LTC3108_12 Datasheet, PDF (10/22 Pages) Linear Technology – Ultralow Voltage Step-Up Converter

LTC3108
OPERATION
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.
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