Texas Instruments 36V, 2A PowerWise® Adjustable Frequency Synchronous Buck Regulator Evaluation Module LM20242EVAL/NOPB LM20242EVAL/NOPB Fiche De Données
Codes de produits
LM20242EVAL/NOPB
A
M
f
SW
/2
0 dB
FREQUENCY (Hz)
G
AI
N
(
d
B
)
Error Amp Zero, f
Z(EA)
Complex Double Pole, f
P(MOD)
Optional Error Amp
Pole, f
P2(EA)
0 dB
0 dB
A
EA
+ A
M
Erro
r
Amp
lif
ie
r
T
ra
n
sf
e
r
F
u
n
ct
io
n
Mo
d
u
la
to
r
a
n
d
O
u
tp
u
t
F
ilt
e
r
T
ra
n
sf
e
r
F
u
n
ct
io
n
C
o
mp
e
n
sa
te
d
C
lo
se
d
L
o
o
p
T
ra
n
sf
e
r
F
u
n
ct
io
n
A
EA
Error Amp Pole, f
P1(EA)
Complex Double Pole, f
P(MOD)
Output Filter Zero, f
Z(FIL)
Output Filter Pole, f
P(FIL)
f
C
Error Amp Pole, f
P(EA)
R
RT
=
82000
f
SW
- 56
SNVS534E – OCTOBER 2007 – REVISED MARCH 2013
ADJUSTING THE OPERATING FREQUENCY (R
RT
)
The operating frequency of the LM20242 can be adjusted by connecting a resistor from the RT pin to ground.
can be used to calculate the value of R
RT
for a given operating frequency.
(8)
Where, f
SW
is the switching frequency in kHz, and R
RT
is the frequency adjust resistor in k
Ω
. Please refer to the
curve Oscillator Frequency versus R
RT
in the typical performance characteristics section. If the R
RT
resistor is
omitted the device will not operate.
LOOP COMPENSATION (R
C1
, C
C1
)
The purpose of loop compensation is to meet static and dynamic performance requirements while maintaining
adequate stability. Optimal loop compensation depends on the output capacitor, inductor, load and the device
itself.
adequate stability. Optimal loop compensation depends on the output capacitor, inductor, load and the device
itself.
The overall loop transfer function is the product of the power stage and the feedback network transfer functions.
For stability purposes, the objective is to have a loop gain slope that is -20db/decade from a very low frequency
to
For stability purposes, the objective is to have a loop gain slope that is -20db/decade from a very low frequency
to
beyond
the
crossover
frequency.
shows
the
transfer
functions
for
power
stage,
feedback/compensation network, and the resulting closed loop system for the LM20242.
Figure 11. LM20242 Loop Compensation
The power stage transfer function is dictated by the modulator, output LC filter, and load; while the feedback
transfer function is set by the feedback resistor ratio, error amp gain and external compensation network.
transfer function is set by the feedback resistor ratio, error amp gain and external compensation network.
To achieve a -20dB/decade slope, the error amplifier zero, located at f
Z(EA)
, should be positioned to cancel the
output filter pole (f
P(FIL)
). An additional error amp pole, located at f
P2(EA)
, can be added to cancel the output filter
zero at f
Z(FIL)
. Cancellation of the output filter zero is recommended if larger value, non-ceramic output capacitors
are used.
16
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