|LM1086CT-ADJ||1.5-A Low Dropout Positive Regulators|
|LM1086CT-ADJ Datasheet PDF : 30 Pages |
SNVS039J – JUNE 2000 – REVISED APRIL 2015
7.3 Feature Description
7.3.1 Ripple Rejection
Ripple rejection is a function of the open loop gain within the feed-back loop (refer to Figure 13 and Figure 16).
The LM1086 exhibits 75dB of ripple rejection (typ.). When adjusted for voltages higher than VREF, the ripple
rejection decreases as function of adjustment gain: (1+R1/R2) or VO/VREF. Therefore a 5-V adjustment decreases
ripple rejection by a factor of four (−12dB); Output ripple increases as adjustment voltage increases.
However, the adjustable version allows this degradation of ripple rejection to be compensated. The adjust
terminal can be bypassed to ground with a capacitor (CADJ). The impedance of the CADJ should be equal to or
less than R1 at the desired ripple frequency. This bypass capacitor prevents ripple from being amplified as the
output voltage is increased.
1/(2π*fRIPPLE*CADJ) ≤ R1
7.3.2 Load Regulation
The LM1086 regulates the voltage that appears between its output and ground pins, or between its output and
adjust pins. In some cases, line resistances can introduce errors to the voltage across the load. To obtain the
best load regulation, a few precautions are needed.
Figure 14 shows a typical application using a fixed output regulator. Rt1 and Rt2 are the line resistances. VLOAD
is less than the VOUT by the sum of the voltage drops along the line resistances. In this case, the load regulation
seen at the RLOAD would be degraded from the data sheet specification. To improve this, the load should be tied
directly to the output terminal on the positive side and directly tied to the ground terminal on the negative side.
Figure 14. Typical Application Using Fixed Output Regulator
When the adjustable regulator is used (Figure 15), the best performance is obtained with the positive side of the
resistor R1 tied directly to the output terminal of the regulator rather than near the load. This eliminates line drops
from appearing effectively in series with the reference and degrading regulation. For example, a 5V regulator with
0.05-Ω resistance between the regulator and load will have a load regulation due to line resistance of 0.05 Ω × IL.
If R1 (=125 Ω) is connected near the load the effective line resistance will be 0.05 Ω (1 + R2/R1) or in this case,
it is 4 times worse. In addition, the ground side of the resistor R2 can be returned near the ground of the load to
provide remote ground sensing and improve load regulation.
Figure 15. Best Load Regulation Using Adjustable Output Regulator
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