LP38500-ADJ Datasheet, PDF (11/23 Pages) National Semiconductor (TI) – 1.5A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs

LP38500-ADJ, LP38502-ADJ
www.ti.com
SNVS539F – NOVEMBER 2007 – REVISED APRIL 2013
LOAD TRANSIENT RESPONSE
Load transient response is defined as the change in regulated output voltage which occurs as a result of a
change in load current. Many applications have loads which vary, and the control loop of the voltage regulator
must adjust the current in the pass FET transistor in response to load current changes. For this reason,
regulators with wider bandwidths often have better transient response.
The LP38500/2-ADJ employs an internal feedforward design which makes the load transient response much
faster than would be predicted simply by loop speed: this feedforward means any voltage changes appearing on
the output are coupled through to the high-speed driver used to control the gate of the pass FET along a signal
path using very fast FET devices. Because of this, the pass transistor’s current can change very quickly.
Figure 24 shows the output voltage load transient which occurs on a 1.8V output when the load changes from
0.1A to 1.5A at an average slew rate of 0.5A/µs. As shown, the peak output voltage change from nominal is
about 40 mV, which is about 2.2%.
Figure 25. Load Transient Response
In cases where extremely fast load changes occur, the output capacitance may have to 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.
In general, managing load transients is done by paralleling ceramic capacitance with a larger bulk capacitance.
In this way, the ceramic can source current during the rapidly changing edge and the bulk capacitor can support
the load current after the first initial spike in current.
PRINTED CIRCUIT BOARD LAYOUT
Good layout practices will minimize voltage error and prevent instability which can result from ground loops. The
input and output capacitors should be directly connected to the IC pins with short traces that have no other
current flowing in them (Kelvin connect).
The best way to do this is to place the capacitors very near the IC and make connections directly to the IC pins
via short traces on the top layer of the PCB. The regulator’s ground pin should be connected through vias to the
internal or backside ground plane so that the regulator has a single point ground.
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