ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 23

• Manufacturer Part Number: ltc3455euf-trpbf
• Description: Dual Dc/dc Converter With Usb Power Manager And Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Hot Swap Output
A current limited Hot Swap output is provided for power-
ing memory cards or other external devices that can be
hot-plugged into the system. Typically connected to the
3.3V supply, this output provides isolation to prevent the
external device from disturbing the 3.3V supply when
inserted. The Hot Swap output can only operate when the
LTC3455 is on, and is enabled using the HSON pin. If this
hot-plugging protection is not needed, this output can be
used as a load switch for other devices within the system.
The HSO pin is discharged to ground when the LTC3455
is shut down.
Gain Block
The LTC3455 contains a gain block (pins AI and AO) that
can be used as either a low-battery indicator, or as an LDO
with the addition of an external PNP. Both circuits are
shown in Figure 14. The LDO is convenient for applications
needing a third output (possibly a low current 2.5V or a
quiet 3V supply). The AO pin can sink around 1mA, which
typically limits the LDO current to 100mA or less (due to
the current gain of the PNP). An external PMOS can be
used for the LDO, but a much larger output capacitor is
needed to ensure stability at light loads.
The gain block is alive whenever switcher 1 is enabled, and
is turned off during shutdown to minimize battery drain.
This means that the low-battery detector will not report a
low-battery condition until the LTC3455 is turned on. This
is not a problem for most applications since the LTC3455
U
U
W
U
usually powers the microcontroller and all other intelli-
gence in the system.
PCB Layout Considerations
As with all DC/DC converters, careful attention must be
paid to the printed circuit board (PCB) layout and compo-
nent placement. The V
both inductors must all be placed as close as possible to
the LTC3455. These components, along with both DC/DC
converter output capacitors, should be placed on the same
side of the circuit board as the LTC3455, with their
connections made on that top layer. Place a local, unbro-
ken ground plane below these components that is tied to
the exposed pad of the LTC3455. The exposed pad (pin 25)
must be soldered to the PCB (to system ground) for proper
operation. Figure 15 shows the recommended placement
for the power sections of the LTC3455.
Figure 14. Low-Battery Detector and LDO Using the Gain Block
V
1.8V
LBO
BAT
1M
2.49M
806k
LOW-BATTERY
DECTECTOR
17
16
AO
AI
LTC3455
BAT
10µF
capacitor, V
3.3V
2.5V
169k
80.6k
MAX
LTC3455
LDO
100k
100pF
capacitor, and
17
16
AO
AI
LTC3455
23
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