LTC3108EGN-1#PBF Linear Technology, LTC3108EGN-1#PBF Datasheet - Page 17

• Manufacturer Part Number: LTC3108EGN-1#PBF
• Description: IC VOLTAGE CONVERTER 16-SSOP
• Manufacturer: Linear Technology

Specifications of LTC3108EGN-1#PBF

• Applications: Energy Harvesting
• Current - Supply: 3mA
• Voltage - Supply: 20mV ~ 500mV
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

applicaTions inForMaTion
Design Example 1
This design example will explain how to calculate the
necessary storage capacitor value for V
load applications, such as a wireless sensor/transmit-
ter. In these types of applications, the load is very small
for a majority of the time (while the circuitry is in a low
power sleep state), with bursts of load current occur-
ring periodically during a transmit burst. The storage
capacitor on V
burst, and the long sleep time between bursts allows
the LTC3108-1 to recharge the capacitor. A method for
calculating the maximum rate at which the load pulses
can occur for a given output current from the LTC3108-1
will also be shown.
In this example, V
lowed voltage droop during a transmit burst is 10%, or
0.3V. The duration of a transmit burst is 1ms, with a total
average current requirement of 40mA during the burst.
Given these factors, the minimum required capacitance
on V
Note that this equation neglects the effect of capacitor
ESR on output voltage droop. For most ceramic or low
ESR tantalum capacitors, the ESR will have a negligible
effect at these load currents.
A standard value of 150µF or larger could be used for C
in this case. Note that the load current is the total current
draw on V
these outputs must come from V
contribution from the holdup capacitor on VSTORE is not
considered, since it may not be able to recharge between
bursts. Also, it is assumed that the charge current from
the LTC3108-1 is negligible compared to the magnitude
of the load current during the burst.
To calculate the maximum rate at which load bursts can
occur, determine how much charge current is available
from the LTC3108-1 V
source being used. This number is best found empirically,
C
OUT
OUT
( )
is:
µF
OUT
, V
≥
OUT
40
OUT2
OUT
mA
supports the load during the transmit
0 3
and VLDO, since the current for all of
.
is set to 3V, and the maximum al-
•
V
OUT
1
ms
pin given the input voltage
=
OUT
133
during a burst. Current
µF
OUT
in pulsed
OUT
since there are many factors affecting the efficiency of
the converter. Also determine what the total load cur-
rent is on V
Note that this must include any losses, such as storage
capacitor leakage.
Assume, for instance, that the charge current from the
LTC3108-1 is 50µA and the total current drawn on V
in the sleep state is 17µA, including capacitor leakage. In
addition, use the value of 150µF for the V
The maximum transmit rate (neglecting the duration of
the transmit burst, which is typically very short) is then
given by:
Therefore, in this application example, the circuit can sup-
port a 1ms transmit burst every 1.3 seconds.
It can be determined that for systems that only need to
transmit every few seconds (or minutes or hours), the
average charge current required is extremely small, as
long as the sleep current is low. Even if the available
charge current in the example above was only 10µA and
the sleep current was only 5µA, it could still transmit a
burst every 9 seconds.
The following formula enables the user to calculate the
time it will take to charge the LDO output capacitor and
the V
charge current available from the LTC3108-1 must be
known. For this calculation, it is assumed that the LDO
output capacitor is 2.2µF .
If there were 50µA of charge current available and a 5µA
load on the LDO (when the processor is sleeping), the time
for the LDO to reach regulation would be 107ms.
If V
was 150µF , the time for V
t
t
t
OUT
LDO
VOUT
=
OUT
(
150
50
were programmed to 3V and the V
=
capacitor the first time, from 0V. Here again, the
=
µA
2 2
OUT
I
µF
I
.
CHG
CHG
−
V
• .
during the sleep state (between bursts).
17
3
0 3
• .
−
−
V
2 2
µA
I
I
LDO
•
V
VOUT
150
)
µF
=
OUT
1 36
µF
−
.
to reach regulation would be:
I
LDO
sec
+
LTC3108-1
or f
t
LDO
MAX
OUT
OUT
=
0 73
capacitor.
capacitor
.

H
31081f
OUT
z z