RE46C190S16F Datasheet, PDF (27/42 Pages) Microchip Technology – CMOS Low Voltage Photoelectric Smoke Detector ASIC with Interconnect and Timer Mode

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 Table 5-1, based on the following
parameters:
V_BAT
V_BST1
V_BST2
Boost capacitor size
Boost Efficiency
IRED on time
IRED Current
=3
= 3.6
=9
= 4.70E-06
= 8.50E-01
= 2.000E-04
= 1.000E-01
TABLE 5-1: STANDBY CURRENT CALCULATION
I_DD Component
Voltage Current Duration Energy Period
(V)
(A)
(s)
(J)
(s)
Fixed I_DD
3
Photo Detection Current
Chamber test 3.6
(excluding IR drive)
IR drive during 3.6
Chamber Test
Smoke Detection 3.6
(excluding IR drive)
IR drive during 3.6
Smoke Detection
1.00E-06
—
—
—
1.00E-03 1.00E-02 3.60E-05 43
0.10 2.00E-04 7.20E-05 43
1.00E-03 1.00E-02 3.60E-05 10.75
0.10 2.00E-04 7.20E-05 10.75
Average
Power
(W)
3.00E-06
I_BAT
Contribution
(A)
1.00E-06
9.85E-07
1.97E-06
3.94E-06
7.88E-06
3.28E-07
6.57E-07
1.31E-06
2.63E-06
I_BAT
(µA)
1.0
0.3
0.7
1.3
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
Unloaded Test
—
6.85E-05 344 2.34E-07 7.81E-08
0.1
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
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:
3---.--6--4-V--3---s-_{----0---.-0-1--.-8A---5--}_{-----2--3-0---V0----}_{---s- = 0.657A}
 2010 Microchip Technology Inc.
DS22271A-page 27