Datasheet

LM3420

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SNVS116D –MAY 1998–REVISED MAY 2013

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 disconnects 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 approximately 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 input 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 assure that switch leakage current

will not cause the output to rise when the battery is removed.

The circuit in Figure 34 is very similar to Figure 33, except the switching regulator has been replaced with a low

dropout linear 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 silicon 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 35. The basic

feedback 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 SOT-23-5 package thus allowing a very compact pc board design. Diode D4 prevents the battery 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).

Figure 35. High Efficiency Switching Charger

with High Side Current Sensing

Product Folder Links: LM3420