LTC3456_15 Datasheet, PDF (22/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

LTC3456
APPLICATIO S I FOR ATIO
AC ADAPTER UVLO VOLTAGE PROGRAMMING
The AC wall adapter UVLO voltage can be set by a resistor
divider connected across the AC wall adapter as shown in
Figure 15.
The AC wall adapter UVLO voltage can be set by a resistor
divider according to the formula:
V_ADAPTER(MIN)
=
1.25V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
Choose 1% resistors for better accuracy. When the WALLFB
pin voltage is higher than 1.25V, the LTC3456 will be
powered from the V_EXT pin. The internal USB power switch
is turned off and the power is derived from the AC adapter
through the diode D1. Ensure that the AC adapter UVLO
voltage is set high enough to make V_EXT > 4V during
regular operation.
USB
POWER
AC
ADAPTER
D1
R2
R1
LTC3456
USB
1Ω
4.7µF
EN
V_EXT
10µF
WALLFB
3456 F14
A 4.32k or smaller resistor should be chosen for R1 to
prevent this behavior. When the power is being delivered
from the wall adapter, efficiency is not a big concern and
choosing small value R1 and R2 resistors should be
acceptable.
Connect the WALLFB pin to ground if the wall adapter is
not used.
Rectifier Diode Selection
The diode, D1, shown in Figures 15 and 16 is used to
connect the V_EXT pin to the AC adapter input. The IC is
powered through the diode D1 when running off the AC
adapter. A Schottky diode is recommended to minimize
the voltage drop from the AC adapter to the V_EXT pin.
V_EXT(MIN) = V_ADAPTER(MIN) – V_DIODE(MAX)
Always ensure that V_EXT > 4V during regular operation.
Choose a diode with a current rating high enough to handle
the input current.
Choose a Schottky diode with low reverse leakage current
(as explained in previous section). ON Semiconductor
MBRM120E (20V/1A) is a good choice for a low leakage
Schottky rectifier.
The Zetex ZLLS400 (40V/0.5A) Schottky diode is available
in a small surface mount package and is also a good fit for
this application.
Figure 15. Setting the Wall Adapter UVLO Voltage
When the device is powered from the USB input (AC
adapter is not present), the V_EXT pin is charged close to the
USB voltage. The reverse leakage current of the diode D1
flows through R1 and R2 as shown in Figure 16. If the
leakage current is large enough to pull the WALLFB pin
above 1.25V, then the USB power is switched off. The
LTC3456 will then enter into a hiccup mode with USB
power being turned on and off in a periodic fashion.
USB
POWER
AC ADAPTER
NOT PRESENT
D1
R2 ILKG
R1
LTC3456
USB
1Ω
4.7µF
EN
10µF ^VEXT
WALLFB
3456 F15
Figure 16. Diode D1 Leakage Current Flow in USB Powered Mode
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