LTC3444_15 Datasheet, PDF (17/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

U
OPERATIO
However, due to the output voltage slewing requirements
found in WCDMA applications the output filter capacitor
must be minimized. To maximize the transient response,
while minimizing the output capacitance, a higher band-
width, Type III compensation is required. A Type III
compensation network, refer to Figure 8, has a double zero
to cancel the double pole of the output LC filter and a
double pole to compensate for the ESR zero and RHP zero
of the boost topology. In addition to the double poles,
the Type III network also has a single pole at DC. The
Type III compensation provides a maximum 135° phase
boost and allows the loop crossover to occur at frequen-
cies higher than the output LC. Refer to Figure 9. Referring
to Figure 8, the location of the poles and zeros are given by:
Assume C₂ >> C₃, R₁ >> R₄.
f
POLE1
≅
2•
π
1
• R5 •
Hz
C3
f _POLE2
=
2•
1
Hz
π • R4 • C1
f
ZERO1
=
2•
π
1
• R1•
Hz
C1
f _ZERO2
=
2•
1
Hz
π • R5 • C2
And the unity gain frequency (f_UG) of the Type III compen-
sation is given by:
f _UG
=
2•
π
1
Hz
•R1• C2
where resistance is in ohms and capacitance is in farads.
Note: Bias resistor, R2, does not affect the Pole/Zero
placement.
LTC3444
7 ^VC
C3
C2
R5
C1
R4
FB
R1
8
V_REF
R2
V_OUT
3444 F08
Figure 8, Error Amplifier with Type III Compensation
80
360
60
270
40
180
20
90
f_UO
0
0
–20
–90
–40
–180
–60
–270
–80
1^e
1^e1 1^e2 1^e3 1^e4 1^e5 1^e6
FREQUENCY (Hz)
–360
1^e7 1^e8
3444 G02
Figure 9. Frequency Response for LTC3444 Error
Amplifier with a Typical Type III Compensation Network
3444fb
17