Specifications

LTC4056-4.2
11
405642f
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
APPLICATIO S I FOR ATIO
WUU
U
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 PACKAGE STYLE ZETEX PART NUMBER ROHM PART NUMBER COMMENTS
3 2 x 2MLP ZXT1M322 Very Low V
CESAT
0.5 SOT-23 FMMT549 Low V
CESAT
0.625 SOT-23 FMMT720 Very Low V
CESAT
, High Beta
1 SOT-89 FCX589 or BCX69
1.1 SOT-23-6 ZXT13P12DE6 Very Low V
CESAT
, High Beta, Small
1 to 2 SOT-89 FCX717 Very Low V
CESAT
, High Beta
2 SOT-223 FZT589 Low V
CESAT
2 SOT-223 BCP69 or FZT549
0.75 FTR 2SB822 Low V
CESAT
1 ATV 2SB1443 Low V
CESAT
2 SOT-89 2SA1797 Low V
CESAT
10 (T
C
= 25°C) TO-252 2SB1182 Low V
CESAT
, High Beta