Datasheet

LTC3520
18
3520fa
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
MANUFACTURER WEB SITE PART NUMBER
Taiyo Yuden www.t-yuden.com JMK212BJ226MG-T
22μF, 6.3V
TDK www.component.tdk.com C3216X5ROJ106KB
10μF, 6.3V
Sanyo www.secc.co.jp 6APD10M 10μF, 6.3V
Murata www.murata.com GRM21BR60J226ME39
22μF, 6.3V
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:
f
LC
Hz Buck Mode
f
FILTER POLE
OUT
FILTER PO
_
_
()=
1
2π
LLE
OUT OUT
VLC
Hz Boost Mode=
1
2π
()
where L is the inductance in henries and C
OUT
is the output
capacitance in farads. The output fi lter zero is given by:
f
RC
Hz
FILTER ZERO
ESR OUT
_
=
1
2π
where R
ESR
is the equivalent series resistance of the output
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:
f
V
ILV
Hz
RHPZ
IN
OUT OUT
=
2
2π
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:
f
RC
Hz
UG
P
=
1
21
1
π
Figure 6. Type I Compensation Network
APPLICATIONS INFORMATION
0.782V
R1
R2
3520 F06
FB1
18
V
C1
C
P1
V
OUT
15
–
+