LTC4010_15 Datasheet, PDF (14/24 Pages) Linear Technology – High Efficiency Standalone Nickel Battery Charger

LTC4010
Applications Information
External DC Source
The external DC power source should be connected to the
charging system and the V_CC pin through a power diode
acting as an input rectifier. This prevents catastrophic
system damage in the event of an input short to ground
or reverse-voltage polarity at the DC input. The LTC4010
automatically senses when this input drives the V_CC pin
above BAT. The open-circuit voltage of the DC source
should be between 5.5V and 34V, depending on the num-
ber of cells being charged. In order to avoid low dropout
operation, ensure 100% capacity at charge termination,
and allow reliable detection of battery insertion, removal
or overvoltage, the following equation can be used to
determine the minimum full-load voltage that should be
produced at V_CC when the external DC power source is
connected.
V_CC(MIN) = (n • 2V) + 0.3V
where n is the number of series cells in the battery pack.
The LTC4010 will properly charge over a wide range of V_CC
and BAT voltage combinations. Operating the LTC4010 in
low dropout or with V_CC much greater than BAT will force
the PWM frequency to be much less than 550kHz. The
LTC4010 disables charging and sets a fault if a large V_CC to
BAT differential would cause generation of audible noise.
Load Control
Proper load current control is an important consideration
when fast charging nickel cells. This control ensures that
the system load remains powered at all times, but that
normal system operation and associated load transients
do not adversely affect fast charge termination. The input
protecton detailed in the previous paragraph is an integral
part of the necessary load control.
The battery should also be connected to the raw system
supply by some rectifying means, thus forming a switch
that selects the battery for system power only if an external
DC source is not present.
Battery Chemistry Selection
The desired battery chemistry is selected by program-
ming the CHEM pin to the proper voltage. If it is wired
to GND, a set of parameters specific to charging NiMH
14
cells is selected. When CHEM is left floating, charging is
optimized for NiCd cells. The various charging parameters
are detailed in Table 2.
Programming Charge Current
Charge current is programmed using the following
equation:
R_SENSE
=
100mV
I_PROG
R_SENSE is an external resistor connected between the
SENSE and BAT pins. A 1% resistor with a low temperature
coefficient and sufficient power dissipation capability to
avoid self-heating effects is recommended. Charge rate
should be between approximately C/2 and 2C.
Inductor Value Selection
For many applications, 10µH represents an optimum value
for the inductor the PWM uses to generate charge current.
For applications with I_PROG of 1.5A or greater running
from an external DC source of 15V or less, values between
5µH and 7.5µH can often be selected. For wider operating
conditions the following equation can be used as a guide
for selecting the minimum inductor value.
L > 6.5 • 10^–6 • V_DCIN • R_SENSE, L ≥ 4.7µH
Actual part selection should account for both manufacturing
tolerance and temperature coefficient to ensure this mini-
mum. A good initial selection can be made by multiplying
the calculated minimum by 1.4 and rounding up or down
to the nearest standard inductance value.
Ultimately, there is no substitute for bench evaluation of
the selected inductor in the target application, which can
also be affected by other environmental factors such as
ambient operating temperature. Using inductor values
lower than recommended by the equation shown above
can result in a fault condition at the start of precharge or
top-off charge.
Programming Maximum Charge Times
Connecting the appropriate resistor between the TIMER
pin and GND programs the maximum duration of various
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