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LTC4000IGN#PBF

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LTC4000
19
4000fb
For more information www.linear.com/LTC4000
Battery Instant-On and Ideal Diode External PMOS
Consideration
The instant-on voltage level is determined using the fol-
lowing formula:
V
OUT(INST _ ON)
=
R
OFB1
+R
OFB2
R
OFB2
• 0.974V
Note that R
OFB1
and R
OFB2
are the same resistors that
program the output voltage regulation level. Therefore,
the output voltage regulation level is always 122.5% of
the instant-on voltage level.
During instant-on operation, it is critical to consider the
charging PMOS power dissipation. When the battery volt-
age is below the low battery threshold (V
LOBAT
), the power
dissipation in the PMOS can be calculated as follows:
P
TRKL
= 0.86 • V
FLOAT
– V
BAT
[ ]
• I
CLIM(TRKL)
where I
CLIM(TRKL)
is the trickle charge current limit.
Figure 7. Charging PMOS Overtemperature Detection Circuit
Protecting PMOS from Overheating
applicaTions inForMaTion
On the other hand, when the battery voltage is above the
low battery threshold but still below the instant-on thresh-
old, the power dissipation can be calculated as follows:
P
INST _ON
= 0.86 • V
FLOAT
– V
BAT
[ ]
• I
CLIM
where I
CLIM
is the full scale charge current limit.
For example, when charging a 3-cell Lithium Ion battery
with a programmed full charged current of 1A, the float
voltage is 12.6V, the bad battery voltage level is 8.55V and
the instant-on voltage level is 10.8V. During instant-on
operation and in the trickle charge mode, the worst case
maximum power dissipation in the PMOS is 1.08W. When
the battery voltage is above the bad battery voltage level,
then the worst case maximum power dissipation is 2.25W.
When overheating of the charging PMOS is a concern, it is
recommended that the user add a temperature detection
circuit that pulls down on the NTC pin. This pauses charg-
ing whenever the external PMOS temperature is too high.
A sample circuit that performs this temperature detection
function is shown in Figure 7.
Li-Ion
BATTERY PACK
R
CS
M2
R
NTC1
TO SYSTEM
RISING
TEMPERATURE
THRESHOLD
SET AT 90°C
VISHAY CURVE 2
NTC RESISTOR
THERMALLY COUPLED
WITH CHARGING PMOS
VOLTAGE HYSTERESIS CAN
BE PROGRAMMED FOR
TEMPERATURE HYSTERESIS
86mV ≈ 10°C
CSN
BGATE
BAT
CSP
BIAS
NTC
LTC4000
162k
20k
R3
R4 = R
NTC2
AT 25°C
4000 F07
C
BIAS
R
NTC2
LTC1540
+
–
2N7002L
LTC4000
20
4000fb
For more information www.linear.com/LTC4000
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
LTC4000
21
4000fb
For more information www.linear.com/LTC4000
applicaTions inForMaTion
Notice that with only one degree of freedom (i.e. adjusting
the value of R3), the user can only use one of the formulas
above to set either the cold or hot threshold but not both.
If the value of R3 is set to adjust the cold threshold, the
value of the NTC resistor at the hot threshold is then
equal to 0.179 • R
NTC
at cold_threshold. Similarly, if the
value of R3 is set to adjust the hot threshold, the value
of the NTC resistor at the cold threshold is then equal to
5.571 • R
NTC
at cold_threshold.
Note that changing the value of R3 to be larger than R25
will move both the hot and cold threshold lower and vice
versa. For example, using a Vishay Curve 2 thermistor
whose nominal value at 25°C is 100k, the user can set
the cold temperature to be at 5°C by setting the value of
R3 = 75k, which automatically then sets the hot threshold
at approximately 50°C.
It is possible to adjust the hot and cold threshold indepen-
dently by introducing another resistor as a second degree
of freedom (Figure 10). The resistor R
D
in effect reduces
the sensitivity of the resistance between the NTC pin and
ground.
Therefore, intuitively this resistor will move the hot
threshold to a hotter temperature and the cold threshold
to a colder temperature.
Figure 9. NTC Thermistor Connection
Figure 10. NTC Thermistor Connection with
Desensitizing Resistor R
D
NTC RESISTOR
THERMALLY COUPLED
WITH BATTERY PACK
R3
R
NTC
NTC
BIAS
BAT
LTC4000
4000 F09
C
BIAS
NTC RESISTOR
THERMALLY COUPLED
WITH BATTERY PACK
R3
R
NTC
NTC
BIAS
BAT
LTC4000
R
D
4000 F10
C
BIAS
Battery Temperature Qualified Charging
To use the battery temperature qualified charging feature,
connect an NTC thermistor, R
NTC
, between the NTC pin
and the GND pin, and a bias resistor, R3, from the BIAS
pin to the NTC pin (Figure 9). Thermistor manufacturer
datasheets usually include either a temperature lookup
table or a formula relating temperature to the resistor
value at that corresponding temperature.
In a simple application, R3 is a 1% resistor with a value
equal to the value of the chosen NTC thermistor at 25°C
(R25). In this simple setup, the LTC4000 will pause charg-
ing when the resistance of the NTC thermistor drops to
0.54 times the value of R25. For a Vishay Curve 2 therm-
istor, this corresponds to approximately 41.5°C. As the
temperature drops, the resistance of the NTC thermistor
rises. The LTC4000 is also designed to pause charging
when the value of the NTC thermistor increases to three
times the value of R25. For a Vishay Curve 2 thermistor,
this corresponds to approximately –1.5°C. With Vishay
Curve 2 thermistor, the hot and cold comparators each
have approximately 5°C of hysteresis to prevent oscillation
about the trip point.
The hot and cold threshold can be adjusted by changing
the value
of R3. Instead of simply setting R3 to be equal to
R25,
R3 is set according to one of the following formulas:
R3 =
R
NTC
at cold_ threshold
3
or
R3
=
1.857 • R
NTC
at hot_ threshold
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LTC4000IGN#PBF

Mfr. #:
Buy LTC4000IGN#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management High Voltage, High Current Controller for Battery Charging and Power Management
Lifecycle:
New from this manufacturer.
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