LTC3109 Linear Technology, LTC3109 Datasheet - Page 10

• Manufacturer Part Number: LTC3109
• Description: Ultralow Voltage Step-Up Converter and Power Manager
• Manufacturer: Linear Technology

LTC3109
operaTion
V
step load on the LDO output, current can come from the
main V
ceramic capacitor for stability. Larger capacitor values
can be used without limitation, but will increase the time
it takes for all the outputs to charge up. The LDO output
is current limited to 5mA minimum.
V
The main output voltage on V
supply, and is user-programmed to one of four regulated
voltages using the voltage select pins VS1 and VS2, ac-
cording to Table 2. Although the logic-threshold voltage
for VS1 and VS2 is 0.85V typical, it is recommended that
they be tied to ground or VAUX.
Table 2
When the output voltage drops slightly below the regulated
value, the charging current will be enabled as long as VAUX
is greater than 2.5V. Once V
value, the charging current is turned off. The resulting
ripple on V
The internal programmable resistor divider, controlled by
VS1 and VS2, sets V
high value external resistors that are susceptible to noise
pickup and board leakages.
In a typical application, a reservoir capacitor (typically a
few hundred microfarads) is connected to V
as VAUX exceeds 2.5V, the V
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 15mA
typical. Note that for very low input voltages, this current
may be in the range of 1µA to 1000µA.
0
OUT
OUT
storage capacitor is still charging. In the event of a
OUT
VAUX
VAUX
GND
GND
VS2
OUT
reservoir capacitor. The LDO requires a 2.2µF
is typically less than 20mV peak to peak .
(Refer to the Block Diagram)
OUT
, eliminating the need for very
VAUX
VAUX
GND
GND
VS1
OUT
OUT
OUT
is charged from the VAUX
has reached the proper
capacitor will begin to
OUT
2.35V
V
3.3V
4.1V
5V
OUT
. As soon
PGOOD
A power good comparator monitors the V
The PGOOD pin is an open-drain output with a weak pull-
up (1MΩ) to the LDO voltage. Once V
to within 7.5% of its programmed voltage, the PGOOD
output will go high. If V
programmed voltage, PGOOD will go low. The PGOOD
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. The PGOOD pin can also be pulled low in
a wire-OR configuration with other circuitry.
V
V
host using the V
nected to V
This output, controlled by a host processor, can be used
to power external circuits such as sensors and amplifiers,
that don’t have a low power “sleep” or shutdown capabil-
ity. V
they are needed.
Minimizing the amount of decoupling capacitance on
V
ing shorter pulse times and therefore smaller duty cycles
in applications such as a wireless sensor/transmitter. A
small V
will be wasted in charging the capacitor every time V
is enabled.
V
current to 0.3A typical.
The V
with 100mV of hysteresis, making it logic compatible. If
V
is low, V
on the V
Note that while V
cuitry for V
from V
OUT2
OUT2
OUT2
OUT2
OUT2_EN
OUT2
OUT2
is an output that can be turned on and off by the
enables it to be switched on and off faster, allow-
OUT
has a current limiting circuit that limits the peak
OUT2
OUT2
OUT2
(which has an internal 5M pull-down resistor)
can be used to power these circuits only when
. This added current draw has a negligible effect
OUT
OUT2
enable input has a typical threshold of 1V
capacitor will also minimize the energy that
output.
will be off. Driving V
through a 1Ω P-channel MOSFET switch.
OUT2_EN
draws an extra 8µA of quiescent current
OUT2_EN
OUT
pin. When enabled, V
is high, the current limiting cir-
drops more than 9% from its
OUT2_EN
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OUT
high will turn
has charged
OUT
OUT2
voltage.
is con-
OUT2
3109f