Specifications

LTC4056-4.2
8
405642f
APPLICATIO S I FOR ATIO
WUU
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Undervoltage Lockout
An internal undervoltage lockout (UVLO) circuit monitors
the input voltage and keeps the charger in shutdown mode
until V
CC
rises above the UVLO threshold (V
UVLOI
is
typically 4.4V). Approximately 50mV of hysteresis is built
in to prevent oscillation around the threshold level. In
undervoltage lockout, battery drain current is very low
(<1µA) and supply current is approximately 40µA.
Undervoltage Charge Current Limiting
The LTC4056 includes undervoltage charge current limit-
ing that prevents full charge current until the input supply
voltage reaches a threshold value (V
UVCL
). This feature is
particularly useful if the LTC4056 is powered from a
supply with long leads (or any relatively high output
impedance).
For example, USB powered systems tend to have highly
variable source impedances (due primarily to cable quality
and length). A transient load combined with such an
impedance can easily trip the UVLO threshold and turn the
charger off unless undervoltage charge current limiting is
implemented.
Consider a situation where the LTC4056 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
170mV above the rising undervoltage lockout threshold,
V
UVLOI
), undervoltage charge current limiting will begin to
reduce the charge current in an attempt to maintain V
UVCL
at the V
CC
input of the IC. The LTC4056 will continue to
operate at the reduced charge current until the input
supply voltage is increased or voltage mode reduces the
charge current further.
Trickle Charge and Defective Battery Detection
At the beginning of a charge cycle, if the battery voltage is
low (below V
TRIKL
of about 2.8V) the charger goes into
trickle charge mode reducing the charge current to ap-
proximately 2% of the full-scale current. If the low battery
voltage persists for one quarter of the total charge time,
the battery is assumed to be defective, the charge cycle is
terminated and the CHRG pin output transitions from a
strong pull-down to a 12µA pull-down. To restart the
charge cycle, remove the input voltage and reapply it or
momentarily force the TIMER/SHDN pin to ground.
Programming Charge Current
When in constant current mode, the full-scale charge
current is programmed using a single external resistor
between the PROG pin and ground, R
PROG
. The current
delivered to the I
SENSE
pin (flowing from V
CC
through the
internal 110mΩ sense resistor) will be 915 times the
current in R
PROG
. Because the LTC4056 provides a virtual
1V source at the PROG pin, the charge current is given by:
II
V
R
or
R
V
I
CHG PROG
PROG
PROG
CHG
=
()
=






=






••
•
915
1
915
1
915
Under trickle charge conditions, this current is reduced to
approximately 2% of the full-scale value. The actual bat-
tery charge current (I
BAT
) is slightly lower than the ex-
pected charge current because the charger forces the
emitter current and the battery charge current will be
reduced by the base current. In terms of β (I
C
/I
B
), I
BAT
can
be calculated as follows:
IA I
V
R
BAT PROG
PROG
()
=
+






=
+






915
1
915
1
••
β
β
β
β
If β = 50, then I
BAT
is 2% low. If desired, reducing R
PROG
by 2% can compensate for the 2% drop.
For example, if 700mA charge current is required, calcu-
late:
R
V
mA
k
PROG
=






=
1
700
915 1 3•.
If a low β needs to be compensated for, say β = 50,
calculate:
R
V
mA
k
PROG
=
+






=
915
700
50
50 1
127•.
For best stability over temperature and time, 1% metal-
film resistors are recommended.