LTC3455 Datasheet, PDF (21/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
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 13. 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
LOW-BATTERY
DECTECTOR
1.8V
3.3V
1M
17
LBO
AO
V_BAT
LTC3455
2.49M
16
AI
806k
2.5V
10µF
LDO
100k
17
AO
169k
80.6k
100pF _LTC3455
16
AI
3455 F14
Figure 13. Low-Battery Detector and LDO Using the Gain Block
is not a problem for most applications since the LTC3455
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_BAT capacitor, V_MAX capacitor, and
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.
TYPICAL APPLICATIO S
Standalone USB Power Supply
with Temporary Backup Power
Although designed primarily for Li-Ion powered portable
applications, the LTC3455 is also a good choice for
systems that are always powered by a USB supply or wall
adapter. The battery charger can then be used to charge up
a large capacitor or backup battery, which briefly provides
power to the system after the external power has been
removed. This gives the microcontroller enough time to
follow proper shutdown procedures when the main power
source is abruptly removed. Figure 14 shows a standalone
power supply for USB high power applications (500mA
maximum USB current) using the LTC3455. The total
system power should be kept below 1.8W to ensure clean
operation even under worst-case USB conditions. With
the resistor values shown, the low-battery indicator (AI
and AO pins) triggers when the V_MAX pin voltage drops to
4V, notifying the microcontroller of an impending dropout
condition. The 1MΩ resistor connected between the AI
3455f
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