LTC3456 Datasheet, PDF (21/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

LTC3456
APPLICATIO S I FOR ATIO
Input Capacitor Selection
The LTC3456 can be powered from three different power
sources: battery, USB or the AC wall adapter. Choose a
4.7µF 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.
V_USB
(2V/DIV)
V_EXT
(2V/DIV)
0.1mS/DIV
Figure 13(a). Hot-Plugging the USB Power (5.5V Input) with
a 4.7µF Ceramic Capacitor Used for Bypassing.
V_USB
(2V/DIV)
USB
INPUT
AC
ADAPTER
1Ω
4.7µF
USB
1Ω
4.7µF ^LTC3456
10µF
V_EXT
3456 F12
Figure 12. Bypassing USB and AC Wall Adapter Inputs
Figure 13(a) shows the voltage waveforms at the USB and
V_EXT 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 1Ω 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.
V_EXT
(2V/DIV)
0.1mS/DIV
Figure 13(b). Hot-Plugging the USB Power (5.5V Input) with a
4.7µF 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:
V_CORE
=
0.8V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
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.
0.8V ≤ V_CORE ≤ V_BATT(MIN)
R2
FB1
LTC3456
R1
AGND
3456 F13
Figure 14. Setting the Core Output Voltage
3456fa
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