LP38501TS-ADJ Datasheet, PDF (10/22 Pages) Texas Instruments – LP38501/3-ADJ, LP38501A/3A-ADJ 3A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs

LP38501-ADJ, LP38503-ADJ
SNVS522H – AUGUST 2007 – REVISED APRIL 2013
www.ti.com
When the load current changes much more quickly, the output voltage will show more change because the loop
and internal feedforward circuitry are not able to react as fast as the load changes. In such cases, it is the output
capacitor which must supply load current during the transition until the loop responds and changes the pass
transistor’s drive to deliver the new value of load current. As an example, the slew rate of the load current will be
increased to 75A/µs and the same test will be performed. In Figure 19, it can be seen that the peak excursion of
the output voltage during the transient has now increased to about 200 mV, which is just slightly over 11% for the
1.8V output.
Figure 19. Load Transient Response: 10 µF Ceramic, 75A/µs di/dt
A better understanding of the load transient can be obtained when the load’s rising edge is expanded in time
scale (Figure 20).
Figure 20. Rising Edge, 10 µF Ceramic, 75A/µs di/dt
It can be seen from the figure that the output voltage starts “correcting” back upwards after less than a micro
second, and has fully reversed direction after about 1.2 µs. This very rapid reaction is a result of the maximum
loop bandwidth (full load is being delivered) and the feedforward effect kicking on the drive to the FET before
feedback gets fully around the loop.
In cases where extremely fast load changes occur, and output voltage regulation better than 10% is required, the
output capacitance must be increased. When selecting capacitors, it must be understood that the better
performing ones usually cost the most. For fast changing loads, the internal parasitics of ESR (equivalent series
resistance) and ESL (equivalent series inductance) degrade the capacitor’s ability to source current quickly to the
load. The best capacitor types for transient performance are (in order):
1. Multilayer Ceramic: with the lowest values of ESR and ESL, they can have ESR values in the range of a few
milli Ohms. Disadvantage: capacitance values above about 22 µF significantly increase in cost.
2. Low-ESR Aluminum Electrolytics: these are aluminum types (like OSCON) with a special electrolyte which
provides extremely low ESR values, and are the closest to ceramic performance while still providing large
amounts of capacitance. These are cheaper (by capacitance) than ceramic.
3. Solid tantalum: can provide several hundred µF of capacitance, transient performance is slightly worse than
OSCON type capacitors, cheaper than ceramic in large values.
4. General purpose aluminum electrolytics: cheap and provide a lot of capacitance, but give the worst
performance.
10
Submit Documentation Feedback
Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LP38501-ADJ LP38503-ADJ