Datasheet

1
1
2
ff
ff
C
fz Rp
=
× × ×
1
2
Roll
C Roll
fp
R Cp
=
× × ×
Re
Roll
C
sr Co
C
R
×
=
Re
Roll
C
sr Co
C
R
×
=
1
2
O L
fp
C R p
=
× ×
L
c
c
R Co
C
R
×
=
L
c
c
R Co
C
R
×
=
2
C
M ps
fc Vo Co
R
g Vref gm
p × × ×
=
× ×
2
C
M ps
fc Co
R
g gm
p × ×
=
×
TPS65250
SLVSAA3C –JUNE 2010–REVISED OCTOBER 2012 www.ti.com
To calculate the external compensation components follow the following steps:
TYPE II CIRCUIT TYPE III CIRCUIT
Select switching frequency that is appropriate for
application depending on L, C sizes, output ripple, EMI
concerns and etc. Switching frequencies between 500 kHz
Use type III circuit for switching
and 1 MHz give best trade off between performance and
frequencies higher than 500 kHz.
cost. When using smaller L and Cs, switching frequency
can be increased. To optimize efficiency, switching
frequency can be lowered.
Select cross over frequency (fc) to be less than 1/5 to 1/10 Suggested Suggested
of switching frequency. fc = fs/10 fc = fs/10
Set and calculate R
c
.
Calculate C
c
by placing a compensation zero at or before
the converter dominant pole
Add C
Roll
if needed to remove large signal coupling to high
impedance COMP node. Make sure that
is at least twice the cross over frequency.
Calculate C
ff
compensation zero at low frequency to boost
the phase margin at the crossover frequency. Make sure
NA
that the zero frequency (fz
ff
is smaller than soft start
equivalent frequency (1/T
ss
).
Slope Compensation
The device has a built-in slope compensation ramp. The slope compensation can prevent sub harmonic
oscillations in peak current mode control.
Power Good
The PGOOD pin is an open drain output. The PGOOD pin is pulled low when any buck converter is pulled below
85% of the nominal output voltage. The PGOOD is pulled up when Buck 1 and 3 converters’ outputs are more
than 90% of its nominal output voltage. The default reset time is 1000 ms. The polarity of the PGOOD is active
high.
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