LM3477EVAL National Semiconductor, LM3477EVAL Datasheet - Page 3

• 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: -

Increasing Current Limit
The R
slope compensation. Slope compensation affects the mini-
mum inductance for stability (see the Slope Compensation
section in the LM3477 datasheet), but also helps determine
the current limit and hysteretic threshold. As an example,
R
so that no extra slope compensation is added to the current
sense waveform to increase the current limit. A more con-
ventional way to adjust the current limit is to change R
R
simplicity and to demonstrate the dependence of current
limit to R
tions may be met:
Magnitude = 20 dB/Decade
Phase = 45˚/Decade
Layout Fundamentals
Good layout for DC-DC converters can be implemented by
following a few simple design guidelines:
1. Place the power components (catch diode, inductor, and
2. Use wide traces between the power components and for
3. Connect the ground pins of the input and output filter
4. Arrange the power components so that the switching
SL
SL
filter capacitors) close together. Make the traces be-
tween them short.
power connections to the DC-DC converter circuit.
capacitors and catch diode as close as possible using
generous
pseudo-ground plane. Then, connect this to the
ground-plane with several vias.
current loops curl in the same direction.
can be disconnected and replaced by a 0 ohm resistor
is used here to change current limit for the sake of
SL
resistor offers flexibility in choosing the ramp of the
SL
. By changing R
component-side
SL
to 0 ohm, the following condi-
Phase Margin = 42˚
Bandwidth = 55.3kHz
copper
FIGURE 6. Open Loop Frequency Response
V
fill
IN
= 5V, V
as
SN
OUT
a
.
= 3.3V, I
3
4.5V ≤ V
V
0A ≤ I
The current limit is a weak function of slope compensation
and a strong function of the sense resistor. By decreasing
R
current limit increases. The hysteretic mode threshold will
also increase to about 1A (see Figure 5).
Given below is a bode plot of LM3477 open loop frequency
response using the modified (R
achieve higher output current capability.
5. Route high-frequency power and ground return as direct
6. Separate noise sensitive traces, such as the voltage
7. Ensure a good low-impedance ground for the converter
8. Place the supporting components for the converter IC,
9. Place noise sensitive circuitry, such as radio-modem IF
OUT
SL
, slope compensation is decreased, and as a result the
OUT
continuous parallel paths.
feedback path, from noisy traces associated with the
power components.
IC.
such as compensation, frequency selection and
charge-pump components, as close to the converter IC
as possible but away from noisy traces and the power
components. Make their connections to the Converter IC
and it’s pseudo-ground plane as short as possible.
blocks, away from the DC-DC converter, CMOS digital
blocks, and other noisy circuitry.
= 3.3V
OUT
20016306
= 3A
IN
≤ 3A
≤ 15V
SL
= 0Ω) components to
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