LTC4000
20
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applicaTions inForMaTion
Figure 8. Possible Voltage Ranges for V
OUT
and
V
OUT(INST_ON)
in Ideal Scenario
Similar to the input external PMOS, the charging external
PMOS must be able to withstand a gate to source voltage
greater than V
BGATE(ON)
(15V maximum) or the maximum
regulated voltage at the CSP pin, whichever is less. Consider
the expected maximum current, power dissipation and
instant-on voltage drop when selecting this PMOS. The
PMOS suggestions in Table 1 are an appropriate starting
point depending on the application.
Float Voltage, Output Voltage and Instant-On Voltage
Dependencies
The formulas for setting the float voltage, output voltage
and instant-on voltage are repeated here:
V
FLOAT
=
R
BFB1
+
R
BFB2
R
BFB2
• 1.136V
V
OUT
=
R
OFB1
+
R
OFB2
R
OFB2
• 1.193V
V
OUT(INST _ ON)
=
R
OFB1
+
R
OFB2
R
OFB2
• 0.974V
In the typical application, V
OUT
is set higher than V
FLOAT
to ensure that the battery is charged fully to its intended
float voltage. On the other hand, V
OUT
should not be
programmed too high since V
OUT(INST_ON)
, the minimum
voltage on CSP, depends on the same resistors R
OFB1
and
R
OFB2
that set V
OUT
. As noted before, this means that the
output voltage regulation level is always 122.5% of the
instant-on voltage. The higher the programmed value of
V
OUT(INST_ON)
, the larger the operating region when the
charger PMOS is driven in the linear region where it is
less efficient.
If R
OFB1
and R
OFB2
are set to be equal to R
BFB1
and R
BFB2
respectively, then the output voltage is set at 105% of
the float voltage and the instant-on voltage is set at 86%
of the float voltage. Figure 8 shows the range of possible
output voltages that can be set for V
OUT(INST_ON)
and V
OUT
with respect to V
FLOAT
to ensure the battery can be fully
charged in an ideal scenario.
Taking into account possible mismatches between the
resistor dividers as well as mismatches in the various
regulation loops, V
OUT
should not be programmed to
be less than 105% of V
FLOAT
to ensure that the battery
can be fully charged. This automatically means that the
instant-on voltage level should not be programmed to be
less than 86% of V
FLOAT
.
NOMINAL OUTPUT VOLTAGE
POSSIBLE
OUTPUT
VOLTAGE RANGE
75%
86%
4000 F08
POSSIBLE
INSTANT-ON
VOLTAGE RANGE
105%
100%
100%
81.6%
NOMINAL FLOAT VOLTAGE
100%
NOMINAL INSTANT-ON VOLTAGE
MINIMUM PRACTICAL
OUTPUT VOLTAGE
MINIMUM PRACTICAL
INSTANT-ON VOLTAGE
