SC2440 SEMTECH [Semtech Corporation], SC2440 Datasheet - Page 20

• Manufacturer Part Number: SC2440
• Description: 2.5 MHz Dual Switching Regulator with Integrated 2A Switches
• Manufacturer: SEMTECH [Semtech Corporation]

In addition C
The
band gain (between z
overall loop gain T(s) is the product of the control-to-
output and the output-to-control transfer functions. To
simplify
assumed to be resistive. If the overall loop gain is to
cross 0dB at one tenth of the switching frequency
(
(between z
This is also equal to
Re-arranging,
Notice that R
converter. Increasing R
POWER MANAGEMENT
Applications Information
v
v
z1
COMP
2005 Semtech Corp.
1 z
OUT
C
R
C
G
is shown to be less than
MP
5
5
Z
p
1
c
10
2
R
p
S
2
OUT
C
v
output-to-control
1
COMP
v
gives a first-order estimate of C
1
T
C
140
R
FB
1
G
R
j (
nR
1
5
10
OUT
and p
5
R
MA
5
f
n
R
R
1
5
C
S
and R
OUT
)
1
2
5
) at –20dB/decade, then its mid-band gain
determines the mid-band loop gain of the
R
(
Krads
5
MIN
v
5
v
OUT
Bode plot, the feedback network is
10
FB
)
2
R
) will be
nG
5
1
is also shown in Figure 13. Its mid-
15
R
form a zero with angular frequency:
G
S
1
S
2
1
C
10
MP
MP
5
C
R
4 .
1
and p
increases the mid-band gain and
1
2
R
G
R
n
K
OUT
OUT
MA
22
1
G
.
KHz
470
3
MA
) is
C
10
R
1
p2
transfer
R
5
n
pF
OUT
in Figure 13. Making
G
R
MA
1
.
R
R
2
5
R
2
5
R
:
1
R
. Therefore
2
R
function
2
. The
(11)
(12)
20
the crossover frequency. However it reduces the phase
margin. An estimate of R
(11) and (12) with n=1. The compensation is then checked
by measuring the loop gain and the phase or by observing
the inductor current and the output voltage during load
transient. Choose the largest R
give at least 45 of phase margin. The corresponding
load transient should not show any ringing or excessive
overshoot (see Figures 14(c), 14(d), 17(b) and 17(c)). C
is a small ceramic capacitor (10-47pF) to roll off the loop
gain at high frequency. Feedforward capacitor C
phase margin over a limited frequency range and is
sometimes used to improve loop response. C
more effective if
Example: Determine the compensation components for
the 1.3MHz 12V to 5V and 3.3V converter in Figure 1.
For both channels,
For the 3.3V output:
For the 5V channel:
C
margin. Load transient responses of both channels are
observed using these values. There is very little inductor
current overshoot even with C
and 220pF respectively (Figure 14). The measured overall
loop gain and phase plots of the converter are also
shown.
C
6
1
and C
R
C
R
C
5
7
10
5
8
25
16
(
(
9
F
1
1
) 1
) 1
(both 10pF) are then added to increase gain
. n is assumed to be 1 in (11) and (12).
5 .
9 .
10
10
25
16
K
K
40
23
10
10
5
5 .
9 .
5
3 .
2 .
K
K
K
3
K
K
K
5
R
3 .
1
V
2 (
2 (
10
10
V
S
) A
) A
R
(
(
1
) 1
) 1
5
8
8
8
1
and C
2 .
1
2 .
2 .
nF
R
(
(
nF
Mrads
. 5
. 5
2
5
.
10
10
) 7
) 7
and C
5
5
and the smallest C
6
6
2 (
2 (
can be obtained from
10
10
1
8 .
8 .
8
,
reduced to 470pF
I
5
5
OUT
10
10
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SC2440
(
MAX
4
4
)
)
)
11
11
2
A
boosts
will be
and
5
to
6