LP38500-ADJ Datasheet, PDF (10/16 Pages) National Semiconductor (TI) – 1.5A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs

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FIGURE 3. Gain-Bandwidth Plot for No Load
The reduction in unity-gain bandwidth as load current is re-
duced is normal for any LDO regulator using a P-FET or PNP
pass transistor, because they have a pole in the loop gain
function given by:
This illustrates how the pole goes to the highest frequency
when R_L is minimum value (maximum load current). In gen-
eral, LDO’s have maximum bandwidth (and lowest phase
margin) at full load current. In the case of the LP38500/2-ADJ,
it can be seen that it has good phase margin even when using
ceramic capacitors with ESR values of only a few milli Ohms.
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 cou-
pled through to the high-speed driver used to control the gate
of the pass FET along a signal path using very fast FET de-
vices. Because of this, the pass transistor’s current can
change very quickly.
Figure 3 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%.
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FIGURE 4. Load Transient Response
In cases where extremely fast load changes occur, the output
capacitance may have to be increased. When selecting ca-
pacitors, it must be understood that the better performing
ones usually cost the most. For fast changing loads, the in-
ternal 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 chang-
ing 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.
The external resistors which set the output voltage must also
be located very near the IC with all connections directly tied
via short traces to the pins of the IC (Kelvin connect). Do not
connect the resistive divider to the load point or DC error will
be induced.
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