LTC3521 Datasheet, PDF (14/20 Pages) Linear Technology – Wide VIN, 1A Buck-Boost DC/DC and Dual 600mA Buck DC/DC Converters

LTC3521
applications information
Buck Input Capacitor Selection
The PV_IN2 pin provides current to the buck converter power
switch and is the supply pin for the IC’s internal circuitry.
It is recommended that a low ESR ceramic capacitor with
a value of at least 4.7µF 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 output voltage is set by a resistive divider, according
to the following formula:
V_OUT2,3
=
0.6V ^{}1+
R2
R1
The external divider is connected to the output, as shown in
Figure 3. It is recommended that a feedforward capacitor,
C_FF , be placed in parallel with resistor R2 to improve the
noise immunity of the feedback node. Table 3 provides
the recommended resistor and feedforward capacitor
combinations for common output voltage options.
Table 3. Buck Resistor Divider Values
V_OUT
R1
R2
C_FF
0.6V
–
0
–
0.8V
200k
69.8k
22pF
1.0V
118k
80.6k
22pF
1.2V
100k
102k
22pF
1.5V
78.7k
121k
22pF
1.8V
68.1k
137k
22pF
2.7V
63.4k
226k
33pF
3.3V
60.4k
274k
33pF
Buck-Boost Output Voltage Programming
The buck-boost output voltage is set by a resistive divider
according to the following formula:
V_OUT1
=
0.6V ^{}1+
R2
R1
The external divider is connected to the output, as shown
in Figure 4. The buck-boost converter utilizes voltage
mode control and the value of R2 plays an integral role
in the dynamics of the feedback loop. In general, a larger
value for R2 will increase stability and reduce the speed of
the transient response. A smaller value of R2 will reduce
stability but increase the transient response speed. A good
starting point is to choose R2 = 1MΩ, then calculate the
required value of R1 to set the desired output voltage ac-
cording to the above formula. If a large output capacitor
is used, the bandwidth of the converter is reduced. In
such cases R2 can be reduced to improve the transient
response. If a large inductor or small output capacitor is
utilized, the loop will be less stable and the phase margin
can be improved by increasing the value of R2.
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
0.6V b V_OUT2 b 5.25V
0.6V b V_OUT3 b 5.25V
R2
FB2
FB3
R1
LTC3521
GND
R2
R1
3521 F03
Figure 3. Setting the Buck Output Voltage
1.8V b V_OUT1 b 5.25V
FB1
LTC3521
GND
R2
R1
3521 F04
Figure 4. Setting the Buck-Boost Output Voltage
3521f
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