LM3420 datasheet(14/17 Pages) NSC

14 / 17 page

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

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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