LTC4000_15 Datasheet, PDF (20/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

LTC4000
Applications Information
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
NOMINAL FLOAT VOLTAGE ^100%
POSSIBLE POSSIBLE
OUTPUT INSTANT-ON
VOLTAGE RANGE VOLTAGE RANGE
105%
100%
100%
MINIMUM PRACTICAL
OUTPUT VOLTAGE
86%
NOMINAL INSTANT-ON VOLTAGE
75%
MINIMUM PRACTICAL
INSTANT-ON VOLTAGE
81.6%
4000 F08
Figure 8. Possible Voltage Ranges for V_OUT and
V_{OUT(INST_ON)} in Ideal Scenario
20
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