Datasheet

LTC3521
15
3521fb
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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 capaci-
tor, 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
applicaTions inForMaTion
LTC3521
GND
0.6V ≤ V
OUT3
≤ 5.25V0.6V ≤ V
OUT2
≤ 5.25V
FB3
R1
3521 F03
R2
FB2
R1
R2
Figure 3. Setting the Buck Output Voltage
LTC3521
GND
1.8V ≤ V
OUT1
≤ 5.25V
FB1
R1
3521 F04
R2
Figure 4. Setting the Buck-Boost Output Voltage