|Description||5-A Low Dropout Positive Regulators|
|LM1084ISX-3.3/NOPB Datasheet PDF : 29 Pages |
SNVS037G – SEPTEMBER 1999 – REVISED JANUARY 2015
Feature Description (continued)
7.3.3 Overload Recovery
Overload recovery refers to regulator's ability to recover from a short circuited output. A key factor in the recovery
process is the current limiting used to protect the output from drawing too much power. The current limiting circuit
reduces the output current as the input to output differential increases. Refer to short circuit curve in the Typical
During normal start-up, the input to output differential is small because the output follows the input. But, if the
output is shorted, then the recovery involves a large input to output differential. Sometimes during this condition
the current limiting circuit is slow in recovering. If the limited current is too low to develop a voltage at the output,
the voltage will stabilize at a lower level. Under these conditions it may be necessary to recycle the power of the
regulator in order to get the smaller differential voltage and thus adequate start up conditions. Refer to Typical
Characteristics section for the short circuit current vs. input differential voltage.
7.4 Device Functional Modes
7.4.1 Output Voltage
The LM1084 adjustable version develops a 1.25-V reference voltage, (VREF), between the output and the adjust
pin. As shown in Figure 14, this voltage is applied across resistor R1 to generate a constant current I1. This
constant current then flows through R2. The resulting voltage drop across R2 adds to the reference voltage to
sets the desired output voltage.
The current IADJ from the adjustment terminal introduces an output error. But because it is small (120 uA max), it
becomes negligible when R1 is in the 100 Ω range.
For fixed voltage devices, R1 and R2 are integrated inside the devices.
Figure 14. Basic Adjustable Regulator
7.4.2 Stability Consideration
Stability consideration primarily concerns the phase response of the feedback loop. In order for stable operation,
the loop must maintain negative feedback. The LM1084 requires a certain amount series resistance with
capacitive loads. This series resistance introduces a zero within the loop to increase phase margin and thus
increase stability. The equivalent series resistance (ESR) of solid tantalum or aluminum electrolytic capacitors is
used to provide the appropriate zero (approximately 500 kHz).
Aluminum electrolytics 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 10-uF tantalum or a 50-uF aluminum. These values will
assure stability for the majority of applications.
The adjustable versions allow 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 22 uF for tantalum or to 150 uF for aluminum.
Capacitors other than tantalum or aluminum can be used at the adjust pin and the input pin. A 10-uF capacitor is
a reasonable value at the input. See Ripple Rejection section regarding the value for the adjust pin capacitor.
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