L4945 datasheet(4/6 Pages) STMICROELECTRONICS

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EXTERNAL COMPENSATION

Since the purpose of a voltage regulator is to sup-

ply and load variations, the open loop gain of the

regulators must be very high at low frequencies.

This may cause instability as a result of the vari-

ous poles present in the loop. To avoid this insta-

bility dominant pole compensation is used to re-

duce phase shift due to other poles at the unity

gain frequency. The lower the frequency of these

others poles at the unity gain frequency. The

lower the frequency of these other poles, the

greater must be capacitor esed to create the

dominant pole for the same DC gain.

Where the output transistor is a lateral PNP type

there is a pole in the regulation loop at a fre-

quencybtoo low to be compensated by a capaci-

tor which can be integrated. An external compen-

sation is therefore necessary so a very high value

capacitor must be connected from the output to

ground.

The paeassitic equivalent series resistance of the

capacitor used adds a zero to the regulation loop.

This zero may compromise the stability of the

system since its effect tends to cancel the effect

of the pole added. In regulators this ESR must be

less than 3

Ω and the minimum capacitor value is

µF.

Figure 1: Typical Dropout Voltage vs. Tj

(Io = 500mA).

L4945

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