LTC3520 Datasheet, PDF (18/24 Pages) Linear Technology – Synchronous 1A Buck-Boost and 600mA Buck Converters

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 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 filter exhibits a double
pole response as given by the following equations:
f _{FILTER_POLE} = _2π
1
Hz(Buck Mode)
LC_OUT
where L is the inductance in henries and C_OUT is the output
capacitance in farads. The output filter zero is given by:
f _{FILTER_ ZERO}
=
1
2πR_ESR C_OUT
Hz
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 _RHPZ
=
V_IN2
2πI_OUT LV_OUT
Hz
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 sufficient
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 amplifier
with Type I compensation is given by:
f _UG
=
1
2πR1C_P1
Hz
f _{FILTER_POLE}
=
1
2π V_OUT
LC_OUT
Hz(Boost Mode)
0.782V
FB1
18
V_C1
C_P1
15
V_OUT
R1
R2
3520 F06
Figure 6. Type I Compensation Network
3520f
18