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參數(shù)資料
| 型號(hào)： | LTC3456 |
| 廠商： | Linear Technology Corporation |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | 2-Cell, Multi-Output DC/DC Converter with USB Power Manager |
| 中文描述： | 2節(jié)電池，多輸出DC / DC轉(zhuǎn)換器與USB電源管理器 |
| 文件頁(yè)數(shù)： | 22/28頁(yè) |
| 文件大小： | 358K |
| 代理商： | LTC3456 |
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LTC3456
3456f
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 15. 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.
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 14 and 15 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)
APPLICATIU
W
U
U
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.
BOARD LAYOUT CONSIDERATION
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To prevent electromagnetic interference (EMI) problems,
proper layout of high frequency switching paths is essen-
tial. Minimize the length and area of all traces connected to
the switching node pins (SW1, SW2_BK, SW2_BST).
Keep the feedback pins FB1 and AIN away from the
switching nodes. The power traces shown as bold lines in
Figure 16 should be kept short, direct and wide.
Figure 15. Diode D1 Leakage Current Flow in USB Powered Mode
USB
POWER
AC ADAPTER
NOT PRESENT
R2
1
I
LKG
D1
R1
4.7
μ
F
10
μ
F
3456 F15
USB
WALLFB
EN
LTC3456
V
EXT
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