AN865 Datasheet, PDF (3/7 Pages) Silicon Laboratories – APPLYING DUTY-CYCLE CONTROL TO SAVE POWER

AN865
VBATT
1.8V - 2.5V
R1
10M
R2
2.2M
C1
0.1uF
R3
10M
3+
-
4
5
1
2 _U1
TSM9119
R4 14M
R5 100K
Q1
2N3906
R7
R8
1M
220K
Q2
R6
2N3904
1M
VOUT
D1
photodiode
3⁺
-
4
5
1
2 U2
TS1001
R9 33K
(adjust for photodiode)
C2
0.001uF
MEASURED LIGHT
V
lux
Power on :
2msec
Power off:
1sec
5
3
+
1
4
-
vref
1.25v
2
TSM9117
U3
TS9117
uC
INTERRUPT
(when enough light)
Figure 2. Nanopower Solar Detector Implementing Duty-Cycle Control to Keep
Power Consumption Low
Figure 2 shows a scenario in which a solar monitor checks the availability of sunlight for a low-power solar-
powered system that should not turn on until enough light power is available. The duty-cycling clock circuit
provides power to the solar detector, comprised of U2 and U3 and associated passives, at intervals of about 1
second. In the solar detector, photodiode D1 senses available light, and a TS1001 op amp sinks the resulting
photodetected current through its VSS power supply pin, providing a positive polarity signal. In the op amp loop,
the op amp must consume power supply current according to the photodetected current (effectively cancelling it);
this current can be higher than 30 µA. However, since the solar detector does not need to be continuously on, the
duty cycling clock circuit reduces this current by the factor of the duty cycle (approximately 500), resulting in a net
average current consumption of the detector to less than 100 nA. The TSM9117 is a voltage detector IC whose
output goes high when the input rises above 1.25 V, indicating sufficient light is present and interrupting the
processor. The desired sensitivity trip threshold voltage can be set by choosing R9.
The “duty-cycler” can be further configured to provide burst power at a boosted output voltage to the load. Here,
the idea is to generate short bursts of higher voltage utilizing a simple voltage doubling boost capacitor integrated
into the clock design. The boost cap provides the higher voltage only for the short duration for which it is actually
needed, eliminating the need for a separate boost converter and its associated overhead quiescent currents and
delays. In fact, some converters’ shutdown current exceeds the currents utilized by this duty-cycling clock.
VIN
1.8V
R1
10M
R2
2.2M
R3
10M
3+
-
4
1
U1
TSM9119
R4 14 M
C1
0.1uF
R5 22K
R7
1M
R6
1M
Q1
2N3906
R8
220K
Q2
2N3904
C2
Q3
100µF BSH205
3.3V
1.8V
D1
1N5817
Power burst : 3.3V@
25mA, 500usec
OUT
C3
100µF
U3
74AUP2G14
A
R9 15K
C4 4700pF
C5 4700pF
R10 15 K
B
U2
74AUP2G00
Q4
BSH205
R11
10K
R12
100K
Q5
2N3904
VOUT
vcc
433
SAW
RESONATOR _MO-SAWR-A
TX
OPTIONAL
LOAD CUTOFF
BURST ON
LOGIC PATTERN
GENERATOR
Figure 3. Technique to Boost Voltage Using Duty-Cycle Control
Rev. 1.0
3