LTC3566-2_15 Datasheet, PDF (25/28 Pages) Linear Technology – High Efficiency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566/LTC3566-2
APPLICATIONS INFORMATION
A Type III compensation network attempts to introduce
a phase bump at a higher frequency than the LC double
pole. This allows the system to cross unity gain after the
LC double pole, and achieve a higher bandwidth. While
attempting to cross over after the LC double pole, the
system must still cross over before the boost right-half
plane zero. If unity gain is not reached sufficiently before
the right-half plane zero, then the –180° of phase lag from
the LC double pole combined with the –90° of phase lag
from the right-half plane zero will result in negating the
phase bump of the compensator.
The compensator zeros should be placed either before
or only slightly after the LC double pole such that their
positive phase contributions offset the –180° that occurs
at the filter double pole. If they are placed at too low of a
frequency, they will introduce too much gain to the system
and the crossover frequency will be too high. The two high
frequency poles should be placed such that the system
crosses unity gain during the phase bump introduced by
the zeros and before the boost right-half plane zero and
such that the compensator bandwidth is less than the
bandwidth of the error amp (typically 900 kHz). If the gain
of the compensation network is ever greater than the gain
of the error amplifier, then the error amplifier no longer
acts as an ideal op amp, another pole will be introduced
and at the same point.
Recommended Type III compensation components for a
3.3V output:
R1: 324kΩ
R_FB: 105kΩ
C1: 10pF
R2: 15kΩ
C2: 330pF
R3: 121kΩ
C3: 33pF
C_OUT: 22μF
L_OUT: 2.2μH
Printed Circuit Board Layout Considerations
In order to be able to deliver maximum current under all
conditions, it is critical that the Exposed Pad on the back-
side of the LTC3566 family package be soldered to the PC
board ground. Failure to make thermal contact between
the Exposed Pad on the backside of the package and the
copper board will result in higher thermal resistances.
Furthermore, due to its high frequency switching circuitry,
it is imperative that the input capacitors, inductors, and
output capacitors be as close to the LTC3566 family as
ERROR
AMP
0.8V
FB1
V_OUT1
R1
V_C1 ^CP1
R_FB
3566 F06
Figure 6. Error Amplifier with Type I Compensation
ERROR
AMP
0.8V
FB1
V_OUT1
R3
R1
C3
V_C1
C2
R2
R_FB
C1
3566 F07
Figure 7. Error Amplifier with Type III Compensation
3566fb
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