LTC4060_15 Datasheet, PDF (14/20 Pages) Linear Technology – Standalone Linear NiMH/NiCd Fast Battery Charger

LTC4060
APPLICATIO S I FOR ATIO
temperature) at approximately 0°C (5°C – 5°C hysteresis
at V_CC = 5V) and then resume when the thermistor falls
below 27k (rising temperature) which will be approxi-
mately 5°C (V_NTC = V_CLD = 0.86 • V_CC typ).
Many thermistors with an R_COLD to R_HOT ratio of approxi-
mately 7 will work. For lower power dissipation higher
values of thermistor resistance can be used. The Murata
NTH4G series offers resistances of up to 100k at 25°C.
It is important that the thermistor be placed in close
contact with the battery and away from the external PNP
pass transistor to avoid excessive temperature errors on
the sensed battery temperature. Furthermore, since V_CC is
a high current path into the LTC4060, it is essential to
minimize voltage drops between the V_CC supply pin and
the top of R_HOT by Kelvin connecting R_HOT directly to the
V_CC pin.
Power Requirements
The DC power input to the V_CC pin must always be within
proper limits while charging a battery. Voltages beyond
the absolute maximum ratings may damage the charger
and voltages falling below the UVLO entry thresholds, as
programmed by the SEL0 and SEL1 pins, will likely cause
the charger to enter the shutdown state (when the UVLO
exit threshold is exceeded charging will begin anew). While
the LTC4060 is designed to reject 60Hz or 120Hz supply
ripple, certain precautions are required. The instantaneous
ripple voltage must always be within the above mentioned
limits. Ripple voltage seen across the collector-base junc-
tion of the external PNP pass transistor will slightly modu-
late its beta and hence its base current. Since the emitter
current is precisely regulated by the LTC4060, any modu-
lation of base current will appear at the collector. This
slightly modulated battery charge current into a battery
will usually produce an insignificant modulation voltage at
the battery. However, if excessive wire impedance to the
battery from the PNP exists, then it may be helpful to Kelvin
connect the BAT pin to a convenient point closest to the
battery to reduce ripple magnitude entering the LTC4060’s
battery monitoring circuits. The battery ground imped-
ance should also be managed to limit ripple voltage at the
BAT pin. Excessive ripple into the BAT pin may cause the
charger to deviate from specified performance.
14
V_CC Bypass Capacitor
A 1µF capacitor located close to the LTC4060 will usually
provide adequate input bypassing. However, caution must
be exercised when using multilayer ceramic capacitors.
Because of the self-resonance and high Q characteristics
of some types of ceramic capacitors, along with wiring
inductance, high voltage transients can be generated
under some conditions such as connecting or disconnect-
ing a supply input to a hot power source. To reduce the Q
and prevent these transients from exceeding the absolute
maximum voltage rating, consider adding about 1Ω of
resistance in series with the ceramic input capacitor.
BAT Bypass Capacitor
This optional capacitor, connected between BAT and GND,
can be used to help filter excessive contact bounce during
the battery monitoring or charging process. The value will
depend upon the contact bounce open duration, but is typi-
cally 10µF. Another purpose of this capacitor is to bypass
transient battery load events that might otherwise disrupt
monitoring or charging. Should the battery connections not
be subject to excessive contact bounce or excessive bat-
tery voltage transients, then no BAT pin capacitor is re-
quired. The same caution mentioned above for the V_CC by-
pass capacitor applies.
External PNP Transistor
The external PNP pass transistor must have adequate beta
and breakdown voltages, low saturation voltage and suf-
ficient power dissipation capability that may include heat
sinking.
To provide 2A of charge current with the minimum avail-
able base current drive of 40mA (I_DRV min) requires a
minimum PNP beta of 50.
The transistor’s collector to emitter breakdown voltage
must be high enough to withstand the difference between
the maximum supply voltage and minimum battery volt-
age. Almost any transistor will meet this requirement.
Additionally, when no power is supplied to the charger
(V_IN = 0V and V_SENSE = 0V), the transistor’s emitter to base
breakdown voltage must be high enough to prevent a
leakage path at the maximum battery voltage while not
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