LTC3108
10
3108fc
For more information www.linear.com/LTC3108
In a typical application, a storage capacitor (typically a few
hundred microfarads) is connected to V
OUT
. As soon as
VAUX exceeds 2.5V, the V
OUT
capacitor will be allowed to
charge up to its regulated voltage. The current available
to charge the capacitor will depend on the input voltage
and transformer turns ratio, but is limited to about 4.5mA
typical.
PGOOD
A power good comparator monitors the V
OUT
voltage.
The PGD pin is an open-drain output with a weak pull-up
(1MΩ) to the LDO voltage. Once V
OUT
has charged to within
7.5% of its regulated voltage, the PGD output will go high.
If V
OUT
drops more than 9% from its regulated voltage,
PGD will go low. The PGD output is designed to drive a
microprocessor or other chip I/O and is not intended to
drive a higher current load such as an LED. Pulling PGD
up externally to a voltage greater than VLDO will cause a
small current to be sourced into VLDO. PGD can be pulled
low in a wire-OR configuration with other circuitry.
V
OUT2
V
OUT2
is an output that can be turned on and off by the
host, using the V
OUT2_EN
pin. When enabled, V
OUT2
is
connected to V
OUT
through a 1.3Ω P-channel MOSFET
switch. This output, controlled by a host processor, can
be used to power external circuits such as sensors and
amplifiers, that do not have a low power sleep or shutdown
capability. V
OUT2
can be used to power these circuits only
when they are needed.
Minimizing the amount of decoupling capacitance on
V
OUT2
will allow it to be switched on and off faster, allowing
shorter burst times and, therefore, smaller duty cycles in
pulsed applications such as a wireless sensor/transmit
-
ter. A small V
OUT2
capacitor will also minimize the energy
that will be wasted in charging the capacitor every time
V
OUT2
is enabled.
V
OUT2
has a soft-start time of about 5µs to limit capacitor
charging current and minimize glitching of the main output
when V
OUT2
is enabled. It also has a current limiting circuit
that limits the peak current to 0.3A typical.
The V
OUT2
enable input has a typical threshold of 1V
with 100mV of hysteresis, making it logic-compatible. If
V
OUT2_EN
(which has an internal pull-down resistor) is
low, V
OUT2
will be off. Driving V
OUT2_EN
high will turn on
the V
OUT2
output.
Note that while V
OUT2_EN
is high, the current limiting cir-
cuitry for V
OUT2
draws an extra 8µA of quiescent current
from V
OUT
. This added current draw has a negligible effect
on the application and capacitor sizing, since the load on
the V
OUT2
output, when enabled, is likely to be orders of
magnitude higher than 8µA.
VSTORE
The VSTORE output can be used to charge a large storage
capacitor or rechargeable battery after V
OUT
has reached
regulation. Once V
OUT
has reached regulation, the VSTORE
output will be allowed to charge up to the VAUX voltage.
The storage element on VSTORE can be used to power
the system in the event that the input source is lost, or
is unable to provide the current demanded by the V
OUT
,
V
OUT2
and LDO outputs. If VAUX drops below VSTORE,
the LTC3108 will automatically draw current from the stor
-
age element. Note that it may take a long time to charge
a large capacitor, depending on the input energy available
and the loading on V
OUT
and VLDO.
Since the maximum current from VSTORE is limited to a
few milliamps, it can safely be used to trickle-charge NiCd
or NiMH rechargeable batteries for energy storage when
the input voltage is lost. Note that the VSTORE capacitor
cannot supply large pulse currents to V
OUT
. Any pulse load
on V
OUT
must be handled by the V
OUT
capacitor.
Short-Circuit Protection
All outputs of the LTC3108 are current limited to protect
against short-circuits to ground.
Output Voltage Sequencing
A timing diagram showing the typical charging and
voltage sequencing of the outputs is shown in Figure 1.
Note: time not to scale.
operaTion
