AN865 Datasheet, PDF (5/7 Pages) Silicon Laboratories – APPLYING DUTY-CYCLE CONTROL TO SAVE POWER

AN865
At hot temperatures, the circuit’s only vulnerability is the parasitic leakage currents of Q3, a BSH205 MOSFET. If
this is a concern, the BSH205 may be substituted with a PNP transistor at the expense of slightly reduced boost
voltage.
V_IN
1V (1.5V max)
R1
10M
R2
2.2M
R3
10M
3+
1
-
4
U1
TS1001
R4
5.6M
C1
0.1uF
R5
4.7K
R7
1M
R6
10k
Q1
2N3906
R8
10K
Q2
2N3904
1.8V
1V
Power burst : 1.8V@
350µA, 10ms
D1
1N5817
C2
C3
R9
Cin
Cout
22K
VOUT
(bursted 1.8V
standby 0.9V))
Q3
BSH205
R10
220K
Q4
2N3904
Microcontroller
OPTIONAL
LOAD CUTOFF
DONE
Figure 5. Applying Duty-cycle Control to Boost Power from a Single 1 V Cell
The circuit of Figure 5 shows how to use the duty-cycled boost concept to solve the basic problem of powering a
microcontroller-based system from a single coin-type alkaline battery cell. The circuit is similar to the circuit shown
in Figure 3, but utilizes a very low-voltage, very low-supply-current TS1001 op amp as a comparator, replacing the
TSM9119. Also, the circuit has been simplified and provides lower currents.
The circuit operates from battery voltages as low as 1 V and provides power bursts at 1.8 V suitable for “burst
powering” a microcontroller at its specified VDD = 1.8 V in short durations. During the “off–time,” the circuit
provides the microcontroller a standby voltage of at least 0.9 V specified in microcontroller spec sheets as the
“RAM retention voltage”. Alternatively, the load (microcontroller) may be disconnected entirely with the optional
load cutoff circuitry ensuring that the microcontroller is completely off with zero leakage.
There is no fixed on-time with this circuit; instead a DONE control input is added. The microcontroller, when done
with its operations, pulls the DONE line high, effectively “cutting the power cord” and shutting down the burst by
resetting capacitor C1 (note that Q2’s collector and emitter are reversed to ensure a low Vcesat for headroom
reasons). The charge pump in this circuit is directly driven by op amp U1’s output, which supports the 350 µA burst
currents. If higher currents are needed, the circuit may be modified with the additional circuitry of Figure 3.
Rev. 1.0
5