LTC4010 Datasheet, PDF (11/20 Pages) Linear Integrated Systems – High Efficiency Standalone Nickel Battery Charger

U
OPERATIO
current threshold for CC such that the desired average
current through R_SENSE is maintained. The current com-
parator output does this by switching the state of the SR
latch at the appropriate time.
At the beginning of each oscillator cycle, the PWM clock
sets the SR latch and the external P-channel MOSFET is
switched on (N-channel MOSFET switched off) to refresh
the current carried by the external inductor. The inductor
current and voltage drop across R_SENSE begin to rise
linearly. During normal operation, the PFET is turned off
(NFET on) during the cycle by CC when the voltage
difference across R_SENSE reaches the peak value set by
the output of EA. The inductor current then ramps down
linearly until the next rising PWM clock edge. This closes
the loop and maintains the desired average charge current
in the external inductor.
Low Dropout Charging
After charging is initiated, the LTC4010 does not require
that V_CC remain at least 500mV above BAT because
situations exist where low dropout charging might occur.
LTC4010
In one instance, parasitic series resistance may limit PWM
headroom (between V_CC and BAT) as 100% charge is
reached. A second case can arise when the DC adapter
selected by the end user is not capable of delivering the
current programmed by R_SENSE, causing the output volt-
age of the adapter to collapse. While in low dropout, the
LTC4010 PWM runs near 100% duty cycle with a fre-
quency that may not be constant and can be less than
550kHz. The charge current will drop below the pro-
grammed value to avoid generating audible noise, so the
actual charge delivered to the battery may depend prima-
rily on the LTC4010 charge timer.
Internal Die Temperature
The LTC4010 provides internal overtemperature detec-
tion to protect against electrical overstress, primarily at
the FET driver outputs. If the die temperature rises above
this thermal limit, the LTC4010 stops switching and
indicates a fault as previously discussed.
APPLICATIO S I FOR ATIO
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 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 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 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 current load 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
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