lm21215mhx National Semiconductor Corporation, lm21215mhx Datasheet - Page 15

• Manufacturer Part Number: lm21215mhx
• Description: 15a High Efficiency Synchronous Buck Regulator
• Manufacturer: National Semiconductor Corporation

INDUCTOR SELECTION
The inductor (L) used in the application will influence the ripple
current and the efficiency of the system. The first selection
criteria is to define a ripple current, ΔI
it is typically selected to run between 20% to 30% of the max-
imum output current.
standard buck converter operating in continuous conduction
mode. Larger ripple current will result in a smaller inductance
value, which will lead to a lower series resistance in the in-
ductor, and improved efficiency. However, larger ripple cur-
rent will also cause the device to operate in discontinuous
conduction mode at a higher average output current.
Once the ripple current has been determined, the appropriate
inductor size can be calculated using the following equation:
OUTPUT CAPACITOR SELECTION
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
LM21215 that provide various advantages. The best perfor-
mance 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
FIGURE 7. Switch and Inductor Current Waveforms
OUT
OUT
(F) is the output capacitance used in the design.
(V) is the amount of peak to peak voltage ripple
Figure 7
OUT
SW
, filters the inductor ripple current
(Hz) is the switching frequency,
ESR
shows the ripple current in a
(Ω) is the series resistance
L
. In a buck converter,
30103707
15
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
Quality input capacitors are necessary to limit the ripple volt-
age at the VIN pin while supplying most of the switch current
during the on-time. Additionally, they help minimize input volt-
age droop in an output current transient condition. In general,
it is recommended to use a ceramic capacitor for the input as
it provides both a low impedance and small footprint. Use of
a high grade dielectric for the ceramic capacitor, such as X5R
or X7R, will provide improved over-temperature performance
and also minimize the DC voltage derating that occurs with
Y5V capacitors. The input capacitors C
placed as close as possible to the PVIN and PGND pins.
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.
When operating at low input voltages (3.3V or lower), addi-
tional capacitance may be necessary to protect from trigger-
ing an under-voltage condition on an output current transient.
This will depend on the impedance between the input voltage
supply and the LM21215, as well as the magnitude and slew
rate of the output transient.
The AVIN pin requires a 1 µF ceramic capacitor to AGND and
a 1Ω resistor to PVIN. This RC network will filter inherent
noise on PVIN from the sensitive analog circuitry connected
to AVIN.
PROGRAMMABLE CURRENT LIMIT
A resisitor from the ILIM pin to GND will set the internal current
limit on the LM21215. The current limit should be pro-
grammed such that the peak inductor current (I
trigger the current limit in normal operation. This requires set-
ting the resistor from the ILIM pin to GND (R
appropriate value to allow the maximum ripple current,
ΔI
LMAX
(A) is the load step change, R
plus the DC output current through the high-side FET
OUT
(F) is the minimum required output capacitance,
IN
(V) is the input voltage, and V
DROOP
ESR
IN1
and C
(Ω) is the output
(V) is the output
IN2
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L
ILIM
) does not
OUT
should be
) to the
(V) is
OUT-