Datasheet

LTC3522
14
3522fa
Table 2. Buck Output Capacitor Range
V
OUT
C
MIN
C
MAX
0.6V 15F 300F
0.8V 15F 230F
1.2V 10F 150F
1.8V 6.8F 90F
2.7V 6.8F 70F
3.3V 6.8F 50F
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:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
0 594 1
2
1
.
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:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
11
2
1
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
APPLICATIONS INFORMATION
LTC3522
GND
0.6V ≤ V
OUT
≤ 5.25V
FB2
R1
3522 F03
R2 C
FF
Figure 3. Setting the Buck Output Voltage
LTC3522
GND
2.2V ≤ V
OUT
≤ 5.25V
FB1
R1
3522 F04
R2
Figure 4. Setting the Buck-Boost Output Voltage