User's Manual

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USER MANUAL 1.34

STM 300 / STM 300C / STM 300U

Charge switcher

The charge switcher connects both short term storage and long term storage parallel to the

energy source as soon as the STM 300 supply voltage reaches the typical VON threshold of

2.45 V. Supposing VDD then falls below VON, the energy source will be switched back to

short term storage alone, for faster recharging. As long as the voltage on long term storage

remains below VON, the charge switcher will continuously switch the energy source be-

tween short term and long term storage, trying to ensure continuous device operation. That

is because of the higher resistance and capacitance of long term storage, which would lead

to much too long charging (i.e. non-operative time). In addition short term storage cannot

be charged over this threshold until the voltage on long term storage exceeds VON. Charge

switcher is the PMOS transistor Q1, driven from the STM 300 charge control output CCO

over T1A. To start with, as long as the STM 300 VDD voltage is below the VON threshold,

only the small storage (C1) is filled over D3. Once the threshold is reached, the CCO control

signal goes High, T1B and Q2 are turned on and the long term storage (C2) will be filled

over Q2.

Overvoltage protection

All of these long term storage solutions have a rated operating voltage that must be not

exceeded. After reaching this limit the energy source is automatically separated from stor-

age to avoid any damage. Overvoltage protection is implemented by the S-1000C32-M5T1x

voltage detector from Seiko (SII) or the NCP300LSN30T1G series (ON Semiconductor),

which limits the maximum charging voltage to 3.3 V to avoid damaging long term energy

storage. In case a different voltage limit is required, this device has to be replaced by a

suitable voltage variant. As soon as the voltage on D2 anode or the voltage detector input

exceeds the selected threshold, the voltage detector delivers a High level on its output con-

nected to the T1A emitter. The T1A base is consequently lower polarized than its emitter

and the transistor is turned off. That means Q1 is turned off too — the energy source is

switched off and long term storage is protected.

The selected voltage detector must have a very low quiescent current in the operating

range, and an appropriate threshold voltage, corresponding to the selected long term ener-

gy storage voltage (e.g. threshold nominally 3.2 V for a 3.3 V capacitor). If the selected

threshold is too low, e.g. 3.0 V, a relatively high amount of energy corresponding to a use-

ful voltage difference of 0.3 V would be wasted. If the nominal threshold is too high, e.g.

exactly 3.3 V (not forgetting that this could reach 3.4 V as a result of additional manufac-

turer tolerances), it could be critical for energy storage life expectation. The S-1000C32-

M5T1x voltage detector consequently looks like the best compromise here (rated 3.2 V)

Undervoltage protection

PAS capacitors should not be deep discharged to voltages below 1.5 V. To avoid long term

degradation of their capacity and lifetime, an undervoltage protection block is added.

Undervoltage protection is also implemented through Q2. In normal operation, when VDD

reaches the VON threshold, the STM 300 charge control CCO goes high, T1B rapidly dis-

charges C3 to GND and Q2 turns on long term storage. The C3 charge recovers very slowly

over R6, so Q2 cannot turn off long term storage immediately. Only if VDD falls below VOFF

for a longer time does C3 have time to recover and finally to turn off Q2 and thus the long

term storage path (over D4) from the STM 300, avoiding deep discharge.

For more details and alternative circuits please refer to application note

AN208.