LM1117/LM1117I
Application Note
1.0 EXTERNAL CAPACITORS/STABILITY
1.1 Input Bypass Capacitor
An input capacitor is recommended. A 10µF tantalum on the input is a suitable input bypassing for almost all applications.
1.2 Adjust Terminal Bypass Capacitor
The adjust terminal can be bypassed to ground with a bypass capacitor (CADJ) to improve ripple rejection. This bypass capacitor
prevents ripple from being amplified as the output voltage is increased. At any ripple frequency, the impedance of the CADJ should
be less than R1 to prevent the ripple from being amplified:
1/(2π*fRIPPLE*CADJ) < R1
The R1 is the resistor between the output and the adjust pin. Its value is normally in the range of 100-200Ω. For example, with R1
= 124Ω and fRIPPLE = 120Hz, the CADJ should be > 11µF.
1.3 Output Capacitor
The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both minimum amount
of capacitance and ESR (Equivalent Series Resistance). The minimum output capacitance required by the LM1117 is 10µF, if a
tantalum capacitor is used. Any increase of the output capacitance will merely improve the loop stability and transient response.
The ESR of the output capacitor should range between 0.3Ω - 22Ω. In the case of the adjustable regulator, when the CADJ is used,
a larger output capacitance (22µf tantalum) is required.
2.0 OUTPUT VOLTAGE
The LM1117 adjustable version develops a 1.25V reference voltage, VREF, between the output and the adjust terminal. As shown
in Figure 1, this voltage is applied across resistor R1 to generate a constant current I1. The current IADJ from the adjust terminal
could introduce error to the output. But since it is very small (60µA) compared with the I1 and very constant with line and load
changes, the error can be ignored. The constant current I1 then flows through the output set resistor R2 and sets the output voltage
to the desired level.
For fixed voltage devices, R1 and R2 are integrated inside the devices.
10091917
FIGURE 1. Basic Adjustable Regulator
3.0 LOAD REGULATION
The LM1117 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 2, shows a typical application using a fixed output regulator. The Rt1 and Rt2 are the line resistances. It is obvious that the
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.
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