Datasheet
LTC4000-1
19
40001fa
For more information www.linear.com/LTC4001-1
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.
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:
Figure 7. Charging PMOS Overtemperature Detection Circuit
Protecting PMOS from Overheating
applicaTions inForMaTion
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
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
LTC4000-1
162k
20k
R3
R4 = R
NTC2
AT 25°C
40001 F07
C
BIAS
R
NTC2
LTC1540
+
–
2N7002L