LTC3456 Datasheet, PDF (17/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

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OPERATIO
The LTC3456 incorporates additional features like output
short-circuit protection and thermal regulation. When the
core regulator output (V_CORE) is shorted to ground, the
LTC3456’s unique control scheme prevents inductor cur-
rent runaway.
Dropout Operation
The LTC3456 is capable of operating at 100% duty cycle
when powered from the battery. If the input supply voltage
decreases to a value close to the output voltage, the core
regulator will run at 100% duty cycle. The output voltage
is then determined by the input voltage minus the voltage
drop across the PMOS switch and the inductor. When
running off the USB or the AC adapter, this situation never
arises (there is plenty of voltage headroom).
When the LTC3456 operates with low input supply volt-
age, say 1.8V (fully discharged two AA cells) the maximum
allowable output current gets reduced. Figure 7 shows the
reduction in the maximum output current as a function of
input voltage for various output voltages.
600
L = 10µH
(BATTERY POWERED)
500
V_CORE = 1.5V
400
V_CORE = 1.8V
300
200
100
0
1.8 2
2.2 2.4 2.6 2.8
V_BATT (V)
3 3.2
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Figure 7. Maximum Output Current vs Battery Voltage
V_MAX OUTPUT
A special internal PowerPath controller monitors the V_BATT,
V_INT, V_EXT and USB voltages and passes the highest
available supply voltage to the V_MAX pin. This pin is used
LTC3456
to power some of the internal circuitry of the IC. Connect
a 1µF bypass capacitor from this pin to ground. It can be
used to supply a maximum of 1mA output load.
The V_MAX output voltage stays alive even when the IC is in
shutdown. During shutdown, the V_MAX output will be the
highest of V_BATT, V_EXT and USB voltages. V_MAX output can
be used to supply power to a critical block like the real-time
clock, which needs to stay alive even during shutdown.
POWER SEQUENCING
Power On/Off
The LTC3456 can turned on in two different ways:
• Pulling the PWRKEY pin low
• Pulling the PWRON pin high
Pulling the PWRKEY pin low is usually the first step in
turning on the LTC3456. When PWRKEY is pulled low, it
powers on the bandgap reference. Onboard voltage moni-
tors check the status of the AC adapter and USB supply.
The IC is powered from either the AC adapter, USB or the
battery, in that order of preference. The V_INT voltage
powers up, followed by the Core, Main and Hot Swap
outputs.
At initial power up, the RESET pin is held low. When the
Core output comes into regulation, the RESET timer is
started. After a 262ms timeout, the RESET pin is released.
This allows the microprocessor to turn on. The micropro-
cessor in turn pulls PWRON high. After the PWRON pin is
pulled high, the PWRKEY pin can be released. There is a
400kΩ pull-up resistor on the PWRKEY pin.
The PWRKEY pin serves the dual purpose of turning on
and off the IC. During regular operation, if PWRKEY is
pressed low, the microprocessor detects this by monitor-
ing the status of PBSTAT pin. The PBSTAT pin goes low
whenever the PWRKEY pin is pulled low. The micropro-
cessor then goes into shutdown mode and pulls the
PWRON pin low. This results in powering off the LTC3456.
Figure 8 shows the device power-on/power-off sequence.
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