lm21215a-1 National Semiconductor Corporation, lm21215a-1 Datasheet - Page 15

• Manufacturer Part Number: lm21215a-1
• Description: 15a High Efficiency Synchronous Buck Regulator With Frequency Synchronization
• 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 lower inductor series resistance, and
improved efficiency. However, larger ripple current 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
LM21215A-1 that provide various advantages. 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
FIGURE 8. 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 8
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,
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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 PVIN 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 performance over temperature
and also minimize the DC voltage derating that occurs with
Y5V capacitors. The input capacitors should be 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 LM21215A-1, 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.
CONTROL LOOP COMPENSATION
The LM21215A-1 incorporates a high bandwidth amplifier be-
tween the FB and COMP pins to allow the user to design a
compensation network that matches the application. This
section will walk through the various steps in obtaining the
open loop transfer function.
(A) is the load step change, R
OUT
(F) is the minimum required output capacitance,
IN
(V) is the input voltage, and V
DROOP
ESR
(Ω) is the output
(V) is the output
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OUT
(V) is
OUT-