LM20333EVAL National Semiconductor, LM20333EVAL Datasheet - Page 5

• 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: -

Component Selection
This section provides a walk-through of the design process of
the LM20333 evaluation board. Unless otherwise indicated all
equations assume units of amps (A) for current, farads (F) for
capacitance, henries (H) for inductance, and volts (V) for volt-
ages.
INPUT CAPACITORS: C1, C2, C3
The required RMS current rating of the input capacitor for a
buck regulator can be estimated by the following equation:
The variable D refers to the duty cycle, and can be approxi-
mated by:
From this equation, it follows that the maximum I
quirement will occur at a full 3A load current with the system
operating at 50% duty cycle. Under this condition, the maxi-
mum I
Ceramic capacitors feature a very large I
footprint, making a ceramic capacitor ideal for this application.
A two 4.7 µF, X5R, 25V ceramic capacitor(C2, C3) from Mu-
rata are used to provide the necessary input capacitance for
the evaluation board. An additional 22uF, X5R, 25V capacitor
is used to provide additional input capacitance to counter ca-
bling inductance to the input.
INDUCTOR: L1
The value of the inductor was selected to allow the device to
achieve a 12V to 3.3V conversion at 250kHz to provide a peak
to peak ripple current 957mA, which is about 32% of the max-
imum output current. To have an optimized design, generally
the peak to peak inductor ripple current should be kept to
within 20% to 40% of the rated output current for a given input
voltage, output voltage and operating frequency. The peak to
peak inductor ripple current can be calculated by the equa-
tion:
Once an inductance value is calculated, an actual inductor
needs to be selected based on a trade-off between physical
size, efficiency, and current carrying capability. For the
LM20333
ER10R0M11 inductor offers a good balance between effi-
ciency (28 mΩ DCR), size, and saturation current rating (7.1A
I
OUTPUT CAPACITOR: C9
The value of the output capacitor in a buck regulator influ-
ences the voltage ripple that will be present on the output
voltage, as well as the large signal output voltage response
to a load transient. Given the peak-to-peak inductor current
SAT
rating).
CIN(RMS)
evaluation
is given by:
board,
a
Vishay
RMS
rating in a small
IHLP4040DZ-
CIN(RMS)
re-
5
ripple (ΔI
by the equation:
The variable R
pacitor. As can be seen in the above equation, the ripple
voltage on the output can be divided into two parts, one of
which is attributed to the AC ripple current flowing through the
ESR of the output capacitor and another due to the AC ripple
current actually charging and discharging the output capaci-
tor. The output capacitor also has an effect on the amount of
droop that is seen on the output voltage in response to a load
transient event.
For the evaluation board, a Sanyo 150 µF POSCAP output
capacitor was selected to provide good transient and DC per-
formance in a relatively small package. From the technical
specifications of this capacitor, the ESR is roughly 35 mΩ, and
RMS ripple current rating is 1.4A. With these values, the worst
case peak to peak voltage ripple on the output when operating
from a 12V input can be calculated to be 37 mV.
SOFT-START CAPACITOR: C6
A soft-start capacitor can be used to control the startup time
of the LM20333 voltage regulator. The startup time of the
regulator when using a soft-start capacitor can be estimated
by the following equation:
For the LM20333, I
board, the soft-start time has been designed to be roughly 15
ms, resulting in a C
VCC BYPASS: C5
The capacitor C5 is used to bypass the internal 4.5V sub-
regulator. A value of 1 µF is sufficient for most applications.
BOOT CAPACITOR: C4
C4 is the boot capacitor which is used to provide the charge
needed to drive the high-side FET. An optimal value for this
capacitor is 0.1 µF.
COMPENSATION CAPACITOR: C8
The capacitor C8 is used to set the crossover frequency of
the LM20333 control loop. Since this board was optimized to
be stable over the full input and output voltage range, the val-
ue of C8 was selected to be 1.5 nF. Once the operating
conditions for the device are known, the transient response
can be optimized by reducing the value of C8 and calculating
the value for R7 as outlined in the next section.
COMPENSATION RESISTOR: R7
Once the value of C8 is known, resistor R7 is used to place a
zero in the control loop to cancel the output filter pole. This
resistor can be sized according to the equation:
P-P
) the output voltage ripple can be approximated
ESR
above refers to the ESR of the output ca-
SS
SS
capacitor value of 100 nF.
is nominally 5 µA. For the evaluation
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