LTC3789 LINER [Linear Technology], LTC3789 Datasheet - Page 20

• Manufacturer Part Number: LTC3789
• Description: High Voltage High Current Controller for Battery Charging and Power Management
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
LTC4000
Battery Instant-On and Ideal Diode External PMOS
Consideration
The instant-on voltage level is determined using the fol-
lowing formula:
Note that R
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
dissipation in the PMOS can be calculated as follows:
where I
20
P
V
TRKL
OUT(INST _ ON)
CLIM(TRKL)
= 0.86 • V
[
OFB1
and R
=
is the trickle charge current limit.
FLOAT
R
OFB1
OFB2
R
OFB2
+ R
– V
are the same resistors that
BAT
LTC4000
OFB2
BATTERY PACK
Figure 7. Charging PMOS Overtemperature Detection Circuit
Protecting PMOS from Overheating
BGATE
]
BIAS
CSN
CSP
NTC
BAT
• I
• 0.974V
CLIM(TRKL)
Li-Ion
LOBAT
R3
R
NTC1
), the power
C
R
BIAS
CS
M2
TO SYSTEM
WITH CHARGING PMOS
THERMALLY COUPLED
TEMPERATURE HYSTERESIS
VOLTAGE HYSTERESIS CAN
VISHAY CURVE 2
NTC RESISTOR
BE PROGRAMMED FOR
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:
where I
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.
2N7002L
R
NTC2
86mV ≈ 10°C
P
LTC1540
INST _ ON
CLIM
R4 = R
AT 25°C
NTC2
is the full scale charge current limit.
= 0.86 • V
–
+
[
162k
20k
RISING
TEMPERATURE
THRESHOLD
SET AT 90°C
4000 F07
FLOAT
– V
BAT
]
• I
CLIM
4000f