eorex
Input Capacitor
Output Capacitor
EP1117
An input capacitor of 10μF or greater is recommended. Tantalum or aluminum electrolytic capacitors can
be used for bypassing. Larger Values will improve ripple rejection by bypassing the input to the regulator.
The EP1117 requires an output capacitor to maintain stability and improve transient response. The
EP1117 output capacitor selection is dependent upon the ESR (equivalent series resistance) of the
output capacitor to maintain stability. When the output capacitor is 10μF or greater, the output capacitor
should have an ESR less than 1Ω. This will improve transient response as well as promote stability. A
low-ESR solid Tantalum capacitor works extremely well and provides good transient response and
stability over temperature. Aluminum electrolytes can also be used, as long as the ESR of the
capacitor is less than 1Ω. The value of the output capacitor can be increased without limit. Higher
capacitance values help to improve transient response and ripple rejection and reduce output noise.
Ripple Rejection
The curves for Ripple Rejection were generated using an adjustable device with the FB pin bypassed.
With a 22μF bypassing capacitor 75dB ripple rejection is obtainable at any output level. The impedance
of the FB pin capacitor, at the ripple frequency, should be<R1. R1 is normally in the range of 100Ω-200Ω.
The size of the required FB pin capacitor is a function of the input ripple frequency. At 120Hz, with
R1=100Ω, the FB pin capacitor should be 13μF. For fixed voltage devices, and adjustable devices
without a FB pin capacitor, the output ripple will increase as the ratio of the output voltage to the
reference voltage (V
OUT
/V
REF
).
Thermal Considerations
The EP1117 regulators have thermal protection to limit junction temperature to 150ºC. However, device
functionality is only guaranteed to a maximum junction temperature of +125ºC. A heat sink may be
required depending on the maximum power dissipation and maximum ambient temperature of the
application. Figure 3 and 4 show for the SOT-223 and TO-252 the measured values
θ
(JA)
for different
copper area sizes using a 2 layers, 1.6mm, and 6Sq. cm FR-4 PCB with 2oz. copper and a ground plane
layer on the backside area used for heat sinking. It can be used as a rough guideline in estimating
thermal resistance. Both the SOT-223 and TO-252 packages use a copper plane on the PCB and the
PCB itself as a heat sink. To optimize the heat sinking ability of the plane and PCB, solder the tab of the
package to the plane.
Figure 3-θ
(JA)
vs copper area for SOT-223 package
Jul. 2006
7/10
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