Hi,
We are driving two fans (one rated for 0.6A and the other rated for 0.39A - both are
12V fans) and a 5V LDO, with max load of ~160 mA. The Vout setpoint is set to 13.45V
(R5 = 10k, R6 = 1k). The two fans are connected in parallel. The fan positive leads
are connected to the output of the LM5575 regulator via a 4.7 nH inductor and the
negative leads are connected to ground via a MOSFET that is switching at 25 kHz. The
gate signal duty cycle that drives the MOSFET is varied between 10% and 90%. We
notice that when we perform a duty sweep, the output voltage is not regulating.
Here is the data collected:
% DC Vout
---- -----
10 13.43
20 13.42
30 13.32
40 13.17
50 13.06
60 12.96
70 12.9
80 12.86
90 12.82
97 12.7
This is below the 1.5A limit of the regulator as stated in the datasheet.
The compensator values are as follows:
C5 - 1.5 nF
C6 - 68 pF
R4 - 35k
Additionally, we also have the following component values:
C3 = 470 pF (the output inductor is 47 uH)
Rramp = 330 k (per TI forum stating that this should be in the 100ks range)
C7 - 0.022 uF
Can you please advise as to why this device does not regulate.
Thank you.
Regards,
Saul
