LTC4000_15 Datasheet, PDF (19/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

LTC4000
Applications Information
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.
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.
CSP
LTC4000
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
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
4000 F07
Figure 7. Charging PMOS Overtemperature Detection Circuit
Protecting PMOS from Overheating
For more information www.linear.com/LTC4000
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