LP38500-ADJ, LP38502-ADJ

SNVS539H – NOVEMBER 2007 – REVISED SEPTEMBER 2015

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Functional Block Diagrams (continued)

Figure 13. LP38502-ADJ WSON Block Diagram

7.3 Feature Description

7.3.1 Stability And Phase Margin

Any regulator which operates using a feedback loop must be compensated in such a way as to ensure adequate

phase margin, which is defined as the difference between the phase shift and –180 degrees at the frequency

where the loop gain crosses unity (0 dB). For most LDO regulators, the ESR of the output capacitor is required to

create a zero to add enough phase lead to ensure stable operation. The LP38500-ADJ and LP38502-ADJ each

have a unique internal compensation circuit which maintains phase margin regardless of the ESR of the output

capacitor, so any type of capacitor may be used.

Figure 14 shows the gain/phase plot of the LP38500-ADJ and LP38502-ADJ with an output of 1.2 V, a 10-µF

ceramic output capacitor, delivering 1.5 A of load current. It can be seen that the unity-gain crossover occurs at

150 kHz, and the phase margin is about 40° (which is very stable).

Figure 14. Gain-Bandwidth Plot for 1.5-A Load

Figure 15 shows the gain and phase with no external load. In this case, the only load is provided by the gain

setting resistors (about 12 k

Ω total in this test). It is immediately obvious that the unity-gain frequency is

significantly lower (dropping to about 500 Hz), at which point the phase margin is 125°.

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