LTC3520 Linear Technology, LTC3520 Datasheet - Page 16

• Manufacturer Part Number: LTC3520
• Description: Synchronous 1A Buck-Boost and 600mA Buck Converters
• Manufacturer: Linear Technology

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LTC3520
APPLICATIONS INFORMATION
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
Buck Input Capacitor Selection
The PV
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:
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
16
V
OUT
IN2
V
0.8V
1.2V
1.8V
2.7V
3.3V
OUT
=
pin provides current to the buck converter PMOS
Figure 4. Setting the Buck Output Voltage
0 790
.
V
LTC3520
⎛
⎝ ⎜
GND
1
+
FB2
R
R
30µF
15µF
10µF
C
2
1
0.8V ≤ V
7µF
6µF
MIN
⎞
⎠ ⎟
3520 F04
OUT
R2
R1
≤ 5.25V
27pF
100µF
C
50µF
30µF
22µF
20µF
MAX
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:
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:
V
∆
∆
OUT
I
I
L P P BUCK
L P P BO
,
,
Figure 5. Setting the Buck-Boost Output Voltage
−
−
=
,
,
0 782
.
O O ST
=
V
=
LTC3520
fL
⎛
⎝ ⎜
GND
1
fL
1
1
+
V
FB1
V
OUT
R
R
IN
2.2V ≤ V
2
1
(
⎞
⎠ ⎟
(
V
V
3520 F05
V
OUT
IN
V
OUT
OUT
IN
−
R2
R1
≤ 5.25V
−
V
OUT
V
IN
)
)
3520f