
and V
OUT
are both below regulation: In this case
FBIN is below 1.205V while FBOUT is below 1.207V.
Normally this condition causes V
C
to fall due to FBIN
being below 1.205V. The LT8708 responds by in-
creasing the reverse current and power being drawn
from V
OUT.
This can be an undesirable response, for example, if
V
OUT
is connected to a battery or supercapacitor sup-
plying power to V
IN
. The solution is to use VOUTLO-
MON to detect the minimum V
OUT
and disable reverse
conduction by pulling
RVSOFF
low.
If VINHIMON rises above its activation threshold or VOUT-
LOMON falls below its activation threshold (see Electrical
Characteristics), the LT8708 will pull the
RVSOFF
pin low
and not allow M4 switch to turn on if the inductor cur-
rent is negative. In addition to the 24mV (typical) voltage
hysteresis, the VINHIMON pin will source 1μA (typical)
current and the VOUTLOMON pin will sink 1μA (typical)
current as current hysteresis.
There are two ways to configure the VINHIMON and
VOUTLOMON pins. Method (1) uses dedicated resistor
dividers for VINHIMON and VOUTLOMON respectively,
while method (2) uses common resistor dividers for
VINHIMON and FBIN as well as for VOUTLOMON and
FBOUT, allowing improved tracking with the FBOUT and
FBIN regulation voltages, respectively.
1. Connect a resistor divider between V
IN
, VINHIMON
and GND to configure the V
IN
overvoltage threshold.
Connect a resistor divider between V
OUT
, VOUTLO-
MON and GND to configure the V
OUT
undervoltage
threshold. (see Figure 12). Use the following equa-
tions to calculate the resistor values:
R
HIMON1
=
V
OVIN
+
– 1.207
I
FBDIV
R
HIMON2
=
1.207
I
FBDIV
R
HIMON3
=
1.207 – V
HYSMON
I
HYSMON
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
–
R
HIMON1
•R
HIMON2
R
HIMON1
+R
HIMON2
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
–
V
OVIN
–
•R
HIMON2
I
HYSMON
•(R
HIMON1
+R
HIMON2
)
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
R
LOMON1
=
V
UVOUT
–
– 1.207
I
FBDIV
R
LOMON2
=
1.207
I
FBDIV
R
LOMON3
=
V
UVOUT
+
•R
LOMON2
I
HYSMON
•(R
LOMON1
+R
LOMON2
)
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
–
R
LOMON1
•R
LOMON2
R
LOMON1
+R
LOMON2
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
–
1.207 + V
HYSMON
I
HYSMON
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
where:
I
FBDIV
is the desired current through the resistor string.
50μA – 100μA is a good value.
V
OVIN
+ and V
OVIN
– are the rising and falling V
IN
over-
voltage thresholds.
V
UVOUT
+ and V
UVOUT
– are the rising and falling V
OUT
undervoltage thresholds.
R
HIMON1-3
and R
LOMON1-3
V
HYSMON
is the VINHIMON and VOUTLOMON hyster-
esis voltage. Typical value is 24mV.
I
HYSMON
is the VINHIMON and VOUTLOMON hysteresis
current. Typical value is 1μA.
APPLICATIONS INFORMATION
