LP38501-ADJ datasheet(11/16 Pages) NSC

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30028135

FIGURE 6. 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 ca-

pacitance 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 tran-

sient performance are (in order):

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.

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.

Solid tantalum: can provide several hundred µF of

capacitance, transient performance is slightly worse than

OSCON type capacitors, cheaper than ceramic in large

values.

General purpose aluminum electrolytics: cheap and

provide a lot of capacitance, but give the worst

performance.

As a first example, larger values of ceramic capacitance will

be tried to show how much reduction can be obtained from

the 200 mV output change (Figure 6) which was seen with

only a 10 µF ceramic output capacitor. In Figure 7, the 10 µF

output capacitor is increased to 22 µF. The 200 mV transient

is reduced to about 160 mV, which is from about 11% of

OUT down to about 9%.

30028137

FIGURE 7. 22 µF Ceramic Output Capacitor

In Figure 8, the output capacitance is increased to 47 µF ce-

ramic. It can be seen that the output transient is further

reduced down to about 120 mV, which is still about 6.6% of

the output voltage. This shows that a 5X increase in ceramic

capacitance from the original 10 µF only reduced the peak

voltage transient amplitude by about 40%.

30028138

FIGURE 8. 47 µF Ceramic Output Capacitor

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.

In the next test, the same 10 µF ceramic capacitor will be

paralleled with a general purpose (cheap) aluminum elec-

trolytic whose capacitance is 220 µF. As shown in Figure 9,

there is a small improvement over the 200 mV peak seen with

the 10 µF ceramic alone. By adding the 220 µF aluminum

capacitor, the peak is reduced to about 160 mV (the same

peak value as seen with a 22 µF ceramic capacitor alone).

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