LP38501TS-ADJ Datasheet, PDF (9/22 Pages) Texas Instruments – LP38501/3-ADJ, LP38501A/3A-ADJ 3A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs

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LP38501-ADJ, LP38503-ADJ
SNVS522H – AUGUST 2007 – REVISED APRIL 2013
Figure 17. 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:
(2)
This illustrates how the pole goes to the highest frequency when R_L is minimum value (maximum load current).
In general, LDO’s have maximum bandwidth (and lowest phase margin) at full load current. In the case of the
LP38501, it can be seen that it has good phase margin even when using ceramic capacitors with ESR values of
only a few milli Ohms.
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 employs an internal feedforward 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.
Figure 18 shows the output transient response resulting from a change in load current of 0.1A – 3A, and then
3A – 0.1A with a load current slew rate of 500 mA/µs. As shown in the plots, the resulting change in output
voltage is only about 40 mV (peak), which is just slightly over 2% for the 1.8V output used for this test. This is
excellent performance for such a small output capacitor.
Figure 18. Load Transient Response: 10 µF Ceramic, 0.5A/µs di/dt
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