LTC1734L Datasheet, PDF (9/12 Pages) Linear Technology – Lithium-Ion Linear Battery Charger in ThinSOT

LTC1734L
APPLICATIONS INFORMATION
Monitoring Charge Current
The voltage on the PROG pin indicates the charge current
as a proportion of the maximum current set by the
program resistor. The charge current is equal to 250 •
(V_PROG/R_PROG) amps. This feature allows a microcontrol-
ler with an ADC to easily monitor charge current and if
desired, manually shut down the charger at the appropri-
ate time. The minimum PROG pin current is about 3µA
(I_PROGPU).
Errors in the charge current monitor voltage on the PROG
pin and in the full-scale charge current are inversely
proportional to battery current and can be statistically
approximated as follows:
One Sigma Error(%) ≅ 1 + 0.08/I_BAT(A)
Dynamic loads on the battery will cause transients to
appear on the PROG pin. Should they cause excessive
errors in charge current monitoring, a simple RC filter as
shown in Figure 2 can be used to filter the transients. The
filter will also quiet the PROG pin to help prevent momen-
tary entry into the manual shutdown mode.
Because the PROG pin is in a closed-loop signal path the
pole frequency must be kept high enough to maintain
adequate AC stability. This means that the maximum
resistance and capacitance presented to the PROG pin
must be limited. See the Stability section for more details.
Constant Current Source
The LTC1734L can be used as a constant current source
by disabling the voltage control loop as shown in Figure 3.
This is done by pulling the BAT pin below the preset float
voltage of 4.2V by grounding the BAT pin. The program
resistor will determine the output current. The output
current range can be between approximately 10mA and
180mA, depending on the maximum power rating of the
external PNP pass transistor.
External PNP Transistor
The external PNP pass transistor must have adequate
beta, low saturation voltage and sufficient power dissipa-
tion capability (including any heat sinking, if required).
To provide 180mA of charge current with the minimum
available base drive of approximately 20mA requires a
PNP beta greater than 9.
With low supply voltages, the PNP saturation voltage
(V_CESAT) becomes important. The V_CESAT must be less
than the minimum supply voltage minus the maximum
voltage drop across the internal sense resistor and bond
wires (0.3Ω) and battery float voltage. If the PNP transis-
tor can not achieve the low saturation voltage required,
base current will dramatically increase. This is to be
avoided for a number of reasons: output drive may reach
current limit resulting in the charger’s characteristics to
go out of specifications, excessive power dissipation may
force the IC into thermal shutdown, or the battery could
become discharged because some of the current from the
DRIVE pin could be pulled from the battery through the
forward biased collector base junction.
For example, to program a charge current of 100mA with
a minimum supply voltage of 4.75V, the minimum operat-
ing V_CE is:
V_CE(MIN)(V) = 4.75 – (0.1)(0.3) – 4.2 = 0.52V
The actual battery charge current (I_BAT) is slightly less
than the expected charge current because the charger
senses the emitter current and the battery charge current
will be reduced by the base current. In terms of β (I_C/I_B),
I_BAT can be calculated as follows:
I_BAT(A) = 250 • I_PROG[β/(β + 1)]
If β = 50, then I_BAT is 2% low. If desired, the 2% loss can
be compensated for by increasing I_PROG by 2%.
Another important factor to consider when choosing the
PNP pass transistor is the power handling capability. The
transistor’s 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_DD(MAX) – V_BAT(MIN))
V_DD(MAX) is the maximum supply voltage and V_BAT(MIN) is
the minimum battery voltage when discharged.
1734lf
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