LTC4121/LTC4121-4.2
12
4121fc
For more information www.linear.com/LTC4121
OPERATION
Overview
The LTC4121 is a synchronous step-down (buck)
monolithic battery charger with maximum power-point
tracking (MPPT) control of the source voltage. The
LTC4121/LTC4121-4.2 serves as a constant-current/
constant-voltage battery charger with the following built-in
charger functions: programmable charge current, battery
precondition with ½ hour timeout, precision shutdown/
run control, NTC thermal protection, a 2-hour safety ter-
mination timer, and automatic recharge. The LTC4121/
LTC4121-4.2 also provides output pins to indicate state
of charge and fault status.
Maximum Power Point Tracking
The LTC4121 employs an MPPT algorithm that compares a
stored open-circuit input voltage measurement against the
instantaneous input voltage while charging. The LTC4121
automatically reduces the charge current if the input voltage
falls below the user defined percentage of the open-circuit
voltage. This algorithm lets the LTC4121 optimize power
transfer for a variety of different input sources including
first order temperature compensation of a solar panel.
The LTC4121 periodically pauses charging to measure
the open-circuit voltage allowing the LTC4121 to track
fluctuations in the available power. About once every 30
seconds the LTC4121 pauses charging and waits about
36ms (PW
MP
) for the input voltage to recover to its
open-circuit potential. At the end of this recovery time,
the LTC4121 samples the input voltage divided by 10 (1/
K
F
), and stores this value on a digital to analog converter
(DAC). When charging resumes, the DAC voltage is com-
pared against the MPPT pin voltage that is programmed
with a resistive divider
. If the MPPT voltage falls below
the DAC voltage, the charge current is reduced to regulate
the input voltage at that level. This regulation loop serves
to maintain the input voltage at or above a user defined
level that corresponds to the peak power available from
the applied source.
A timing diagram illustrating the sampling of the open-
circuit voltage is shown below. The charge current drops
to zero and the LTC4121 waits PW
MP
and then samples
the open-circuit voltage. When charging resumes the
input voltage collapses if the source cannot support the
demanded charge current. When the input voltage drops
to V
MP
, the charge current is reduced so as to maintain
V
IN
at V
MP
.
Figure 3. MPPT Timing Diagram
4121 F03
TIME
TIME
T
MP
= 30s
PW
MP
= 36ms
SAMPLE V
IN(OC)
STORE
IN DAC: 23µs
V
IN
RECOVERS
PAUSE CHARGER
I
CHG
V
OC
V
IN
I
BAT
V
MP
Connect the MPPT pin to a resistive input voltage divider,
as shown in Figure 4, to program the fraction (K
R
) of the
input voltage where the input voltage regulation loop
reduces available charge current. The LTC4121 reduces
charge current if the MPPT pin voltage falls below the fixed
fraction (K
F
) of the open-circuit voltage (V
OC
). The ratio of
(K
F
/K
R
) defines the maximum power voltage (V
MP
) of the
applied power source as a ratio to the open-circuit voltage
(V
OC
) following the relation:
V
MP
V
=
K
F
K
=
0.1
K
=
0.1• R
MPPT1
+R
MPPT2
R
where the MPPT pin resistive divider gain is K
R
= R
MPPT2
/
(R
MPPT1
+ R
MPPT2
). These equations can be rearranged to
solve for R
MPPT2
in terms of K
F
(0.1) and the maximum power
voltage divided by the open circuit voltage, (V
MP
/V
OC
) as:
R
MPPT2
=
V
MP
V
OC
−0.1
•R
MPPT
This function serves to maintain the input voltage at or
above the peak power voltage while the LTC4121 charges
a battery.
Because MPPT operation involves large changes of input
voltage, it is important to ensure that the programmed
maximum power voltage does not violate minimum input
operating conditions: 4.4V or 160mV above the battery
voltage, whichever is higher.
