Datasheet

LTC4081
16
4081f
APPLICATIO S I FOR ATIO
WUU
U
BATTERY CHARGER
Programming Charge Current
The battery charge current is programmed using a single
resistor from the PROG pin to ground. The charge current
is 400 times the current out of the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R
V
I
I
V
R
PROG
BAT
BAT
PROG
==400
1
400
1
•, •
The charge current out of the BAT pin can be determined
at any time by monitoring the PROG pin voltage and using
the following equation:
I
V
R
BAT
PROG
PROG
= •400
Stability Considerations
The LTC4081 battery charger contains two control loops:
constant-voltage and constant-current. The constant-
voltage loop is stable without any compensation when a
battery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1
μ F from BAT to
GND. Furthermore, a 4.7μF capacitor with a 0.2Ω to 1Ω
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
In constant-current mode, the PROG pin voltage is in
the feedback loop, not the battery voltage. Because of
the additional pole created by PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the battery
charger is stable with program resistor values as high
as 25k. However, additional capacitance on this node
reduces the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above 100kHz.
Therefore, if the PROG pin is loaded with a capacitance,
C
PROG
, the following equation should be used to calculate
the maximum resistance value for R
PROG
:
R
kHz C
PROG
PROG
≤
π
1
2 100••
Average, rather than instantaneous, battery current may be
of interest to the user. For example, when the switching
regulator operating in low-current mode is connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
fi lter can be used on the PROG pin to measure the average
battery current as shown in Figure 3. A 10k resistor has
been added between the PROG pin and the fi lter capacitor
to ensure stability.
Undervoltage Charge Current Limiting (UVCL)
USB powered systems tend to have highly variable source
impedances (due primarily to cable quality and length). A
transient load combined with such impedance can easily trip
the UVLO threshold and turn the battery charger off unless
undervoltage charge current limiting is implemented.
Consider a situation where the LTC4081 is operating under
normal conditions and the input supply voltage begins to
sag (e.g. an external load drags the input supply down).
If the input voltage reaches V
UVCL
(approximately 300mV
above the battery voltage, ΔV
UVCL
), undervoltage charge
current limiting will begin to reduce the charge current in
an attempt to maintain ΔV
UVCL
between V
CC
and BAT. The
LTC4081 will continue to operate at the reduced charge
current until the input supply voltage is increased or volt-
age mode reduces the charge current further.
Figure 3. Isolating Capacitive Load
on PROG Pin and Filtering
4081 F03
C
FILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
R
PROG
LTC4081
PROG
GND
10k