MC33761

http://onsemi.com

8

Understanding the Load Transient Improvement

The MC33761 features a novel architecture which allows

the user to easily implement the regulator in burst systems

where the time between two current shots is kept very small.

The quality of the transient response time is related to

many parameters, among which the closed−loop bandwidth

with the corresponding phase margin plays an important

role. However, other characteristics also come into play like

the series pass transistor saturation. When a current

perturbation suddenly appears on the output, e.g. a load

increase, the error amplifier reacts and actively biases the

PNP transistor. During this reaction time, the LDO is in

open−loop and the output impedance is rather high. As a

result, the voltage brutally drops until the error amplifier

effectively closes the loop and corrects the output error.

When the load disappears, the opposite phenomenon takes

place with a positive overshoot. The problem appears when

this overshoot decays down to the LDO steady−state value.

During this decreasing phase, the LDO stops the PNP bias

and one can consider the LDO asleep (Figure 8). If by

misfortune a current shot appears, the reaction time is

incredibly lengthened and a strong undershoot takes place.

This reaction is clearly not acceptable for line sensitive

devices, such as VCOs or other Radio−Frequency parts.

This problem is dramatically exacerbated when the output

current drops to zero rather than a few mA. In this later case,

the internal feedback network is the only discharge path,

accordingly lengthening the output voltage decay period

(Figure 9).

The MC33761 cures this problem by implementing a

clever design where the LDO detects the presence of the

overshoot and forces the system to go back to steady−state

as soon as possible, ready for the next shot. Figure 10 and 11

show how it positively improves the response time and

decreases the negative peak voltage.

Figure 8. A Standard LDO Behavior when the Load

Current Disappears

Figure 9. A Standard LDO Behavior when the Load

Current Appears in the Decay Zone

Figure 10. Without Load Transient Improvement

Figure 11. MC33761 with Load Transient Improvement