ltc3521fe Linear Technology Corporation, ltc3521fe Datasheet - Page 14

• Manufacturer Part Number: ltc3521fe
• Description: Ltc3521 Wide Vin, 1a Buck-boost Dc/dc And Dual 600ma Buck Dc/dc Converters Features
• Manufacturer: Linear Technology Corporation

LTC3521
applicaTions inForMaTion
Buck Input Capacitor Selection
The PV
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:
The external divider is connected to the output, as shown in
Figure 3. It is recommended that a feedforward capacitor,
C
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

FF
V
, be placed in parallel with resistor R2 to improve the
OUT2 3
V
0.6V
0.8V
1.0V
1.2V
1.5V
1.8V
2.7V
3.3V
OUT
IN2
,
pin provides current to the buck converter power
Figure 3. Setting the Buck Output Voltage
=
0.6V V
0 6
.
V
OUT2

 
78.7k
68.1k
63.4k
60.4k
200k
118k
100k
1
R1
R2
R1
–
+
5.25V
R
R
FB2
2
1
LTC3521

 
GND
0.6V V
FB3
69.8k
80.6k
102k
121k
137k
226k
274k
R2
0
OUT3
3521 F03
R2
R1
5.25V
22pF
22pF
22pF
22pF
22pF
33pF
33pF
C
–
FF
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 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
V
OUT1
Figure 4. Setting the Buck-Boost Output Voltage
=
0 6
.
V

 
1
+
R
R
LTC3521
2
1
GND

 
1.8V V
FB1
OUT1
3521 F04
R2
R1
5.25V
3521f