LTC4060_15 Datasheet, PDF (15/20 Pages) Linear Technology – Standalone Linear NiMH/NiCd Fast Battery Charger

LTC4060
APPLICATIO S I FOR ATIO
charging (the DRIVE pin is internally switched to the BAT
pin). Most transistors will meet this requirement as well.
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 current sense resistor and
bond wires (approximately 0.08Ω) and maximum battery
voltage presented to the charger accounting for wire I • R
drops.
V_CESAT (V) < V_DD(MIN) – (I_BAT(MAX) • 0.08Ω + V_BAT(MAX))
For example, if it were desired to have a programmed
charge current of 2A with a minimum supply voltage of
4.75V and a maximum battery voltage of 3.6V (2 series
cells at 1.8V each), then the minimum operating V_CESAT is:
V_CESAT (V) = 4.75 – (2 • 0.08 + 3.6) = 0.99V
If the PNP transistor cannot achieve the saturation voltage
required, base current will dramatically increase. This is to
be avoided for a number of reasons: DRIVE pin current
may reach current limit resulting in the LTC4060 charac-
teristics going out of specifications, excessive power
dissipation may force the IC into thermal shutdown, or the
battery could discharge because some of the current from
the DRIVE pin could be pulled from the battery through the
forward biased PNP collector base junction.
The actual battery fast charge current (I_BAT) is slightly less
than the regulated 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)
=
930
•
I_PROG^{⎛⎝⎜} _β
β
+
⎞
1⎠⎟
If β = 100 then I_BAT is 1% low. The 1% loss can be easily
compensated for by increasing I_PROG by 1%.
Another important factor to consider when choosing the
PNP pass transistor is its 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_MAX(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, but not
less than 0.9V/cell since less than 0.9V/cell invokes
precharge current levels.
Thermal Considerations
Internal overtemperature protection is provided to prevent
excessive LTC4060 die temperature during a fault condi-
tion. If the internal die temperature exceeds approximately
145°C, charging stops and the part enters the shutdown
state. The faults can be generated from insuffient heat
sinking, a shorted DRIVE pin or from excessive DRIVE pin
current to the base of an external PNP transistor if it’s in
deep saturation from a very low V_CE. Once in the shutdown
state, charge qualification can be reinitiated only by re-
moving and replacing the battery or toggling the SHDN pin
low to high or removing and reapplying power to the
charger. This protection is not designed to prevent over-
heating of the PNP pass transistor. Indirectly though, self-
heating of the PNP thermally conducting to the LTC4060
can result in the IC’s junction temperature rising above
145°C, thus cutting off the PNP’s base current. This action
will limit the PNP’s junction temperature to some tempera-
ture well above 145°C. The user should insure that the
maximum rated junction temperature is not exceeded
under any normal operating condition. See Package/Order
Information for the θ_JA of the LTC4060 Exposed Pad
packages. The actual thermal resistance in the application
will vary depending on forced air cooling, use of the
Exposed Pad and other heat sinking means, especially the
amount of copper on the PCB to which the LTC4060 is
attached. The majority of the power dissipated within the
LTC4060 is in the current sense resitor and DRIVE pin
driver as given below:
P_D = (I_BAT)² • 0.08 + I_DRIVE (V_CC – V_EB)
T_J = T_A + θ_JA • P_D
V_EB is the emitter to base voltage of the external PNP.
4060f
15