Datasheet

LM1086
www.ti.com SNVS039H –JUNE 2000–REVISED MAY 2013
The Aluminum electrolytic are less expensive than tantalums, but their ESR varies exponentially at cold
temperatures; therefore requiring close examination when choosing the desired transient response over
temperature. Tantalums are a convenient choice because their ESR varies less than 2:1 over temperature.
The recommended load/decoupling capacitance is a 10uF tantalum or a 50uF aluminum. These values will
assure stability for the majority of applications.
The adjustable versions allows an additional capacitor to be used at the ADJ pin to increase ripple rejection. If
this is done the output capacitor should be increased to 22uF for tantalums or to 150uF for aluminum.
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 applications that entail large changes in load current
(microprocessors 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:
ΔV = ΔI 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
Ripple rejection is a function of the open loop gain within the feed-back loop (refer to Figure 18 and Figure 19).
The LM1086 exhibits 75dB of ripple rejection (typ.). When adjusted for voltages higher than V
REF
, the ripple
rejection decreases as function of adjustment gain: (1+R1/R2) or V
O
/V
REF
. Therefore a 5V 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 (C
ADJ
). The impedance of the C
ADJ
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*f
RIPPLE
*C
ADJ
) ≤ R
1
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 20 shows a typical application using a fixed output regulator. Rt1 and Rt2 are the line resistances. V
LOAD
is less than the V
OUT
by the sum of the voltage drops along the line resistances. In this case, the load regulation
seen at the R
LOAD
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 20. Typical Application using Fixed Output Regulator
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