LM3477EVAL National Semiconductor, LM3477EVAL Datasheet - Page 2

• 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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Performance
Following are some benchmark data taken from the circuit
above on the LM3477 evaluation board. This evaluation
board may also be used to evaluate a buck regulator circuit
optimized for a different operating point, or to evaluate a
trade-off between cost and some performance parameter.
For example, the conversion efficiency may be increased by
using a lower R
ered with lower ESR output capacitors, and the hysteretic
threshold may be changed as a function of the R
resistors.
The conversion efficiency may be increased by using a lower
R
creases. The efficiency reduces because of increased diode
conduction time and increased switching losses. Switching
losses are due to the Vds
charge losses, both of which may be lowered by using a FET
with low gate capacitance. At low duty cycles, where most of
the power loss in the FET is from the switching losses,
trading off higher R
increase efficiency.
DS(ON)
MOSFET, however it drops as input voltage in-
FIGURE 2. Efficiency vs Load
FIGURE 3. Efficiency vs V
DS(ON)
V
OUT
DS(ON)
MOSFET, ripple voltage may be low-
V
= 3.3V, I
*
OUT
Id transition losses and to the gate
for lower gate capacitance will
= 3.3V
OUT
= 2A
IN
20016304
20016303
SN
and R
SL
2
Given below is a bode plot of LM3477 open loop frequency
response using the external components listed in Table 1.
Magnitude = 20 dB/Decade
Phase = 45˚/Decade
Hysteretic Mode
As the load current is decreased, the LM3477 will eventually
enter a ’hysteretic’ mode of operation. When the load current
drops below the hysteretic mode threshold, the output volt-
age rises slightly. The over voltage protection (OVP) com-
parator senses this rise and causes the power MOSFET to
shut off. As the load pulls current out of the output capacitor,
the output voltage drops until it hits the low threshold of the
OVP comparator and the part begins switching again. This
behavior results in a lower frequency, higher peak-to-peak
output voltage ripple than with the normal pulse width modu-
lation scheme. The magnitude of the output voltage ripple is
determined by the OVP threshold levels, which are referred
to the feedback voltage and are typically 1.25V to 1.31V (see
Electrical Characteristics table in the LM3477 datasheet). In
the case of a 3.3V output, this translates to a regulated
output voltage between 3.27V and 3.43V. The hysteretic
mode threshold point is a function of R
shows the Hysteretic Threshold vs. V
evaluation board with and without R
FIGURE 4. Open Loop Frequency Response
V
IN
= 5V, V
FIGURE 5. I
OUT
= 3.3V, I
TH
Phase Margin = 41˚
Bandwidth = 39.8kHz
vs V
SL
OUT
.
SN
IN
IN
20016307
= 1.5A
and R
for the LM3477
20016302
SL
. Figure 5