LTC3108 Linear Technology, LTC3108 Datasheet - Page 8

• Manufacturer Part Number: LTC3108
• Description: Ultralow Voltage Step-Up Converter and Power Manager
• Manufacturer: Linear Technology

www.DataSheet4U.com
OPERATION
LTC3108
Oscillator
The LTC3108 utilizes a MOSFET switch to form a resonant
step-up oscillator using an external step-up transformer
and a small coupling capacitor. This allows it to boost input
voltages as low as 20mV high enough to provide multiple
regulated output voltages for powering other circuits. The
frequency of oscillation is determined by the inductance of
the transformer secondary winding and is typically in the
range of 20kHz to 200kHz. For input voltages as low as
20mV, a primary-secondary turns ratio of about 1:100 is
recommended. For higher input voltages, this ratio can be
lower. See the Applications Information section for more
information on selecting the transformer.
Charge Pump and Rectifi er
The AC voltage produced on the secondary winding of
the transformer is boosted and rectifi ed using an external
charge pump capacitor (from the secondary winding to pin
C1) and the rectifi ers internal to the LTC3108. The rectifi er
circuit feeds current into the VAUX pin, providing charge
to the external VAUX capacitor and the other outputs.
VAUX
The active circuits within the LTC3108 are powered from
VAUX, which should be bypassed with a 1μF capacitor.
Larger capacitor values will reduce the ripple on VAUX but
increase the time it takes for VAUX to rise and the other
outputs to become active. Once VAUX exceeds 2.5V, the
main V
An internal shunt regulator limits the maximum voltage
on VAUX to 5.25V typical. It shunts to GND any excess
current into VAUX when there is no load on the converter
or the input source is generating more power than is
required by the load.
Voltage Reference
The LTC3108 includes a precision, micropower reference,
for accurate regulated output voltages. This reference
becomes active as soon as VAUX exceeds 2V.
8
OUT
is allowed to start charging.
Synchronous Rectifi ers
Once VAUX exceeds 2V, synchronous rectifi ers in parallel
with each of the internal diodes take over the job of rectify-
ing the input voltage, improving effi ciency.
Low Dropout Linear Regulator (LDO)
The LTC3108 includes a low current LDO to provide a
regulated 2.2V output for powering low power proces-
sors or other low power ICs. The LDO is powered by the
higher of VAUX or V
as soon as VAUX has charged to 2.3V, while the V
age capacitor is still charging. In the event of a step load
on the LDO output, current can come from the main V
capacitor if VAUX drops below V
a 2.2μF 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 4mA typical.
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. Regulated Voltage Using Pins VS1 and VS2
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 internal programmable resistor divider sets V
eliminating the need for very high value external resistors
that are susceptible to board leakage.
OUT
VAUX
VAUX
GND
GND
VS2
OUT
. This enables it to become active
VAUX
VAUX
GND
GND
VS1
OUT
OUT
is charged from the VAUX
has reached the proper
OUT
. The LDO requires
2.35V
V
3.3V
4.1V
5V
OUT
OUT
stor-
OUT
3108p
OUT
,