LTC4000-1 Datasheet, PDF (20/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging with Maximum Power Point Control

LTC4000-1
Applications Information
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:
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
CSP
LTC4000-1
CSN
BGATE
BAT
TO SYSTEM
VISHAY CURVE 2
NTC RESISTOR
THERMALLY COUPLED
R_CS
WITH CHARGING PMOS
R_NTC2
M2
R4 = R_NTC2
AT 25°C
20
BIAS
R3
NTC
C_BIAS
Li-Ion
BATTERY PACK
R_NTC1
2N7002L
LTC1540
VOLTAGE HYSTERESIS CAN
BE PROGRAMMED FOR
TEMPERATURE HYSTERESIS
86mV ≈ 10°C
162k
RISING
TEMPERATURE
THRESHOLD
SET AT 90°C
20k
40001 F07
Figure 7. Charging PMOS Overtemperature Detection Circuit
Protecting PMOS from Overheating
40001f