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LTC3586-2 Datasheet, PDF (31/36 Pages) – | |||
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LTC3586-2/LTC3586-3
APPLICATIONS INFORMATION
Table 7. Recommended Inductors for Buck-Boost Regulator
MAX MAX
INDUCTOR L IDC DCR
TYPE
(µH) (A) (Ω)
SIZE IN mm
(L Ã W Ã H) MANUFACTURER
LPS4018 3.3 2.2 0.08 3.9 Ã 3.9 Ã 1.7 Coilcraft
2.2 2.5 0.07 3.9 Ã 3.9 Ã 1.7 www.coilcraft.com
D53LC
2.0 3.25 0.02 5.0 Ã 5.0 Ã 3.0 Toko
www.toko.com
7440430022 2.2 2.5 0.028 4.8 à 4.8 à 2.8 Würth-Elektronik
www.we-online.com
CDRH4D22/ 2.2 2.4 0.044 4.7 Ã 4.7 Ã 2.4 Sumida
HP
www.sumida.com
SD14
2.0 2.56 0.045 5.2 Ã 5.2 Ã Cooper
1.45
www.cooperet.com
Buck-Boost Regulator Input/Output Capacitor
Selection
Low ESR ceramic capacitors should be used at both the
buck-boost regulator output (VOUT3) as well as the buck-
boost regulator input supply (VIN3). Again, only X5R or
X7R ceramic capacitors should be used because they
retain their capacitance over wider voltage and temperature
ranges than other ceramic types. A 22µF output capacitor is
sufficient for most applications. The buck-boost regulator
input supply should be bypassed with a 2.2µF capacitor.
Refer to Table 6 for recommended ceramic capacitor
manufacturers.
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 full
scale output voltage is programmed using a resistor divider
from the VOUT3 pin connected to the FB3 pin such that:
VOUT3
=
VFB3

ï£ï£¬
R1
R2
+
1
where VFB3 is 0.8V. See Figure 8 or 9.
Closing the Feedback Loop
The LTC3586-2/LTC3586-3 incorporate 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 filter exhibits a double pole response
given by:
fFILTER _POLE = 2 â¢ Ï â¢
1
L ⢠COUT
Hz
where COUT is the output filter capacitor.
The output filter zero is given by:
fFILTER _ ZERO
=
2
â¢
Ï
1
⢠RESR
â¢
COUT
Hz
where RESR is the capacitor equivalent series resistance.
A troublesome feature in boost mode is the right-half plane
zero (RHP), and is given by:
fRHPZ
=
VIN2
2 â¢ Ï â¢ IOUT ⢠L ⢠VOUT
Hz
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network (as shown in
Figureâ¯8) 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 decade before the LC double pole.
The unity-gain frequency of the error amplifier with the
Type I compensation is given by:
fUG
=
2
â¢
Ï
â¢
1
R1â¢
CP1
Hz
Most applications demand an improved transient response
to allow a smaller output filter 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 VOUT3 overshoot
seen during a start-up condition.
358623f
31
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