TPS70751MPWPREP datasheet(32/41 Pages) TI1

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LDO

ESR

LOAD

Conclusion

TPS70751-EP

DUAL-OUTPUT LOW-DROPOUT VOLTAGE REGULATOR

WITH POWER-UP SEQUENCING FOR SPLIT-VOLTAGE DSP SYSTEMS

SLVS718 – DECEMBER 2006

Figure 41 shows the output capacitor and its parasitic impedances in a typical LDO output stage.

Figure 41. LDO Output Stage With Parasitic Resistances ESR

In steady state (dc state condition), the load current is supplied by the LDO (solid arrow) and the voltage across

the capacitor is the same as the output voltage [V(CO) = VO]. This means no current is flowing into the CO

branch. If IO suddenly increases (transient condition), the following occurs.

The LDO is not able to supply the sudden current need due to its response time (t1 in Figure 42). Therefore,

capacitor CO provides the current for the new load condition (dashed arrow). CO now acts like a battery with an

internal resistance, ESR. Depending on the current demand at the output, a voltage drop occurs at RESR. This

voltage is shown as VESR in Figure 41.

When CO is conducting current to the load, initial voltage at the load will be VO = V(CO) – VESR. Due to the

discharge of CO, the output voltage VO drops continuously until the response time t1 of the LDO is reached and

the LDO resumes supplying the load. From this point, the output voltage starts rising again until it reaches the

regulated voltage. This period is shown as t2 in Figure 42.

The figure also shows the impact of different ESRs on the output voltage. The left brackets show different levels

of ESRs, where number 1 displays the lowest and number 3 displays the highest ESR.

From the previous paragraphs, these conclusions can be drawn:

• The higher the ESR, the larger the droop at the beginning of load transient.

• The smaller the output capacitor, the faster the discharge time and the bigger the voltage droop during the

LDO response period.

To minimize the transient output droop, capacitors must have a low ESR and be large enough to support the

minimum output voltage requirement.

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