LM3420 Datasheet, PDF (11/26 Pages) Texas Instruments – 8.4-V Li-Ion Battery Charge Controller

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LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
Feature Description (continued)
• R_f ≈ 181 kΩ
(1)
The resistor (R_f) in the formula is an internal resistor located on the die. Since this resistor value affects the
phase margin, the worst case maximum and minimum values are important when analyzing closed loop stability.
The minimum and maximum room temperature values of this resistor are specified in the LM3420 Electrical
Characteristics section of this data sheet, and a curve showing the temperature coefficient is shown in the curves
section. Minimum values of R_f result in lower phase margins.
8.3.2 V_REG External Voltage Trim
The regulation voltage (V_REG) of the LM3420 can be externally trimmed by adding a single resistor from the
COMP pin to the +IN pin or from the COMP pin to the GND pin, depending on the desired trim direction. Trim
adjustments up to ±10% of V_REG can be realized, with only a small increase in the temperature coefficient. (See
temperature coefficient curve shown in Figure 11.)
Figure 11. Normalized Temperature Drift With Output Externally Trimmed
Decreasing V_REG
Figure 12. Increasing V_REG
Figure 13. Changing V_REG
Formula for selecting trim resistor values is shown in Equation 2 and Equation 3, based on the percent of
increase (%incr) or percent of decrease (%decr) of the output voltage from the nominal voltage.
R_increase = 26 × 10⁵/%incr
(2)
R_decrease = (154 × 10⁵/%decr) − 181 × 10³
(3)
8.4 Device Functional Modes
8.4.1 Operation as Control Section
The LM3420 is monolithic integrated circuits, which is suitable for charging and end-of-charge control for Lithium-
Ion rechargeable batteries. In this application, LM3420 is the reference and control section in the overall
feedback loop. The regulated voltage is sensed between the IN pin and GND pin. If the voltage at the IN pin is
less than the regulating voltage (V_REG), the OUT pin sources no current. As the voltage at the IN pin approaches
the V_REG, the OUT pin begins sourcing current, which can drive a feedback device or a power device.
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