LTC3520 Datasheet, PDF (16/24 Pages) Linear Technology – Synchronous 1A Buck-Boost and 600mA Buck Converters

LTC3520
APPLICATIONS INFORMATION
output capacitors. Larger value output capacitors can
be accommodated provided they have sufficient 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
0.8V
30µF
1.2V
15µF
1.8V
10µF
2.7V
7µF
3.3V
6µF
C_MAX
100µF
50µF
30µF
22µF
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:
V_OUT
=
0.790V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
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
0.8V ≤ V_OUT ≤ 5.25V
LTC3520
FB2
GND
R2
R1
3520 F04
27pF
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:
V_OUT
=
0.782V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
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 efficiency, 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:
∆I_{L,P−P,BUCK}
=
1
fL
V_OUT (V_IN −
V_IN
V_OUT )
∆I_{L,P−P,BOOST}
=
1
fL
V_IN (V_OUT −
V_OUT
V_IN )
2.2V ≤ V_OUT ≤ 5.25V
LTC3520
R2
FB1
R1
GND
3520 F05
Figure 4. Setting the Buck Output Voltage
Figure 5. Setting the Buck-Boost Output Voltage
3520f
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