LM3420AM5-4.2 National Semiconductor, LM3420AM5-4.2 Datasheet - Page 14

• Manufacturer Part Number: LM3420AM5-4.2
• Description: IC CTRLR LIION BAT CHRG SOT23-5
• Manufacturer: National Semiconductor
• Type: Battery Chargerr

Specifications of LM3420AM5-4.2

• Function: Charge Management
• Battery Type: Lithium-Ion (Li-Ion)
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: SC-74A, SOT-753
• Output Current: 15mA
• Output Voltage: 4.2V
• Operating Supply Voltage (max): 20V
• Operating Temp Range: -40C to 85C
• Package Type: SOT-23
• Mounting: Surface Mount
• Pin Count: 5
• Operating Temperature Classification: Industrial
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM3420AM5-4.2TR; LM3420AM5-4.2; LM3420AM5-4.2TR

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Application Circuits
A switching regulator, constant-current, constant-voltage
two-cell Li-Ion battery charging circuit is shown in Figure 7.
This circuit provides much better efficiency, especially over a
wide input voltage range than the linear topologies. For a 1A
charger an LM2575-ADJ. switching regulator IC is used in a
standard buck topology. For other currents, or other pack-
ages, other members of the SIMPLE SWITCHER
regulator family may be used.
FIGURE 8. Low Dropout Constant Current/Constant
Constant Current/Constant Voltage 2-Cell Charger
FIGURE 7. High Efficiency Switching Regulator
Voltage Li-Ion Battery Charger
(Continued)
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buck
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Circuit operation is as follows. With a discharged battery
connected to the charger, the circuit operates as a constant
current source. The constant-current portion of the charger is
formed by the loop consisting of one half of the LM358 op
amp along with gain setting resistors R3 and R4, current
sensing resistor R5, and the feedback reference voltage of
1.23V. Initially the LM358’s output is low causing the output
of the LM2575-ADJ. to rise thus causing some charging
current to flow into the battery. When the current reaches 1A,
it is sensed by resistor R5 (50 mΩ), and produces 50 mV.
This 50 mV is amplified by the op-amps gain of 25 to
produce 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 disconnects the battery
to ensure that the battery is not discharged. Diode D3 re-
verse 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
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 8 is very similar to Figure 7, 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 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).