lm21215mhx National Semiconductor Corporation, lm21215mhx Datasheet - Page 17

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

The complex poles created by the output inductor and ca-
pacitor cause a 180° phase shift at the resonant frequency as
seen in
to the output capacitor ESR zero. The 180° phase shift must
be compensated out and phase boosted through the error
amplifier to stabilize the closed loop response. The compen-
sation network shown around the error amplifier in
creates two poles, two zeros and a pole at the origin. Placing
these poles and zeros at the correct frequencies will stabilize
the closed loop response. The Compensated Error Amplifier
transfer function is:
The pole located at the origin gives high open loop gain at DC,
translating into improved load regulation accuracy. This pole
occurs at a very low frequency due to the limited gain of the
error amplifier, however, it can be approximated at DC for the
purposes of compensation. The other two poles and two ze-
ros can be located accordingly to stabilize the voltage mode
loop depending on the power stage complex poles and Q.
Figure 10
pensation transfer function will look like.
Figure
is an illustration of what the Error Amplifier Com-
-20
-40
-60
-80
60
40
20
FIGURE 9. Power Train Bode Plot
0
100
9. The phase is boosted back up to -90° due
GAIN
PHASE
1k
FREQUENCY (HZ)
10k
100k
1M
10M
-120
-160
-200
-240
-280
-320
-360
-40
-80
Figure 8
0
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17
As seen in
sation network give a phase boost. This will cancel out the
effects of the phase loss from the output filter. The compen-
sation network also adds two poles to the system. One pole
should be located at the zero caused by the output capacitor
ESR (f
frequency (f
dependancy of the pole and zero locations on the compen-
sation components is described below.
An example of the step-by-step procedure to generate
comensation component values using the typical application
setup, (see
the compensation values are given in the table below.
where ΔV
(nominally 0.8V), and f
FIGURE 10. Type 3 Compensation Network Bode Plot
ESR
Parameter
f
) and the other pole should be at half the switching
CROSSOVER
RAMP
Figure
ΔV
100
-20
SW
Figure
80
60
40
20
V
R
R
0
I
V
f
OUT
C
100
OUT
DCR
SW
ESR
RAMP
/2) to role off the high frequency response. The
L
IN
O
is the oscillator peak-to-peak ramp voltage
10, the two zeros (f
15), is given. The parameters needed for
GAIN
PHASE
1k
CROSSOVER
FREQUENCY (Hz)
10k
100k
is the frequency at which
LC
/2, f
100 kHz
500 kHz
1M
0.56 µH
1.8 mΩ
1.0 mΩ
150 µF
Value
5.0V
1.2V
0.8V
LC
15A
) in the comen-
10M
www.national.com
-135
-180
-45
-90
90
45
0
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