SC2612C Semtech, SC2612C Datasheet - Page 6

• Manufacturer Part Number: SC2612C
• Description: 600kHz/200kHz Step-Down DC/DC Converter
• Manufacturer: Semtech

COMPENSA
COMPENSA
COMPENSA
COMPENSA
COMPENSATION COMPONENTS
ponents have been determined, the compensation com-
ponents 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
G
G
G
POWER MANAGEMENT
MOD
DIV
FMD
2003 Semtech Corp.
Zs
=
=
=
FB
20
19
19
REF
×
+
+
Log
Zf
20
20
+
-
TION COMPONENTS
TION COMPONENTS
TION COMPONENTS
TION COMPONENTS - Once the filter com-
×
æ
ç ç
è
EA
×
Log
Log
R
SC2612 AND FETS
5
R
Zp
æ
ç
è
+
æ
ç
è
COMP
8
V
V
R
5
5
IN
IN
8
ö
÷
ø
ö
÷
ø
ö
÷ ÷
ø
+
MODULATOR
20
×
Log
æ
ç ç
è
R
OUT
A
R
+
B
R
L
B
ö
÷ ÷
ø
Resr
Co
Ra
Rb
VOUT
6
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 Fig2
This is point B in Fig2.
This is point C in Fig2., the line joining B-C slopes at -
40dB/decade, the line joining C-D slopes at -20dB/de-
cade.
For 600kHz switching frequency, crossover is designed
for 100kHz.
Since Fz(esr)<<Fco/4 Type 2 compensation is appropri-
ate.
Fp
Fz
Fp
Fz
G
FMD
(
(
(
lc (
esr
lc
esr
)
)
=
FB
=
=
)
)
19
=
=
2
2
REF
p
p
+
2
2
× p
× p
20
1
1
LCo
LCo
+
-
Co
3000
×
EA
Log
1
SC2612 AND FETS
Cs
Rs
×
=
Re
×
æ
ç
è
10
COMP
2
5
5
1
sr
p
ö
÷
ø
-
6
+
3
×
Cp
20
22
.
MODULATOR
3
×
×
×
10
10
Log
-
-
1
6
3
æ
ç
è
×
3000
=
6
.
2
98
OUT
www.DataSheet4U.com
.
8
4
.
kHz
+
×
06
SC2612A/C
10
Vin=5V
8
3.3uH
.
-
06
6
3000uF
22mOhm
»
ö
= ÷
ø
1
www.semtech.com
.
13
6
kHz
.
6
dB
6.98k
8.06k
VOUT