SC2616 Semtech, SC2616 Datasheet - Page 13

• Manufacturer Part Number: SC2616
• Description: Complete DDR Power Solution
• Manufacturer: Semtech

Compensation Components
Once the filter components have been determined, the
compensation components can be calculated. The goal
of compensation is to modify the frequency response
characteristics of the error amplifier to ensure that the
closed loop feedback system has the highest gain and
bandwidth possible while maintaining stability.
A simplified stability criteria states that the open loop
gain of the converter should fall through 0dB at 20dB/
decade at a frequency no higher than 20-25% of the
switching frequency.
This objective is most simply met by generating asymp-
totic bode plots of the small signal response of the vari-
ous sections of the converter.
It is convenient to split the converter into two sections,
the Error amp and compensation components being one
section and the Modulator, output filter and divider be-
ing the other.
First calculate the DC Filter + Modulator + Divider gain.
The DC filter gain is always 0dB, the Modulator gain is
19dB at 5V in and is proportional to Vin, so modulator
gain at any input voltage is.
the divider gain is given by
So the total Filter + Modulator + Divider DC Gain is
POWER MANAGEMENT
Applications Information (Cont.)
2003 Semtech Corp.
Zs
FB
REF
G
Zf
FMD
+
-
EA
SC2610 AND FETS
19
Zp
COMP
G
G
MOD
20
DIV
Log
MODULATOR
20
19
V
Log
5
IN
20
Log
R
OUT
20
A
R
B
V
R
Log
5
IN
B
L
R
Resr
A
Co
R
B
R
B
Ra
Rb
VOUT
13
Calculate the filter double pole frequency (Fp(lc))
and calculate ESR Zero frequency (Fz(esr))
Choose an open loop crossover frequency (Fco) no higher
than 20% of the switching frequency (Fs).
The proximity of Fz(esr) to the crossover frequency Fco
determines the type of compensation required, if
Fz(esr)>Fco/4, use type 3 compensation, otherwise use
type 2. Type 1 compensation is not appropriate and is
not discussed here.
Type 2 Example
As an example of type 2 compensation, we will use the
Evaluation board schematic.
The total Filter+Modulator+Divider DC Gain is:
This is drawn as the line A-B in Figure 5.
Fp
FB
G
lc (
REF
FMD
)
+
-
2
EA
19
SC2616 AND FETS
Cs
Rs
1
LCo
20
COMP
Fz
Log
(
Fp
Cp
esr
MODULATOR
2
lc (
5
5
)
)
3
20
2
.
3
2
Log
10
Co
OUT
1
LCo
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1
1
6
. 6
Re
Vin=5V
3000
98
3.3uH
. 8
3000uF
sr
22mOhm
06
. 8
10
06
www.semtech.com
SC2616
6
13
6.98k
8.06k
1
6 .
.
6
dB
kHz
VOUT