LM20146MHEVAL National Semiconductor, LM20146MHEVAL Datasheet - Page 4

• Manufacturer Part Number: LM20146MHEVAL
• Description: BOARD EVALUATION FOR LM20146MH
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM20146MHEVAL

• Design Resources: LM20146 Design Spreadsheet
• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 6A
• Voltage - Input: 2.95 ~ 5.5V
• Regulator Topology: Buck
• Board Type: Fully Populated
• Utilized Ic / Part: LM20146
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

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Component Selection
This section provides a walk-through of the design process
for the LM20146 evaluation board. Unless otherwise indicat-
ed all formulas assume units of Amps (A) for current, Farads
(F) for capacitance, Henries (H) for inductance, and Volts (V)
for voltage.
INPUT CAPACITOR
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 6A 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 100µF X5R ceramic capacitor from TDK provides the nec-
essary input capacitance for the evaluation board.
AVIN FILTER
An RC filter should be added to prevent any switching noise
on PVIN from interfering with the internal analog circuitry con-
nected to AVIN. This can be seen on the schematic as com-
ponents R
resistor R
current during startup. If R
drop can trigger the UVLO comparator. For the evaluation
board, a 1.0Ω resistor is used for R
not be triggered after the part is enabled. A 1.0µF capacitor
in conjunction with the 1.0Ω resistor is recommended to filter
the input to AVIN.
INDUCTOR
As per the datasheet recommendations, the inductor value
should initially be chosen to give a peak to peak ripple current
equal to roughly 30% of the maximum output current. The
peak to peak inductor ripple current can be calculated by the
equation:
Rearranging this equation and solving for the inductance re-
veals that for this application (V
750kHz, and I
roughly .68 µH. This results in a peak-to-peak ripple current
of 1.8A and 1.5A when the converter is operating from 5V and
3.3V respectively. Once an inductance value is calculated, an
actual inductor needs to be selected based on a trade-off be-
tween physical size, efficiency, and current carrying capabil-
CIN(RMS)
F
F
as the AVIN pin will draw a short 60mA burst of
and C
is given by:
OUT
F
. There is a practical limit to the value of
= 6A) the nominal inductance value is
F
is too large the resulting voltage
IN
= 5V, V
F
ensuring that UVLO will
RMS
OUT
rating in a small
= 1.2V, f
CIN(RMS)
SW
re-
=
4
ity. For the LM20146 evaluation board, a TDK SPM6530T-
R68M140 inductor offers a good balance between efficiency
(5.39mΩ DCR), size (7.1mm x 6.5mm), and saturation current
rating (14A I
is increased or if the switching frequency is decreased, there
is a chance the device may hit current limit at 6A output. To
avoid current limit with higher output voltages the value of the
inductor should be increased to reduce the ripple current.
OUTPUT CAPACITOR
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
ripple (ΔI
by the equation:
The variable R
tance of the output capacitor. As can be seen in the above
equation, the ripple voltage on the output can be divided in
two parts. One part is attributed to the AC ripple current flow-
ing through the ESR of the output capacitor. The other part is
due to the AC ripple current charging and discharging the
output capacitor. The output capacitor also has an affect on
the amount of droop that is seen on the output voltage in re-
sponse to a load transient event.
For the evaluation board, a TDK 100µF ceramic capacitor is
selected for the output capacitor to provide good transient and
DC performance in a relatively small package. From the tech-
nical specifications of this capacitor, the ESR is roughly
3mΩ and the effective in-circuit capacitance is approximately
60µF (reduced from 100µF due to the 1.2V DC bias and worse
case tolerance). With these values, the peak to peak voltage
ripple on the output when operating from a 5V input can be
calculated to be about 10mV.
C
A soft-start capacitor can be used to control the startup time
of the LM20146. The startup time when using a soft-start ca-
pacitor can be estimated by the following equation:
For the LM20146, I
board, the soft-start time has been designed to be roughly 5
ms, resulting in a C
C
The C
subregulator. This capacitor should be sized equal to or
greater than 1µF but less than 10µF. A value of 1µF is suffi-
cient for most applications.
C
The capacitor C
the LM20146 control loop. Since this board was optimized to
work well over the full input and output voltage range, the val-
ue of C
conditions for the device are known, the transient response
SS
VCC
C1
VCC
C1
P-P
capacitor is necessary to bypass an internal 2.7V
was selected to be 1.2nF. Once the operating
) the output voltage ripple can be approximated
SAT
ESR
). If the output voltage of the evaluation board
C1
above refers to the Effective Series Resis-
is used to set the crossover frequency of
SS
SS
capacitor value of 33nF.
is nominally 5µA. For the evaluation