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LM21215AMHX-1/NOPB Datasheet(PDF) 19 Page - Texas Instruments

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Part No. LM21215AMHX-1/NOPB
Description  15-A Ultra High-Efficiency Synchronous Buck Converter
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM21215AMHX-1/NOPB Datasheet(HTML) 19 Page - Texas Instruments

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background image
L(max)
L(max)
OUT(max)
I
I
I
2
'
OUT
F
L
SW
V
1 D
L
I F
˜
' ˜
VIN
IL AVG = IOUT
'IL
Time
Time
IL
VSW
IN
EN1
EN2
EN
EN2
V
1.35V
R
R
1.35V I
R
˜
˜
LM21215A
www.ti.com
SNOSB87C – MARCH 2011 – REVISED JANUARY 2016
where
REN1 is the resistor from VIN to EN
REN2 is the resistor from EN to AGND
IEN is the internal enable pullup current (2 µA)
1.35 V is the precision enable rising threshold voltage
(4)
Typical values for REN2 range from 10 kΩ to 100 kΩ.
8.2.1.2.3
Filter Inductor Selection
The filter inductor, designated LF, chosen for the application influences the ripple current and the efficiency of the
converter. The first selection criteria is to define the buck converter inductor ripple current
ΔIL, typically selected
between 20% to 40% of the maximum output current. Figure 31 shows the ripple current in a conventional buck
converter operating in continuous conduction mode. Larger ripple current results in a lower inductance, which
leads to lower inductor DC resistance (DCR) and improved efficiency. However, larger ripple current causes the
LM21215A to operate in DCM at a higher average output current.
Figure 31. Switch (SW) Voltage and Inductor Current Waveforms
Once the ripple current has been determined, calculate the appropriate inductance using Equation 5.
(5)
A 0.56-µH inductor with 1.8-m
Ω DCR meets the application requirements here. The peak inductor current at full
load corresponds to the maximum output current plus the ripple current, as shown by Equation 6.
(6)
Choose an inductor with a saturation current rating at maximum operating temperature that is higher than the
overcurrent protection limit. In general, lower inductance is desirable in switching converters because it equates
to faster transient response, lower inductor DCR, and reduced size for more compact designs. However, too low
of an inductance implies large inductor ripple current such that the overcurrent protection circuit is falsely
triggered at the full load. Larger inductor ripple current also implies higher output voltage ripple.
Copyright © 2011–2016, Texas Instruments Incorporated
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