Datasheet

REF LEVEL
0.000 dB
0.0 deg
100 1k
START 100.000 Hz
10k
STOP 100 000.000 Hz
/DIV
10.000 dB
45.000 deg
0
GAIN
PHASE
100k
REF LEVEL
0.000 dB
0.0 deg
100 1k
START 50.000 Hz
10k
STOP 50 000.000 Hz
/DIV
10.000 dB
45.000 deg
0
GAIN
PHASE
LM25575, LM25575-Q1
SNVS479G –JANUARY 2007–REVISED APRIL 2013 www.ti.com
For the design example, a target loop bandwidth (crossover frequency) of 15kHz was selected. The
compensation network zero (f
Z
) should be selected at least an order of magnitude less than the target crossover
frequency. This constrains the product of R4 and C5 for a desired compensation network zero 1 / (2π R4 C5) to
be less than 2kHz. Increasing R4, while proportionally decreasing C5, increases the error amp gain. Conversely,
decreasing R4 while proportionally increasing C5, decreases the error amp gain. For the design example C5 was
selected for 0.01µF and R4 was selected for 49.9kΩ. These values configure the compensation network zero at
320Hz. The error amp gain at frequencies greater than f
Z
is: R4 / R5, which is approximately 10 (20dB).
Figure 10. Error Amplifier Gain and Phase
The overall loop can be predicted as the sum (in dB) of the modulator gain and the error amp gain.
Figure 11. Overall Loop Gain and Phase
If a network analyzer is available, the modulator gain can be measured and the error amplifier gain can be
configured for the desired loop transfer function. If a network analyzer is not available, the error amplifier
compensation components can be designed with the guidelines given. Step load transient tests can be
performed to verify acceptable performance. The step load goal is minimum overshoot with a damped response.
C6 can be added to the compensation network to decrease noise susceptibility of the error amplifier. The value
of C6 must be sufficiently small since the addition of this capacitor adds a pole in the error amplifier transfer
function. This pole must be well beyond the loop crossover frequency. A good approximation of the location of
the pole added by C6 is: f
p2
= fz x C5 / C6.
18 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LM25575 LM25575-Q1