LM3477EVAL National Semiconductor, LM3477EVAL Datasheet - Page 20

• Manufacturer Part Number: LM3477EVAL
• Description: BOARD EVALUATION LM3477
• Manufacturer: National Semiconductor

Specifications of LM3477EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 1.6A
• Voltage - Input: 4.5 ~ 15V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3477
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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Compensation
The strategy taken here for choosing R
crossover frequency with R
with C
set R
f
mended)
R
GM = 1000x10
The compensator zero, f
sient responses are desired, f
possible, however it should not be higher than the selected
crossover frequency f
choose C
power pole f
frequency fc:
In this compensation scheme, the pole created by C
used to cancel out the zero created by the ESR of the output
capacitor. In other schemes such as the methods discussed
in the SAMPLING POLE QUALITY FACTOR subsection, the
ESR zero is used. For the typical case, use C
PLOTTING THE OPEN LOOP RESPONSE
The open loop response is expressed as:
T = A
Where A
A
C
GM
CM
= Crossover frequency in Hertz (20kHz - 50kHz is recom-
= GM x R
= 50x10
C
DC
C1
to:
. Using the selected target crossover frequency, f
DC
x A
C1
CM
and H are given above and
P1
3
such that f
Ω
GM
−6
and
x H x F
A/V
1
⁄
2
C
decade before the selected crossover
p
. The guideline proposed here is to
(s) x F
Z1
Z1
, is set with C
(Continued)
c
, and set the compensator zero
falls somewhere between the
Z1
c
(s)
should be placed as high as
c
and C
C1
. When fast tran-
C1
C2
is to set the
if:
C2
C
is
,
20
One can plot the magnitude and phase of the open loop
response to analyze the frequency response.
EXAMPLE: COMPENSATION DESIGN
4.5V ≤ V
V
I
R
L = 3.3µH
R
C
R
First, calculate the power stage parameters using V
and R
In this example, a crossover frequency of 20kHz is chosen,
so: f
information and the target crossover frequency f
This sets the high frequency gain of the compensator such
that a crossover frequency of f
C
power pole f
over frequency f
OUT
OUT
SN
SL
OUT
ESR
C1
C
= 0Ω
= 0.02Ω
sets the compensator zero, f
= 3A (R = 0.83Ω)
= 2.5V
= 0.01Ω
= 100µF
(MAX)
= 20000. R
IN
:
≤ 5.5V
P1
and the
C
C
:
is now calculated using the power stage
1
⁄
2
decade before the target cross-
C
is obtained. The capacitor
Z2
. Set f
Z2
between the
C
:
IN(MIN)