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LTC3456

3456fa

Input Capacitor Selection

The LTC3456 can be powered from three different power

sources: battery, USB or the AC wall adapter. Choose a

4.7

µF or higher X5R or X7R type ceramic capacitor for

bypassing the input of LTC3456.

However, special care must be taken when bypassing the

USB and AC wall adapter inputs with ceramic capacitors.

Ceramic capacitors with their low ESR can form a resonant

tank circuit with the stray wiring inductance of the power

leads. This can cause large voltage transients at the input

of the device when the power is applied quickly (for

example, plugging the AC adapter output into the portable

device). This voltage spike can be large enough to damage

the LTC3456. A possible solution is to clamp the input

voltage or insert a small resistor in series with the ceramic

capacitor as shown in Figure 12. Please refer to Linear

Technology Application Note AN88 for more details.

APPLICATIO S I FOR ATIO

Figure 12. Bypassing USB and AC Wall Adapter Inputs

ADAPTER

USB

INPUT

Ω

4.7

µF

Ω

4.7

µF

3456 F12

USB

LTC3456

VEXT

Figure 13(a) shows the voltage waveforms at the USB and

VEXT pins resulting from hot-plugging a 5.5V input supply.

As seen in the figure, there is a large voltage transient (in

excess of 8V) at the USB input pin. This voltage spike

exceeds the 6V absolute maximum voltage rating of the

pin, and can cause serious performance degradation or,

even complete failure of the part. The spike can be greatly

reduced by adding a 1

Ω series resisitor with the 4.7µF

ceramic capacitor, as seen in Figure 13(b). The voltage

ringing at the USB pin is completely removed and the

maximum voltage spike at the USB pin is less than the

maximum voltage rating of 6V.

Figure 13(a). Hot-Plugging the USB Power (5.5V Input) with

a 4.7

µF Ceramic Capacitor Used for Bypassing.

Figure 13(b). Hot-Plugging the USB Power (5.5V Input) with a

4.7

µF Ceramic Capacitor and 1Ω Series Resistor Used for

Bypassing

Output Voltage Programming

The output of the core converter can be set by a resistor

divider according to the formula:

CORE =+

⎛

⎝⎜

⎞

⎠⎟

The external resistor divider is connected at the output as

shown in Figure 14. Choose 1% resistors for better

accuracy. R1 should be 80.6k or smaller for better noise

immunity.

Figure 14. Setting the Core Output Voltage

0.8V

≤ VCORE ≤ VBATT(MIN)

3456 F13

FB1

LTC3456

AGND

VUSB

(2V/DIV)

VEXT

(2V/DIV)

0.1mS/DIV

VUSB

(2V/DIV)

VEXT

(2V/DIV)

0.1mS/DIV

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