LMR10530_15 Datasheet, PDF (13/31 Pages) Texas Instruments – SIMPLE SWITCHER 5.5Vin, 3.0A Step-Down Voltage Regulator in WSON-10

LMR10530
www.ti.com
SNVS814A – JUNE 2012 – REVISED APRIL 2013
LOAD STEP RESPONSE
The LMR10530 has a fixed internal loop compensation, which results in a small-signal loop bandwidth highly
related to the output voltage level. In general, the loop bandwidth at low voltage is larger than at high voltage due
to the increased overall loop gain. The limited bandwidth at high output voltage may pose a challenge when loop
step response is concerned. In this case, one effective approach to improving loop step response is to add a
feed-forward capacitor C_FF) in the range of 27nF to 100nF in parallel with the upper feedback resistor (assuming
the lower feedback resistor is 2kΩ), as shown in Figure 27. The feed-forward capacitor introduces a zero-pole
pair which helps compensate the loop. The position of the zero-pole pair is a function of the feedback resistors
and capacitor:
(1)
(2)
Note the factor in parenthesis is the ratio of the output voltage to the feedback voltage. As the output voltage
gets close to 0.6V, the pole moves towards the zero, tending to cancel it out. Consequently, adding C_FF will have
less effect on the step response at lower output voltages.
As an example, Figure 29 shows that at the output voltage of 3.3V, a 47nF of C_FF can boost the loop bandwidth
to 117kHz, from the original 23kHz as shown in Figure 28. Correspondingly, the responses to a load step
between 0.3A and 3A without and with C_FF are shown in Figure 30 and Figure 31 respectively. The higher loop
bandwidth as a result of C_FF reduces the total output excursion by more than half.
Aside from the above approach, increasing the output capacitance is generally also effective to reduce the
excursion in output voltage caused by a load step. This approach remains valid for applications where the
desired output voltages are close to the feedback voltage.
Figure 27. Adding a C_FF Capacitor
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