2010 Microchip Technology Inc. DS22271A-page 27
RE46C190
5.0 APPLICATION NOTES
5.1 Standby Current Calculation and
Battery Life
The supply current shown in the DC Electrical
Characteristics
table is only one component of the
average standby current and, in most cases, can be a
small fraction of the total, because power consumption
generally occurs in relatively infrequent bursts and
depends on many external factors. These include the
values selected for IRED current and integration time,
the V
BST
and IR capacitor sizes and leakages, the V
BAT
level, and the magnitude of any external resistances
that will adversely affect the boost converter efficiency.
A calculation of the standby current for the battery life
is shown in Ta b l e 5 - 1, based on the following
parameters:
The following paragraphs explain the components in
Table 5-1, and the calculations in the example.
5.1.1 FIXED I
DD
The I
DD
is the Supply Current shown in the DC
Electrical Characteristics
table.
5.1.2 PHOTO DETECTION CURRENT
Photo Detection Current is the current draw due to the
smoke testing every 10.75 seconds, and the chamber
test every 43 seconds. The current for both the IR
diode and the internal measurement circuitry comes
primarily from V
BST
, so the average current must be
scaled for both on-time and boost voltage.
The contribution to I
BAT
is determined by first
calculating the energy consumed by each component,
given its duration. An average power is then calculated
based on the period of the event and the boost
converter efficiency (assumed to be 85% in this case).
An I
BAT
contribution is then calculated based on this
average power and the given V
BAT
. For example, the IR
drive contribution during chamber test is detailed in
Equation 5-1:
EQUATION 5-1:
V
BAT
=3
V
BST1
=3.6
V
BST2
=9
Boost capacitor size = 4.70E-06
Boost Efficiency = 8.50E-01
IRED on time = 2.000E-04
IRED Current = 1.000E-01
TABLE 5-1: STANDBY CURRENT CALCULATION
I
DD
Component
Voltage
(V)
Current
(A)
Duration
(s)
Energy
(J)
Period
(s)
Average
Power
(W)
I
BAT
Contribution
(A)
I
BAT
(µA)
Fixed I
DD
3 1.00E-06 — — — 3.00E-06 1.00E-06 1.0
Photo Detection Current
Chamber test
(excluding IR drive)
3.6 1.00E-03 1.00E-02 3.60E-05 43 9.85E-07 3.28E-07 0.3
IR drive during
Chamber Test
3.6 0.10 2.00E-04 7.20E-05 43 1.97E-06 6.57E-07 0.7
Smoke Detection
(excluding IR drive)
3.6 1.00E-03 1.00E-02 3.60E-05 10.75 3.94E-06 1.31E-06 1.3
IR drive during
Smoke Detection
3.6 0.10 2.00E-04 7.20E-05 10.75 7.88E-06 2.63E-06 2.6
Low Battery Check Current
Loaded Test
Load 9 2.00E-02 1.00E-02 1.80E-03 344 6.16E-06 2.05E-06 2.1
Boost V
BST1
to V
BST2
— — 6.85E-05 344 2.34E-07 7.81E-08 0.1
Unloaded Test
Load 3.6 1.00E-04 1.00E-02 3.60E-06 43 9.85E-08 3.28E-08 0.0
Total 8.09E-06 8.1
3.6V 0.1A 200
s
43s 0.85 3V
---------------------------------------------------0.657
A=
