LTC4121/LTC4121-4.2
27
4121fc
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Figure 12. Design Example 4, LTC4121 2-Cell Li-Ion Charger with MPPT Tracking for a Resistive Supply
Figure 13. V
MP
/V
OC
and I
BAT
vs V
IN(OC)
APPLICATIONS EXAMPLES
Design Example 4
Consider the design of a Li-Ion charger from a resistive
supply. With a resistive supply voltage, the maximum
power point is at 50% of the open-circuit voltage. Program
a 50% peak power point using K
R
= 0.199 with R
MPPT1
=
332k and R
MPPT2
= 82.5k. This network keeps the input
voltage at the peak power point for any input resistance
so long as the R-C time constant of R
IN
• C
IN
does not
exceed PW
MP
/5, here C
IN
is 22µF.
With 100Ω of source impedance, the input voltage regu-
lation loop holds the ratio of (V
MP
/V
IN
) at about 49% for
V
IN
ranging from 9V up to 28.3V. For lower input voltages
than 8.7V, the MPPT set point is below DUVLO when
V
BAT
= 4.2V. And above 28.3V, the charger attains the full
programmed charge current of 400mA so MPPT regula-
tion lets go. While the LTC4121 regulates V
IN
, the battery
charge current is automatically scaled to track available
input power. Figure 13 illustrates the circuit performance
measured with V
BAT
held at 4.0V, showing the ratio of
V
MP
/V
OC
and I
BAT
versus V
OC
with R
IN
= 100Ω in series
with the supply.
L
SW
is sized to maintain ripple current below 30% of I
CHG
at V
IN
= 16V. The FB pin network is programmed to set
V
FLOAT
= 4.2V. An NTC network is configured to enable
charging when the battery temperature is between 0°C
and 40°C.
INTV
CC
BOOST
SW
CHGSNS
BAT
FB
FBG
NTC
IN
RUN
MPPT
LTC4121
GND
C
IN
22µF
R
MPPT2
82.5k
R
IN
R
MPPT1
332k
+
–
R
FB1
1.01M
R
FB2
1.35M
C
BST
22nF
L
SW
33µH
FREQ PROG
+
C
BAT
22µF
4121 F12
T = NTCS0402E3103FHT
R
PROG
3.01k
T
10k
Li-Ion
C
INTVCC
2.2µF
V
MP
INTV
CC
V
FLOAT
= 4.2V
V
IN
V
IN(OC)
(V)
5
10
V
MP
/V
OC
(%)
I
BAT
(mA)
40
30
60
50
20
100
80
70
90
0
150
100
250
200
50
450
350
300
400
10
4121 F13
15 20 25 30 35 40
V
BAT
= 4V
R
IN
= 100Ω
