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

• Manufacturer Part Number: LTC3108EGN#PBF
• Description: IC CONV BOOST PROG 4.5MA 16SSOP
• Manufacturer: Linear Technology

Specifications of LTC3108EGN#PBF

• Applications: Energy Harvesting
• Current - Supply: 3mA
• Voltage - Supply: 20mV ~ 500mV
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Primary Input Voltage: 500mV
• No. Of Outputs: 3
• Output Voltage: 5V
• Output Current: 300mA
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +85°C
• Msl: MSL 1 - Unlimited
• Supply Voltage Min: 20mV
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

applicaTions inForMaTion
at which the load pulses can occur for a given output cur-
rent from the LTC3108 will also be shown.
In this example, V
allowed voltage droop during a transmit burst is 10%, or
0.33V. 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 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 V
being used. This number is best found empirically, since
there are many factors affecting the efficiency of the
converter. Also determine what the total load current is
on V
that this must include any losses, such as storage ca-
pacitor leakage.
Assume, for instance, that the charge current from the
LTC3108 is 50µA and the total current drawn on V
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:
C
t
OUT
=
OUT
OUT
150
(
50
( )
is:
µF
during the sleep state (between bursts). Note
OUT
µA
µF
, V
≥
−
• .
40
OUT2
0 33
17
mA
OUT
0 33
µA
OUT
.
and VLDO, since the current for all of
V
)
•
is set to 3.3V, and the maximum
pin given the input voltage source
=
V
1
ms
1 5
. sec
=
OUT
121
or f
during a burst. Current
µF
MAX
OUT
=
0 666
capacitor.
.
OUT
H H z
OUT
in
Therefore, in this application example, the circuit can sup-
port a 1ms transmit burst every 1.5 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 ten seconds.
The following formula enables the user to calculate the
time it will take to charge the LDO output capacitor and
the V
the charge current available from the LTC3108 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
If there were 50µA of charge current available and 5µA of
load on V
the initial application of power would be 12.5 seconds.
Design Example 2
In many pulsed load applications, the duration, magnitude
and frequency of the load current bursts are known and
fixed. In these cases, the average charge current required
from the LTC3108 to support the average load must be
calculated, which can be easily done by the following:
Where I
ternal circuitry in between bursts (including cap leakage),
I
BURST
I
t
t
OUT
CHG
LDO
VOUT
OUT
is the total load current during the burst, t is the
were programmed to 3.3V and the V
Q
≥
=
OUT
is the sleep current on V
capacitor the first time, from 0V. Here again,
=
I
2 2
Q
I
I
.
CHG
CHG
, the time for V
+
V
I
3 3
BURST
• .
.
−
−
2 2
I
V
I
T
LDO
VOUT
•
µF
150
•
OUT
t
−
µF
to reach regulation would be:
I
OUT
LDO
to reach regulation after
+
OUT
t
LDO
required by the ex-
LTC3108
OUT
capacitor

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