ltc3566 Linear Technology Corporation, ltc3566 Datasheet - Page 24

• Manufacturer Part Number: ltc3566
• Description: High Ef?ciency Usb Power Manager Plus 1a Buck-boost Converter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
case. The size of the input overshoot will be determined
by the “Q” of the resonant tank circuit formed by C
the input lead inductance. It is recommended to measure
the input ringing with the selected components to verify
compliance with the Absolute Maximum specifi cations.
Alternatively, the following soft connect circuit (Figure 5)
can be employed. In this circuit, capacitor C1 holds MP1
off when the cable is fi rst connected. Eventually C1 begins
to charge up to the USB input voltage applying increasing
gate support to MP1. The long time constant of R1 and
C1 prevent the current from building up in the cable too
fast thus dampening out any resonant overshoot.
Buck-Boost Regulator Output Voltage Programming
The buck-boost regulator can be programmed for output
voltages greater than 2.75V and less than 5.5V. The output
voltage is programmed using a resistor divider from the
V
where V
Closing the Feedback Loop
The LTC3566 incorporates voltage mode PWM control. The
control to output gain varies with operation region (buck,
boost, buck-boost), but is usually no greater than 20. The
output fi lter exhibits a double pole response given by:
LTC3566
24
OUT1
V
f
FILTER _ POLE
OUT1
5V USB
INPUT
pin connected to the FB1 pin such that:
FB1
= V
USB CABLE
is fi xed at 0.8V (see Figure 6).
FB1
Figure 5. USB Soft Connect Circuit
⎛
⎝ ⎜
=
R
R1
FB
2 • π • L • C
+ 1
C1
100nF
⎞
⎠ ⎟
1
Si2333
MP1
OUT
R1
40k
Hz
C2
10μF
V
GND
LTC3566
BUS
3566 F05
IN
and
Where C
The output fi lter zero is given by:
where R
tance.
A troublesome feature in boost mode is the right-half plane
zero (RHP), and is given by:
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network (as shown in
Figure 6), can be incorporated to stabilize the loop but
at the cost of reduced bandwidth and slower transient
response. To ensure proper phase margin, the loop must
cross unity-gain a decade before the LC double pole.
The unity-gain frequency of the error amplifi er with the
Type I compensation is given by:
Most applications demand an improved transient response
to allow a smaller output fi lter capacitor. To achieve a higher
bandwidth, Type III compensation is required. Two zeros
are required to compensate for the double-pole response.
Type III compensation also reduces any V
seen at start-up.
The compensation network depicted in Figure 7 yields the
transfer function:
f
f
f
s • 1+
V
(
FILTER _ ZERO
RHPZ
UG
1+ sR2C2
V
OUT1
C1
⎛
⎝ ⎜
=
OUT
ESR
=
2 • π • R1• C
=
sR2C1C2
2 • π •I
C1+ C2
R1• C1+ C2
is the output fi lter capacitor.
is the capacitor equivalent series resis-
)
• 1+ s(R1+ R3)C3
(
1
(
=
1
OUT
2 • π • R
V
⎞
⎠ ⎟
IN
P1
• 1+ sR3C3
• L • V
2
(
Hz
)
•
ESR
1
OUT
• C
Hz
OUT
)
)
Hz
OUT1
overshoot
3566p