V20W Datasheet, PDF (22/24 Pages) List of Unclassifed Manufacturers – PIEZOELECTRIC ENERGY HARVESTERS

APPLICATIONS INFORMATION - LOAD ISOLATION EXAMPLE
as the comparator will be ON when approaching this
point. Solving for R3,
R3
R1 R2 VL Vth
R1 Vth R2 Vth R2 VL
The addition of R3 will slightly affect the actual center
voltage; however, this change will typically not be
significant in comparison with the size of the hysteresis
band. For a typical V_c (>3.3V), the final value of V_c will
be lowered. As indicated by Figure 5, however, driving
the piezo to moderately higher open-circuit voltages
than optimal does not have an excessive impact on
efficiency.
Check: Actual V_h and V_l
Once all resistor values are chosen, the actual upper
and lower trip points will be defined by:
Latch-Up Consideration
If diode D8 is removed, the designer must pay attention
to D6, which is necessary to prevent latch-up of the
LT1474’s output switch if the voltage at its SW pin is
held up while V_IN drops. This situation may easily
occur in an application where the load is powered by
multiple sources or includes a storage element, such
as supercapacitor or battery, whose voltage drops
more slowly than that of C1 when no harvested power
is available. The user must then choose D6 so that the
voltage at SW cannot exceed the voltage at V_IN by the
0.6V required for latch-up. Particularly at high C1
voltages, the designer must also take steps to ensure
that reverse leakage over D6 will not allow the voltages
at the V_FB and SW pins to float above their maximum
ratings at no load.
V_h
1.182
R2 R3 R1
R2 R3
V_l 1.182
R1
R3
R2
R2
REVISION N0. 002 REVISION DATE: 01-23-2013
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