LTC3522_15 Datasheet, PDF (14/20 Pages) Linear Technology – Synchronous 400mA Buck-Boost and 200mA Buck Converters

LTC3522
APPLICATIONS INFORMATION
Table 2. Buck Output Capacitor Range
V_OUT
0.6V
C_MIN
15μF
0.8V
15μF
1.2V
10μF
1.8V
6.8μF
2.7V
6.8μF
3.3V
6.8μF
C_MAX
300μF
230μF
150μF
90μF
70μF
50μF
Buck Input Capacitor Selection
The PV_IN2 pin provides current to the buck converter
power switch and is also the supply pin for the IC’s inter-
nal 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_OUT
=
0.594V ^{⎛⎝⎜}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 in order 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
12pF
1.0V
118k
80.6k
12pF
1.2V
100k
102k
12pF
1.5V
78.7k
121k
12pF
1.8V
68.1k
137k
12pF
2.7V
63.4k
226k
18pF
3.3V
60.4k
274k
20pF
Buck-Boost Output Voltage Programming
The buck-boost output voltage is set by a resistive divider
according to the following formula:
V_OUT
=
1V
⎛⎝⎜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 = 1M, and then calculate
the required value of R1 to set the desired output voltage
according to the formula given above. 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
0.6V ≤ V_OUT ≤ 5.25V
FB2
LTC3522
GND
R2
C_FF
R1
3522 F03
2.2V ≤ V_OUT ≤ 5.25V
FB1
LTC3522
GND
R2
R1
3522 F04
Figure 3. Setting the Buck Output Voltage
Figure 4. Setting the Buck-Boost Output Voltage
3522fa
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