LTC3520 Datasheet, PDF (19/24 Pages) Linear Technology – Synchronous 1A Buck-Boost and 600mA Buck Converters

LTC3520
APPLICATIONS INFORMATION
Most applications require a faster transient response than
can be attained using Type I compensation in order to reduce
the size of the output capacitor. To achieve a higher loop
bandwidth, Type III compensation is required, providing
two zeros to compensate for the double pole response of
the output filter. Referring to Figure 7, the location of the
compensation poles and zeros are given as follows:
f_POLE1
≅
1
2π (3 2 0 0 0)R1C_P1
Hz
≅
0Hz
1
f_ZERO1 = _2πRZCP1 Hz
f_ZERO2
=
1
2πR1C_Z1
Hz
f_POLE2
=
1
2πR _Z C_P2
Hz
where all resistances are in ohms and all capacitances
are in farads.
V_OUT
0.782V
FB1
18
V_C1
R_Z
C_P1
15
C_P2
R1
C_Z1
R2
3520 F07
Figure 7. Type III Compensation Network
PCB Layout Considerations
The LTC3520 switches large currents at high frequencies.
Special care should be given to the PCB layout to ensure
stable, noise-free operation. Figure 8 depicts the recom-
mended PCB layout to be utilized for the LTC3520. A few
key guidelines follow:
1. All circulating current paths should be kept as short as
possible. This can be accomplished by keeping the routes
to all bold components in Figure 8 as short and as wide
as possible. Capacitor ground connections should via
down to the ground plane by the shortest route possible.
The bypass capacitors on PV_IN1, PV_IN2, and PV_IN3 should
be placed as close to the IC as possible and should have
the shortest possible paths to ground.
2. The small signal ground pad (SGND) should have a
single-point connection to the power ground. A con-
venient way to achieve this is to short the pin directly
to the Exposed Pad as shown in Figure 8.
3. The components shown in bold and their connections
should all be placed over a complete ground plane to
reduce the cross-sectional area of circulating current
paths.
4. To prevent large circulating currents from disrupting
the output voltage sensing, the ground for each resistor
divider should be returned directly to the small signal
ground pin (SGND).
5. Use of vias in the die attach pad will enhance the ther-
mal environment of the converter especially if the vias
extend to a ground plane region on the exposed bottom
surface of the PCB.
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