|LM1086CS-2.5||1.5A Low Dropout Positive Regulators|
National ->Texas Instruments
|LM1086CS-2.5 Datasheet PDF : 15 Pages |
Application Note (Continued)
Capacitors other than tantalum or aluminum can be used at
the adjust pin and the input pin. A 10uF capacitor is a
reasonable value at the input. See Ripple Rejection section
regarding the value for the adjust pin capacitor.
It is desirable to have large output capacitance for applica-
tions that entail large changes in load current (microproces-
sors for example). The higher the capacitance, the larger the
available charge per demand. It is also desirable to provide
low ESR to reduce the change in output voltage:
∆V = ∆I x ESR
It is common practice to use several tantalum and ceramic
capacitors in parallel to reduce this change in the output
voltage by reducing the overall ESR.
Output capacitance can be increased indefinitely to improve
transient response and stability.
Ripple rejection is a function of the open loop gain within the
feed-back loop (refer to Figure 1 and Figure 2). 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. There-
fore a 5V adjustment decreases ripple rejection by a factor of
four (−12dB); Output ripple increases as adjustment voltage
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 imped-
ance 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 in-
1/(2π*fRIPPLE*CADJ) ≤ R1
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 regula-
tion, a few precautions are needed.
Figure 3 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.
When the adjustable regulator is used (Figure 4), 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 regu-
lation. For example, a 5V regulator with 0.05Ω resistance
between the regulator and load will have a load regulation
due to line resistance of 0.05Ω x IL. If R1 (=125Ω) is con-
nected near the load the effective line resistance will be
0.05Ω (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 sens-
ing and improve load regulation.
FIGURE 4. Best Load Regulation using Adjustable
Under normal operation, the LM1086 regulator does not
need any protection diode. With the adjustable device, the
internal resistance between the adjustment and output ter-
minals limits the current. No diode is needed to divert the
current around the regulator even with a capacitor on the
adjustment terminal. The adjust pin can take a transient
signal of ±25V with respect to the output voltage without
damaging the device.
When an output capacitor is connected to a regulator and
the input is shorted, the output capacitor will discharge into
the output of the regulator. The discharge current depends
on the value of the capacitor, the output voltage of the
regulator, and rate of decrease of VIN. In the LM1086 regu-
lator, the internal diode between the output and input pins
can withstand microsecond surge currents of 10A to 20A.
With an extremely large output capacitor (≥1000 µf), and
with input instantaneously shorted to ground, the regulator
could be damaged. In this case, an external diode is recom-
mended between the output and input pins to protect the
regulator, shown in Figure 5.
FIGURE 3. Typical Application using Fixed Output
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