LTC3455/LTC3455-1
23
3455fc
APPLICATIONS INFORMATION
pin to GND. Since the charge cycle is terminated only by
the charge timer, external charge termination is required
when using either of these methods. Use an external NMOS
to fl oat the PROG pin and disable charging.
Constant-current-only mode is a good choice for systems
that are always powered by a USB supply or wall adapter,
and the charger can be used to charge a super-cap or
backup battery. Disabling the voltage amplifi er allows the
super-cap/backup battery to charge up fully to the available
USB or wall adapter voltage.
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/LTC3455-1 are 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/LTC3455-1 are shut down.
Gain Block
The LTC3455/LTC3455-1 contain 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.
Figure 14. Low-Battery Detector and LDO Using the Gain Block
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/LTC3455-1 are
turned on. This is not a problem for most applications since
the LTC3455/LTC3455-1 usually power the microcontroller
and all other intelligence 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 component
placement. The V
BAT
capacitor, V
MAX
capacitor, and both
inductors must all be placed as close as possible to the
LTC3455/LTC3455-1. These components, along with both
DC/DC converter output capacitors, should be placed on the
same side of the circuit board as the LTC3455/LTC3455-1,
with their connections made on that top layer. Place a local,
unbroken ground plane below these components that is
tied to the Exposed Pad of the LTC3455/LTC3455-1. 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/LTC3455-1.
1.8V
LBO
17
1M
LOW-BATTERY
DECTECTOR
V
BAT
16
2.49M
806k
3.3V
17
100k
3455 F14
LDO
2.5V
16
169k
100pF
80.6k
AO
LTC3455/
LTC3455-1
AI
AO
LTC3455/
LTC3455-1
AI
10μF
