LTC4011 Linear Technology, LTC4011 Datasheet - Page 13

• Manufacturer Part Number: LTC4011
• Description: High Efficiency Standalone Nickel Battery Charger
• Manufacturer: Linear Technology

www.DataSheet4U.com
DataSheet U .com
APPLICATIO S I FOR ATIO
External DC Source
The external DC power source should be connected to the
charging system and the V
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
automatically 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 detec-
tion 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.
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
4
DCIN(MIN) = (n • 2V) + 0.3V
U
U
CC
pin through either a power
W
U
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
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 and BAT pins. A 1% resistor with a low temperature
coefficient and sufficient power dissipation capability to
avoid self-heating effects is recommended.
Programming Maximum Charge Times
Connecting the appropriate resistor between the TIMER
pin and GND programs the maximum duration of various
charging states. To some degree, the value should reflect
how closely the programmed charge current matches the
1C rate of targeted battery packs. The maximum fast
charge period is determined by the following equation:
Some typical timing values are detailed in Table 3. R
should not be less than 15k. The actual time limits used by
the LTC4011 have a resolution of approximately ±30
seconds in addition to the tolerances given the Electrical
Characteristics table. The maximum time period is ap-
proximately 4.3 hours.
SENSE
R
R
SENSE
TIMER
is an external resistor connected between the
( )
=
Ω =
100
I
PROG
mV
t
MAX
30 10
•
(
Hours
–
6
)
LTC4011
13
TIMER
4011p
RT
,