LT3651-8.2 Datasheet, PDF (13/24 Pages) Linear Technology – Monolithic 4A HIgh Voltage 2-Cell Li-Ion Battery Charger

LT3651-8.2/LT3651-8.4
APPLICATIONS INFORMATION
When V_IN is below 10.5V and the BOOST capacitor is
uncharged, the high side switch would normally not have
sufficient head room to start switching. During normal
operation the low side switch is deactivated when charge
current is very low to prevent reverse current in the induc-
tor. However in order to facilitate start-up, the LT3651-
8.2/LT3651-8.4 enable the switch if V_BOOST voltage is
low. This allows initial charging of the BOOST capacitor
which then permits the high side switch to saturate and
efficiently operate. The boost capacitor charges to full
potential after a few cycles.
The design should consider that as the switcher turns on
and input current increases, input voltage drops due to
source input impedance and input capacitance. This po-
tentially allows the input voltage to drop below the internal
V_IN UVLO turn-on and thus disrupt normal behavior and
potentially stall start-up. If an input current sense resis-
tor is used, its drop must be considered as well. These
problems are worsened because input current is largest
at low input voltage. Pay careful attention to drops in the
power path. Adding a soft-start capacitor to the RNG/SS
pin and setting UVLO to 9V with the SHDN pin is required
at low V_IN.
BAT Output Decoupling
It is recommended that the LT3651-8.2/LT3651-8.4 char-
ger output have a decoupling capacitor. If the battery can
be disconnected from the charger output this capacitor is
required. The value of this capacitor (C_BAT) is related to
the minimum operational V_IN voltage such that:
C_BAT
≈
20µF
+



350µF
V_IN(MIN)



The voltage rating on C_BAT must meet or exceed the bat-
tery float voltage.
R_SENSE: Charge Current Programming
The LT3651-8.2/LT3651-8.4 charger is configurable to
charge at average currents as high as 4A (see Figure 1).
If RNG/SS maximum voltage is not limited, the inductor
sense resistor, R_SENSE, has 95mV across it at maximum
charge current so:
R_SENSE
=
0.095V
I_{CHG(MAX )}
where I_CHG(MAX) is the maximum average charge current.
R_SENSE is 24mΩ for a 4A charger.
SW
BOOST
LT3651-8.2
LT3651-8.4
SENSE
BAT
R_SENSE
+
365142 F01
Figure 1. Programming Maximum Charge Current Using R_SENSE
36518284f
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