LTC4011_15 Datasheet, PDF (15/26 Pages) Linear Technology – High Efficiency Standalone Nickel Battery Charger

LTC4011
Applications Information
External DC Source
The external DC power source should be connected to the
charging system and the V_CC pin through either a power
diode or P-channel MOSFET. This prevents catastrophic
system damage in the event of an input short to ground or
reverse-voltage polarity at the DC input. The LTC4011 auto-
matically senses when this input is present. The open-circuit
voltage of the DC source should be between 4.5V and 34V,
depending on the number 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 provided by the external DC power source.
DCIN(MIN) = (n • 2V) + 0.3V
where n is the number of series cells in the battery pack.
The LTC4011 will properly charge over a wide range
of DCIN and BAT voltage combinations. Operating the
LTC4011 in low dropout or with DCIN much greater than
BAT will force the PWM frequency to be much less than
550kHz. The LTC4011 disables charging and sets a fault
if a large DCIN to BAT differential would cause generation
of audible noise.
PowerPath Control
Proper PowerPath 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. For high
efficiency and low dropout applications, the LTC4011 can
provide gate drive from the INFET pin directly to an input
P-channel MOSFET.
The battery should also be connected to the raw system
supply by a switch that selects the battery for system
power only if an external DC source is not present. Again,
for applications requiring higher efficiency, a P-channel
MOSFET with its gate driven from the DC input can be used
to perform this switching function (see Figure 8). Gate
voltage clamping may be necessary on an external PMOS
transistor used in this manner at higher input voltages.
Alternatively, a diode can be used in place of this FET.
Battery Chemistry Selection
The desired battery chemistry is selected by programming
the CHEM pin to the proper voltage. If it is wired to GND,
a set of parameters specific to charging NiMH cells is
selected. When CHEM is left floating or connected to V_RT,
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.
4011fb
15