LTC4121-4.2_15 Datasheet, PDF (24/30 Pages) Linear Technology – 40V 400mA Synchronous Step-Down Battery Charger

LTC4121/LTC4121-4.2
Applications Examples
In this design example, maximum power dissipation is This ratio is obtained by selecting R_MPPT1 and R_MPPT2
calculated during trickle charge as:
following:
P_D = (17V − 2.5V) • 10mA
+ 17V • 2.5mA
R_MPPT1
=
(1− 0.1245)
0.1245
R_MPPT2
=
7.03
• R_MPPT2
+ 0.3Ω • 0.04A²
+ 0.8Ω • ^4.2V 0.04A²
17V
+
0.5Ω
•
_1−
2.5V
17V


•
0.04A²
Using standard 1% resistors, select R_MPPT1 = 715kΩ and
R_MPPT2 = 102kΩ to obtain a K_R of 0.1248 and nominal
MPPT set point of 17.94V. Including the effect of MPPT
error terms, the expected MPPT regulation voltage will
vary between 17.58V to 17.94V or 78.5% to 80.1% of the
open-circuit voltage.
= 0.19W
This dissipated power results in a junction temperature
rise of:
Next, the external feedback divider, R_FB1/R_FB2, is found
using standard 1% values listed in Table 2.
R_FB1 = 2.05MΩ
P_D • Θ_JA = 0.19W • 54C°/W = 10.2°C
Estimating I_{IN(SWITCHING)} at 2.5mA from the I_{IN(SWITCHING)}
Current vs Input Voltage graph at V_IN = 17V, during
regular charging with V_BAT > V_TRKL, the power dissipation
reduces to:
P_D = 17V • 2.5mA
+ 0.3Ω • 0.4A²
+ 0.8Ω • ^4.2V 0.4A²
17V
+
0.5Ω
•
_1−
4.2V
17V


•
0.4A²
= 0.18W
This dissipated power results in a junction temperature
rise of 9.8°C over ambient.
Design Example 2
R_FB2 = 845kΩ
With these resistors, and including the resistance of the
FBG pin, the battery float voltage is 8.22V.
Select the RUN pin divider to turn on the charger when
the solar-cell output reaches 14.7V. This is obtained by
selecting R_RUN1 = 536kΩ, and R_RUN2 = 107kΩ. This selec-
tion turns off the charger if the input falls below 13.52V.
The switching frequency is selected at 1.5MHz which meets
the minimum on-time requirement for battery voltages
as low as 5V.
t_ON
=
5V
1.5MHz • 17.94V
=
186ns
>
t_MIN(ON)
The minimum standard inductance value for a 30% ripple
current is
L_SW
>
(17.94V − 8.2V) • 8.2V
1.5MHz • 17.94V • (30% • 400mA)
=
24.8µH
Consider the design with a 3.5W or greater solar panel with
a maximum input voltage of V_OC = 22.4V and a maximum
power voltage of V_MP = 18V or 80.3% of the open-circuit
voltage. The minimum battery voltage is 5V, and the float
voltage is 8.2V, with a charge current of 400mA.
The nearest standard inductor value greater than this is
33µH. With an inductor of 33µH, the peak inductor current
is 445mA and the ripple current amplitude, ∆I_L, is 90mA.
Select an inductor with a saturation current greater than
the peak inductor current.
The MPPT set point is at 80.3% of the open-circuit volt-
age. So select
K_R = 0.1/0.803 = 0.1245
Select R_PROG = 3.01k, as the nearest standard 1% value
to provide a charge current of 403mA during constant-
current operation.
4121fa
24
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