Datasheet

10
RMP SYNC
mc
S
LB
CT
V k
g Rzc
R
I
N
D ´
£
D ´
4
10 100
CT
LB S
V N
Rzc
S I Rm
´
£
´ ´ D ´
2
S
CT
CXO mc
RMP SYNC B
R
Vout
N
f g Rzc
V k Lp
´
= ´
D ´ ´ ´
1
2
PWM
Cpc
f Rzcp
=
´ ´
UCC28070
SLUS794E – NOVEMBER 2007–REVISED APRIL 2011 www.ti.com
The switching ripple voltage should be attenuated to less than 1/10 of the ΔV
RMP
amplitude so as to be
considered “negligible” ripple.
Thus, CAOx gain at f
PWM
is:
(23)
where ∆I
LB
is the maximum peak-to-peak ripple current in the boost inductor, and g
mc
is the transconductance of
the CA, 100 μS.
(24)
The current-loop cross-over frequency is then found by equating the open loop gain to 1 and solving for f
CXO
:
(25)
C
CZ
is then determined by setting f
ZC
= f
CXO
= 1/(2πxR
ZC
xC
ZC
) and solving for C
ZC
. At f
ZC
= f
CXO
, a phase margin
of 45° is obtained at f
CXO
. Greater phase margin may be had by placing f
ZC
< f
CXO
.
An additional high-frequency pole is generally added at f
PWM
to further attenuate ripple and noise at f
PWM
and
higher. This is done by adding a small-value capacitor, C
pc
, across the R
zc
C
zc
network.
(26)
The procedure above is valid for fixed-value inductors.
NOTE
If a “swinging-choke” boost inductor (inductance decreases with increasing current) is
used, f
CXO
varies with inductance, so C
ZC
should be determined at maximum inductance.
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