Datasheet

10
100 1k 10k 100k 1M
FREQUENCY
-80
-60
-40
-20
0
20
40
60
80
dB
-180
-90
0
90
180
RHP-Zero
Ext (Cf)
gm-Pole
RC-Pole
Vi = 5V
Vo = 12V
Io = 500 mA
Co = 10 mF
Lo = 5 mH
Ext (Cf)-Pole
gm-zero
-Zero
D = 0.625
Cf = 220 pF
Fz-cf = 8 kHz
RHP-Zero = 107 kHz
Fp-rc = 660 Hz
Fp-cf = 77 kHz
10
100 1k 10k 100k 1M
FREQUENCY
-80
-60
-40
-20
0
20
40
60
80
dB
-180
-90
0
90
180
RHP-Zero
gm-Zero
gm-Pole
RC-Pole
Vi = 5V
Vo = 12V
Io = 500 mA
Co = 10 PF
Lo = 5 PH
LMR62421
SNVS734B –OCTOBER 2011–REVISED APRIL 2013 www.ti.com
COMPENSATION
The LMR62421 uses constant frequency peak current mode control. This mode of control allows for a simple
external compensation scheme that can be optimized for each application. A complicated mathematical analysis
can be completed to fully explain the LMR62421’s internal & external compensation, but for simplicity, a
graphical approach with simple equations will be used. Below is a Gain & Phase plot of a LMR62421 that
produces a 12V output from a 5V input voltage. The Bode plot shows the total loop Gain & Phase without
external compensation.
Figure 17. LMR62421 Without External Compensation
One can see that the Crossover frequency is fine, but the phase margin at 0dB is very low (22°). A zero can be
placed just above the crossover frequency so that the phase margin will be bumped up to a minimum of 45°.
Below is the same application with a zero added at 8 kHz.
Figure 18. LMR62421 With External Compensation
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