Datasheet

LM26400Y
www.ti.com SNVS457C –FEBRUARY 2007–REVISED APRIL 2013
An easy strategy to build a stable loop with reasonable phase margin is to try to cross over between 20kHz and
100kHz, assuming the output capacitor is ceramic. When using pure ceramic capacitors at the output, simply use
the following equation to find out the crossover frequency.
(3)
where 22S (22 Siemens) is the equivelant of the 27dBS transfer admittance mentioned above and r is the ratio of
0.6V to the output voltage. Use the same equation to find out the needed output capacitance for a given
crossover frequency. Phase margin is typically between 50° and 60°. Notice the above equation is only good for
a crossover between 20kHz and 100kHz. A crossover frequency outside this range may result in lower phase
margin and less accurate prediction by the above equation.
Example: V
OUT
= 2.5V, C
OUT
= 36µF, find out the crossover frequency.
Assume the crossover is between 20kHz and 100kHz. Then
(4)
The above analysis serves as a starting point. It is a good practice to always verify loop gain on bench.
LOAD STEP RESPONSE
In general, the excursion in output voltage caused by a load step can be reduced by increasing the output
capacitance. Besides that, increasing the small-signal loop bandwidth also helps. This can be achieved by
adding a 27nF or so capacitor (C
FF
) in parallel with the upper feedback resistor (assuming the lower feedback
resistor is 5.9kΩ). See Figure 31 for an illustration.
Figure 31. Adding a C
FF
Capacitor
The responses to a load step between 0.2A and 2A with and without a C
FF
are shown in Figure 32. The higher
loop bandwidth as a result of C
FF
reduces the total output excursion by about 80mV.
Figure 32. C
FF
Improves Load Step Response
Use the following equation to calculate the new loop bandwidth:
(5)
Again, the assumption is the crossover is between 20kHz and 100kHz.
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