Datasheet

LTC3520
16
3520fa
output capacitors. Larger value output capacitors can
be accommodated provided they have suffi cient ESR to
stabilize the loop or by adding a feedforward capacitor in
parallel with the upper feedback resistor.
Table 2. Buck Output Capacitor Range
V
OUT
C
MIN
C
MAX
0.8V 30μF 100μF
1.2V 15μF 50μF
1.8V 10μF 30μF
2.7V 7μF 22μF
3.3V 6μF 20μF
Buck Input Capacitor Selection
The PV
IN2
pin provides current to the buck converter PMOS
power switch. It is recommended that a low ESR ceramic
capacitor with a value of at least 22μF be used to bypass
this pin. The capacitor should be placed as close to the
pin as possible and have a short return to ground.
Buck Output Voltage Programming
The buck converter output voltage is set by a resistive
divider according to the following formula:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
0 790 1
2
1
.
The external divider is connected to the output as shown
in Figure 4. A reasonable compromise between noise
immunity and quiescent current is provided by choosing
R2 = 249k. The required value for R1 can then be solved
via the formula above. It is recommended that a 27pF
Figure 4. Setting the Buck Output Voltage
feedforward capacitor be placed in parallel with R2 in
order to improve the transient response and reduce Burst
Mode ripple.
Buck-Boost Output Voltage Programming
The buck-boost output voltage is set by a resistive divider
according to the following formula:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
0 782 1
2
1
.
The external divider is connected to the output as shown in
Figure 5. In addition to setting the output voltage, the value
of R2 plays an integral role in compensation of the buck-
boost control loop. For more details, see the Closing the
Buck-Boost Feedback Loop section of this datasheet.
Buck-Boost Inductor Selection
To achieve high effi ciency, a low ESR inductor should be
utilized for the buck-boost converter. The inductor must
have a saturation rating greater than the worst case average
inductor current plus half the ripple current. The peak-to-
peak inductor current ripple will be larger in buck and boost
mode than in the buck-boost region. The peak-to-peak
inductor current ripple for each mode can be calculated
from the following formulas, where f is the frequency in
MHz and L is the inductance in μH:
Δ
Δ
I
f
VVV
V
I
LP PBUCK
L
OUT IN OUT
IN
LP PBO
,,
,,
()
−
−
=
−
1
OOST
L
IN OUT IN
OUT
f
VV V
V
=
−
1
()
Figure 5. Setting the Buck-Boost Output Voltage
APPLICATIONS INFORMATION
FB2
0.8V ≤ V
OUT
≤ 5.25V
R2
R1
3520 F04
GND
LTC3520
27pF
FB1
2.2V ≤ V
OUT
≤ 5.25V
R2
R1
3520 F05
GND
LTC3520