LM3424BSTEVAL National Semiconductor, LM3424BSTEVAL Datasheet - Page 27

• Manufacturer Part Number: LM3424BSTEVAL
• Description: LM3424 Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs; Qty per Container: 1/
• Manufacturer: National Semiconductor

And the uncompensated DC loop gain (T
Buck
Boost
Buck-boost
For all topologies, the primary method of compensation is to
place a low frequency dominant pole (ω
that there is ample phase margin at the crossover frequency.
This is accomplished by placing a capacitor (C
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
larger to maintain stability as the LEDs are dimmed to zero.
A high frequency compensation pole (ω
attenuate switching noise and provide better gain margin. As-
suming R
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
FS
= 10Ω, C
FS
is calculated according to the higher
CMP
should be approximately 4x
P2
U0
P3
) which will ensure
) is approximated:
) can be used to
CMP
) from the
27
10. INPUT CAPACITANCE
Set the nominal input voltage ripple (Δv
the required capacitance (C
Buck
Boost
Buck-boost
Use D
solving for C
solving for C
The minimum allowable RMS input current rating (I
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
Buck
Boost
Buck-boost
MAX
to set the worst case input voltage ripple, when
IN
IN
in a buck-boost regulator and D
in a buck regulator.
IN
):
IN-PP
) by solving for
MID
T-MAX
www.national.com
= 0.5 when
):
CIN-RMS
)