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LM21215AMHX-1/NOPB Datasheet(PDF) 24 Page - Texas Instruments

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Part No. LM21215AMHX-1/NOPB
Description  15-A Ultra High-Efficiency Synchronous Buck Converter
Download  38 Pages
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM21215AMHX-1/NOPB Datasheet(HTML) 24 Page - Texas Instruments

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10
100
1k
10k
100k
1M
-50
0
50
100
150
200
-40
-20
0
20
40
60
80
100
120
140
160
FREQUENCY (Hz)
GAIN
PHASE
C3
ESR
C2
1
C
898pF
2
f
R
˜ S ˜
˜
C2
ESR
C2
ESR
LC
R
f
R
166
f
f
˜
:
C1
C2
SW
C1
C1
C
C
71pF
F
R
C
1
S ˜
˜
˜
C1
LC
C1
1
C
1.99nF
f
R
S ˜
˜
crossover
RAMP
C1
FB1
LC
IN
f
V
100kHz 0.8 V
R
R
10k
9.2k
f
V
17.4kHz
5 V
'
˜
˜
˜
˜
:
:
'
LM21215A
SNOSB87C – MARCH 2011 – REVISED JANUARY 2016
www.ti.com
First, choose a resistance for RFB1, a typical value being 10 kΩ. From this, calculate the resistance of RC1 using
Equation 14 to set the mid-band gain such that the desired crossover frequency is achieved.
(14)
Next, calculate the capacitance of CC1 by placing a zero at half of the LC double pole frequency (fLC):
(15)
Now calculate CC2 to place a pole at half of the switching frequency and RC2 to place the second zero at the LC
double pole frequency:
(16)
(17)
Last, derive capacitance of CC3 to place a pole at the same frequency as the output capacitor ESR zero:
(18)
An illustration of the total loop response is seen in Figure 35.
Figure 35. Loop Response
It is important to verify the stability either by observing the load transient response or by using a network
analyzer. A phase margin between 45° and 70° is usually desired for voltage-mode converter circuits. Excessive
phase margin causes slow system response to load transients whereas low phase margin leads to an oscillatory
load transient response. If the peak deviation of the load transient response is larger than required, increasing
fcrossover and recalculating the compensation components may help but usually at the expense of phase margin.
24
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Product Folder Links: LM21215A


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