LTC4121/LTC4121-4.2
20
4121fc
For more information www.linear.com/LTC4121
APPLICATIONS INFORMATION
For a given input and battery voltage, the inductor value
and switching frequency determines the peak-to-peak
ripple current amplitude according to the following formula:
∆I
L
=
V
IN
− V
BAT
• V
BAT
f
OSC
• V
IN
•L
SW
Ripple current is typically set to be within a range of 20%
to 40% of the programmed charge current, I
CHG
. To obtain
a ripple current in this range, select an inductor value us-
ing the nearest standard inductance value available that
obeys the following formula:
L
SW
≥
V
IN(MAX)
− V
FLOAT
• V
FLOAT
f
OSC
• V
IN(MAX)
• 30% •I
CHG
( )
Then select an inductor with a saturation current rating
greater than I
L(PEAK)
.
Input Capacitor
The LTC4121 charger is biased directly from the input
supply at the V
IN
pin. This supply provides large switched
currents, so a high-quality, low ESR decoupling capacitor
is recommended to minimize voltage glitches at V
IN
. Bulk
capacitance is a function of the desired input ripple voltage
(∆V
IN
), and follows the relation:
C
IN(BULK)
=
I
CHG
•
BAT
V
IN
∆V
IN
(µF)
Input ripple voltages (∆V
IN
) above 0.01V are not recom-
mended. 10µF is typically adequate for most charger
applications, with a voltage rating of 40V.
The input capacitor also forms a pole with the source
impedance that supplies power to V
IN
. This R-C network
must settle within the 36ms PW
MP
period for the LTC4121
to accurately sample the open-circuit voltage at V
IN
.
Adequate settling is usually achieved in 3 to 5 R-C time
constants. To allow the LTC4121 to correctly sample the
open-circuit voltage, limit C
IN
to:
C
IN
< PW
MP
/ (5 • R
SOURCE
),
where R
SOURCE
is the impedance of the power source.
For a solar panel this is the impedance of the panel at the
open-circuit voltage. Looking at a panel's I-V curve, the
source impedance is approximated by (V
OC
– V
MP
)/I
MP
.
Typically V
MP
is about 80% of V
OC
, so the solar panels
source impedance can be approximated as:
R
SOURCE
≈ V
OC
/ (5 • I
MP
).
Reverse Blocking
When a fully charged battery is suddenly applied to the
BAT pin, a large in-rush current charges the C
IN
capacitor
through the body diode of the LTC4121 topside power
switch. While the amplitude of this current can exceed
several Amps, the LTC4121 will survive provided the bat-
tery voltage is below about 11V. To completely eliminate
this in-rush current, a blocking P-channel MOSFET should
be placed in series with the BAT pin. When the battery is
the only source of power, this PMOS also serves to de-
crease battery drain current due to any load placed at V
IN
,
conducted through the body diode of the topside power
switch on the LTC4121. The PMOS body diode shown in
Figure8 serves as the blocking component since CHRG is
high impedance when the battery voltage is greater than
the input voltage. When CHRG pulls low, i.e. during most
of a normal charge cycle, the PMOS is on to reduce power
dissipation. The PMOS requires a forward current rating
equal to the programmed charge current and a reverse
breakdown voltage equal to the programmed float voltage.
