LM3420-42 Datasheet, PDF (14/17 Pages) National Semiconductor (TI) – Lithium-Ion Battery Charge Controller

Application Circuits (Continued)
1.23V, which is applied to the feedback pin of the
LM2575-ADJ. to satisfy the feedback loop.
Once the battery voltage reaches 8.4V, the LM3420 takes
over and begins to control the feedback pin of the
LM2575-ADJ. The LM3420 now regulates the voltage across
the battery, and the charger becomes a constant-voltage
charger. Loop compensation network R6 and C3 ensure
stable operation of the charger circuit under both
constant-current and constant-voltage conditions. If the input
supply voltage is removed, diode D2 and the PNP input
stage of the LM358 become reversed biased and discon-
nects the battery to ensure that the battery is not discharged.
Diode D3 reverse biases to prevent the op-amp from sinking
current when the charger changes to constant voltage mode.
The minimum supply voltage for this charger is approxi-
mately 11V, and the maximum is around 30V (limited by the
32V maximum operating voltage of the LM358). If another
op-amp is substituted for the LM358, make sure that the in-
put common-mode range of the op-amp extends down to
ground so that it can accurately sense 50 mV. R1 is included
to provide a minimum load for the switching regulator to as-
sure that switch leakage current will not cause the output to
rise when the battery is removed.
The circuit in Figure 8 is very similar to Figure 7, except the
switching regulator has been replaced with a low dropout lin-
ear regulator, allowing the input voltage to be as low as 10V.
The constant current and constant voltage control loops are
the same as the previous circuit. Diode D2 has been
changed to a Schottky diode to provide a reduction in the
overall dropout voltage of this circuit, but Schottky diodes
typically have higher leakage currents than a standard sili-
con diode. This leakage current could discharge the battery
if the input voltage is removed for an extended period of
time.
Another variation of a constant current/constant voltage
switch mode charger is shown in Figure 9. The basic feed-
back loops for current and voltage are similar to the previous
circuits. This circuit has the current sensing resistor, for the
constant current part of the feedback loop, on the positive
side of the battery, thus allowing a common ground between
the input supply and the battery. Also, the LMC7101 op-amp
is available in a very small SOT23-5 package thus allowing a
very compact pc board design. Diode D4 prevents the bat-
tery from discharging through the charger circuitry if the input
voltage is removed, although the quiescent current of the
LM3420 will still be present (approximately 85 µA).
DS012359-14
FIGURE 9. High Efficiency Switching Charger
with High Side Current Sensing
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