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
