And the uncompensated DC loop gain (T
U0
) is approximated:
Buck
Boost
Buck-boost
For all topologies, the primary method of compensation is to
place a low frequency dominant pole (
ω
P2
) which will ensure
that there is ample phase margin at the crossover frequency.
This is accomplished by placing a capacitor (C
CMP
) from the
COMP pin to GND, which is calculated according to the lower
value of the pole and the RHP zero of the system (shown as
a minimizing function):
If analog dimming is used, C
CMP
should be approximately 4x
larger to maintain stability as the LEDs are dimmed to zero.
A high frequency compensation pole (
ω
P3
) can be used to
attenuate switching noise and provide better gain margin. As-
suming R
FS
= 10Ω
, C
FS
is calculated according to the higher
value of the pole and the RHP zero of the system (shown as
a maximizing function):
The total system loop gain (T) can then be written as:
Buck
Boost and Buck-boost
10. INPUT CAPACITANCE
Set the nominal input voltage ripple (
Δ
v
IN-PP
) by solving for
the required capacitance (C
IN
):
Buck
Boost
Buck-boost
Use D
MAX
to set the worst case input voltage ripple, when
solving for C
IN
in a buck-boost regulator and D
MID
= 0.5 when
solving for C
IN
in a buck regulator.
The minimum allowable RMS input current rating (I
CIN-RMS
)
can be approximated:
Buck
Boost
Buck-boost
11. NFET
The NFET voltage rating should be at least 15% higher than
the maximum NFET drain-to-source voltage (V
T-MAX
):
Buck
Boost
Buck-boost
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LM3424
