LTC4056-4.2_15 Datasheet, PDF (11/16 Pages) Linear Technology – Linear Li-Ion Charger with Termination in ThinSOT

LTC4056-4.2
APPLICATIO S I FOR ATIO
DRIVE pin could be pulled from the battery through the
forward biased collector base junction.
For example, to program a charge current of 500mA with
a minimum supply voltage of 4.75V, the minimum operat-
ing V_CE is:
V_CE(MIN)(V) = 4.75 – (0.5) • (0.2) – 4.2 = 0.45V
Another important factor to consider when choosing the
PNP pass transistor is the power handling capability. The
transistor data sheet will usually give the maximum rated
power dissipation at a given ambient temperature with a
power derating for elevated temperature operation. The
maximum power dissipation of the PNP when charging is:
P_D(MAX)(W) = I_BAT • (V_CC(MAX) – V_BAT(MIN))
V_CC(MAX) is the maximum supply voltage and V_BAT(MIN) is
the minimum battery voltage when discharged.
Once the maximum power dissipation and V_CE(MIN) are
known, Table 1 can be used as a guide in selecting some
PNPs to consider. In the table, very low V_CESAT is less than
0.25V, low V_CESAT is 0.25V to 0.5V and the others are 0.5V
to 0.8V all depending on the current. See the manufac-
turer data sheet for details. All of the transistors are rated
to carry at least 1A continuously as long as the power
dissipation is within limits. In addition, the maximum
supply voltage, minimum battery voltage and chosen
charge current should be checked against the
manufacturer’s data sheet to ensure that the PNP transis-
tor is operating within its safe operating area. The Stabil-
ity section addresses caution in the use of very high beta
PNP transistors.
Should overheating of the PNP transistor be a concern,
protection can be achieved with a positive temperature
coefficient (PTC) thermistor wired in series with the
current programming resistor and thermally coupled to
the transistor. The PTH9C chip series from Murata has a
steep resistance increase at temperature thresholds from
85°C to 145°C making it behave somewhat like a thermo-
stat switch. For example, the model PTH9C16TBA471Q
thermistor is 470Ω at 25°C but abruptly increases its
resistance to 4.7k at 125°C. Below 125°C, the device
exhibits a small negative TC. The 470Ω thermistor can be
added in series with a 976Ω resistor to form the current
programming resistor for a 640mA charger. Should the
thermistor reach 125°C, the charge current will drop to
160mA and inhibit any further increase in temperature.
Stability
The LTC4056 contains two control loops: constant volt-
age and constant current. The constant voltage loop is
stable without any compensation when a battery is con-
nected with low impedance leads. Excessive lead length,
Table 1. PNP Pass Transistor Selection Guide
MAXIMUM P_D(W)
MOUNTED ON BOARD
AT T_A = 25°C
3
PACKAGE STYLE
2 x 2MLP
ZETEX PART NUMBER
ZXT1M322
0.5
SOT-23
FMMT549
0.625
SOT-23
FMMT720
1
SOT-89
FCX589 or BCX69
1.1
SOT-23-6
ZXT13P12DE6
1 to 2
SOT-89
FCX717
2
SOT-223
FZT589
2
SOT-223
BCP69 or FZT549
0.75
FTR
1
ATV
2
SOT-89
10 (T_C = 25°C)
TO-252
ROHM PART NUMBER COMMENTS
Very Low V_CESAT
Low V_CESAT
Very Low V_CESAT, High Beta
Very Low V_CESAT, High Beta, Small
Very Low V_CESAT, High Beta
Low V_CESAT
2SB822
2SB1443
2SA1797
2SB1182
Low V_CESAT
Low V_CESAT
Low V_CESAT
Low V_CESAT, High Beta
405642f
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