LTC3520 Linear Technology, LTC3520 Datasheet - Page 18

• Manufacturer Part Number: LTC3520
• Description: Synchronous 1A Buck-Boost and 600mA Buck Converters
• Manufacturer: Linear Technology

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LTC3520
APPLICATIONS INFORMATION
Capacitor Vendor Information
Both the input and output capacitors used with the LTC3520
must be low ESR and designed to handle the large AC cur-
rents generated by switching converters. The vendors in
Table 4 provide capacitors that are well suited to LTC3520
application circuits.
Table 4. Capacitor Vendor Information
MANU-
FACTURER
Taiyo Yuden www.t-yuden.com
TDK
Sanyo
Murata
Closing the Buck-Boost Feedback Loop
The LTC3520 buck-boost converter employs voltage mode
PWM control. The control to output gain varies with opera-
tional region (buck, boost, or buck-boost), but is usually
no greater than 24dB. The output fi lter exhibits a double
pole response as given by the following equations:
18
f
f
FILTER POLE
FILTER PO
_
_
WEB SITE
www.component.tdk.com C3216X5ROJ106KB
www.secc.co.jp
www.murata.com
L L E
=
=
2π
2π
V
OUT
LC
1
OUT
1
LC
Hz Buck Mode
OUT
PART NUMBER
JMK212BJ226MG-T
22µF, 6.3V
10µF, 6.3V
6APD10M 10µF, 6.3V
GRM21BR60J226ME39
22µF, 6.3V
(
Hz Boost Mode
(
Figure 6. Type I Compensation Network
)
+
–
)
FB1
V
18
15
C1
where L is the inductance in henries and C
capacitance in farads. The output fi lter zero is given by:
where R
capacitor. A challenging aspect of the loop dynamics in
boost mode is the presence of a right half plane zero at
the frequency given by:
The loop gain is typically rolled off to below unity gain
before the worst case right half plane zero frequency.
A simple Type I compensation network as shown in
Figure 6 can be utilized to stabilize the buck-boost
converter. However, this will yield a relatively low band-
width and slow transient response. To ensure suffi cient
phase margin using Type I compensation, the loop must
be crossed over a decade before the LC double pole fre-
quency. The unity-gain frequency of the error amplifi er
with Type I compensation is given by:
0.782V
f
f
f
C
FILTER ZERO
RHPZ
UG
P1
=
ESR
2
=
V
_
π
OUT
is the equivalent series resistance of the output
2π
R C
R1
R2
3520 F06
1
1
I
OUT
P
V
=
1
IN
Hz
2π
LV
2
R
OUT
ESR OUT
1
Hz
C
Hz
OUT
is the output
3520f