Datasheet

C
c2
=
1
f
ESR
2SR
c
f
ESR
=
1
2SC
OUT
ESR
Q
p
=
1
S(m
c
D' ± 0.5)
and
w
n
Sf
s
=
1
1 +
+
s
Q
p
w
n
w
n
2
s
2
)(sF
h
=
LM21305
SNVS639F –DECEMBER 2009–REVISED MARCH 2013 www.ti.com
(26)
where:
(27)
The compensator network transfer function is:
(28)
With the above equations, the loop gain T(s) can be plotted and more accurate loop performance metrics
(crossover frequency and phase margin) can be determined.
HIGH FREQUENCY CONSIDERATIONS
F
h
(s) represents the additional magnitude and phase drop around f
s
/2 caused by the switching behavior of the
current-mode converter. F
h
(s) contains a pair of double poles with quality factor Q
p
at half of the switching
frequency. It is a good idea to check that Q
p
is between 0.15 and 2, ideally around 0.6. If Q
p
is too high, the
resonant peaking at f
s
/2 could become severe and coincide with subharmonic oscillations in the duty cycle and
inductor current. If Q
p
is too low, the two complex poles split, the converter begins to act like a voltage-mode
controlled converter and the compensation scheme used above should be changed.
F
p
(s) also contains the ESR zero of the output capacitors:
(29)
In a typical ceramic capacitor design, f
ESR
is at least three times higher than the desired crossover frequency f
c
. If
f
ESR
is lower than f
s
/2, an additional capacitor C
c2
can be added between the COMP pin and AGND to give a
high-frequency pole:
(30)
C
c2
should be much smaller than C
c1
to avoid affecting the compensation zero.
BOOTSTRAP CAPACITOR
A capacitor is needed between the CBOOT pin and the SW node to supply the gate drive charge when the high-
side switch is turning ON. The capacitor should be large enough to supply the charge without significant voltage
drop. A 0.1 µF ceramic bootstrap capacitor is recommended in LM21305 applications.
5V0 AND 2V5 CAPACITORS
5V0 and 2V5 pins are internal LDO outputs. As previously mentioned, the two LDOs are used for internal circuits
only and should not be substantially loaded. Output capacitors are needed to stabilize the LDOs. Ceramic
capacitors within a specified range should be used to meet stability requirements. The dielectric should be X5R,
X7R, or better and rated for the required operating temperature range. Use the following table to choose a
suitable LDO output capacitor:
Output Voltage NOMINAL Capacitance (Recommended Value and Minimum Voltage Rating)
5V0 4.88V 1 µF ± 20% 16V
2V5 2.47V 0.1 µF ± 20% 10V
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