LM20333EVAL National Semiconductor, LM20333EVAL Datasheet - Page 12

• Manufacturer Part Number: LM20333EVAL
• Description: EVALUATION BOARD FOR THE LM20333
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM20333EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 4.5 ~ 25V
• Regulator Topology: Buck
• Board Type: Fully Populated
• Utilized Ic / Part: LM20333
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

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mation for the current mode control loop. If the ripple current
in the inductor is too low, the control loop will not have suffi-
cient current sense information and can be prone to instability.
OUTPUT CAPACITOR SELECTION (C
The output capacitor, C
and provides a source of charge for transient load conditions.
A wide range of output capacitors may be used with the
LM20333 that provide excellent performance. The best per-
formance is typically obtained using ceramic, SP or OSCON
type chemistries. Typical trade-offs are that the ceramic ca-
pacitor provides extremely low ESR to reduce the output
ripple voltage and noise spikes, while the SP and OSCON
capacitors provide a large bulk capacitance in a small volume
for transient loading conditions.
When selecting the value for the output capacitor, the two
performance characteristics to consider are the output volt-
age ripple and transient response. The output voltage ripple
can be approximated by using the following formula:
where, ΔV
at the power supply output, R
of the output capacitor, f
and C
The amount of output ripple that can be tolerated is applica-
tion specific; however a general recommendation is to keep
the output ripple less than 1% of the rated output voltage.
Keep in mind ceramic capacitors are sometimes preferred
because they have very low ESR; however, depending on
package and voltage rating of the capacitor the value of the
capacitance can drop significantly with applied voltage. The
output capacitor selection will also affect the output voltage
droop during a load transient. The peak droop on the output
voltage during a load transient is dependent on many factors;
however, an approximation of the transient droop ignoring
loop bandwidth can be obtained using the following equation:
where, C
L (H) is the value of the inductor, V
voltage drop ignoring loop bandwidth considerations, ΔI
STEP
capacitor ESR, V
the set regulator output voltage. Both the tolerance and volt-
age coefficient of the capacitor should be examined when
designing for a specific output ripple or transient droop target.
INPUT CAPACITOR SELECTION
Good quality input capacitors are necessary to limit the ripple
voltage at the VIN pin while supplying most of the switch cur-
rent during the on-time. In general it is recommended to use
(A) is the load step change, R
OUT
OUT
OUT
(F) is the output capacitance used in the design.
(F) is the minimum required output capacitance,
(V) is the amount of peak to peak voltage ripple
IN
(V) is the input voltage, and V
OUT
SW
, filters the inductor ripple current
(Hz) is the switching frequency,
ESR
(Ω) is the series resistance
DROOP
ESR
OUT
(Ω) is the output
)
(V) is the output
OUT
(V) is
OUT-
12
a ceramic capacitor for the input as they provide both a low
impedance and small footprint. One important note is to use
a good dielectric for the ceramic capacitor such as X5R or
X7R. These provide better over temperature performance
and also minimize the DC voltage derating that occurs on Y5V
capacitors. The input capacitors C
placed as close as possible to the VIN and GND pins on both
sides of the device.
Non-ceramic input capacitors should be selected for RMS
current rating and minimum ripple voltage. A good approxi-
mation for the required ripple current rating is given by the
relationship:
As indicated by the RMS ripple current equation, highest re-
quirement for RMS current rating occurs at 50% duty cycle.
For this case, the RMS ripple current rating of the input ca-
pacitor should be greater than half the output current. For best
performance, low ESR ceramic capacitors should be placed
in parallel with higher capacitance capacitors to provide the
best input filtering for the device.
SETTING THE OUTPUT VOLTAGE (R
The resistors R
voltage for the device. provides suggestions for R
R
If different output voltages are required, R
lected to be between 4.99 kΩ to 49.9 kΩ and R
calculated using the equation below.
LOOP COMPENSATION (R
The purpose of loop compensation is to meet static and dy-
namic performance requirements while maintaining adequate
stability. Optimal loop compensation depends on the output
capacitor, inductor, load and the device itself. Table 2 below
gives values for the compensation network that will result in
a stable system when using a 150 µF, 6.3V POSCAP output
capacitor (6TPB150MAZB).
FB2
for common output voltages.
TABLE 1. Suggested Values for R
R
FB1
FB1
short
4.99
8.87
12.7
21.5
31.6
52.3
(kΩ)
and R
R
FB2
FB2
open
10.2
10.2
10.2
10.2
C1
10
10
are selected to set the output
(kΩ)
, C
C1
)
IN1
V
FB1
and C
0.8
1.2
1.5
1.8
2.5
3.3
5.0
OUT
FB1
, R
FB2
and R
FB2
IN2
should be se-
)
FB1
should be
FB2
FB1
can be
and