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LTC3522

3522f

Table 2. Buck Output Capacitor Range

VOUT

CMIN

CMAX

0.6V

15μF

300μF

0.8V

15μF

230μF

1.2V

10μF

150μF

1.8V

6.8μF

90μF

2.7V

6.8μF

70μF

3.3V

6.8μF

50μF

Buck Input Capacitor Selection

The PVIN2 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:

OUT =+

⎛

⎝⎜

⎞

⎠⎟

0 594

The external divider is connected to the output as shown in

Figure 3. It is recommended that a feedforward capacitor,

CFF,beplacedinparallelwithresistorR2inordertoimprove

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

VOUT

CFF

0.6V

–

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:

OUT =+

⎛

⎝⎜

⎞

⎠⎟

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

≤ VOUT ≤ 5.25V

FB2

3522 F03

CFF

Figure 3. Setting the Buck Output Voltage

LTC3522

GND

2.2V

≤ VOUT ≤ 5.25V

FB1

3522 F04

Figure 4. Setting the Buck-Boost Output Voltage

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