LP38503-ADJ_15 Datasheet, PDF (10/27 Pages) Texas Instruments – LP3850x-ADJ, LP3850xA-ADJ 3-A FlexCap Low Dropout Linear Regulator for 2.7-V to 5.5-V Inputs

LP38501-ADJ, LP38503-ADJ
SNVS522I – AUGUST 2007 – REVISED AUGUST 2015
Feature Description (continued)
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Figure 13. Gain-Bandwidth Plot for 2-A Load
Figure 14 shows the gain and phase with no external load. In this case, the only load is provided by the gain
setting resistors (about 12 kΩ 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°.
Figure 14. Gain-Bandwidth Plot for No Load
The reduction in unity-gain bandwidth as load current is reduced is normal for any LDO regulator using a P-FET
or PNP pass transistor, because they have a pole in the loop gain function given by:
F_P
1
2 u S u R_L u C_OUT
(1)
Equation 1 calculates how the pole goes to the highest frequency when R_L is minimum value (maximum load
current). In general, LDOs have maximum bandwidth (and lowest phase margin) at full load current. In the case
of the LP38501-ADJ, good phase margin is seen even when using ceramic capacitors with ESR values of only a
few mΩ.
7.3.2 Load Transient Response
Load transient response is defined as the change in regulated output voltage which occurs as a result of a
change in load current. Many applications have loads which vary, and the control loop of the voltage regulator
must adjust the current in the pass FET transistor in response to load current changes. For this reason,
regulators with wider bandwidths often have better transient response.
The LP38501-ADJ employs an internal feed-forward design which makes the load transient response much
faster than would be predicted simply by loop speed; this feedforward means any voltage changes appearing on
the output are coupled through to the high-speed driver used to control the gate of the pass FET along a signal
path using very fast FET devices. Because of this, the pass transistor’s current can change very quickly.
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