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Laurids_PETERSE
In normal operating conditions, following equation describes relation between IOUT and VSENSE:
Design value of Sense Resistor can be calculated from the above equation given the intended voltage at the ADC with the nominal load current and the typical K factor of the device.
The calculated sense resistor implies following considerations:
a) In normal operating conditions, in order not to reach Current sense voltage saturation VSENSE_SAT , with the maximum load current that can be read IOUT_MAX , the RSENSE has to fullfill the following equation:
(eq.2)
Within this maximum current linearity of Current sense is guaranteed. If a lower maximum load current needs to be read, like for example in case a LED string, the RSENSE value must be increased.
b) In normal operating conditions , the maximum sense voltage that can be read with the given RSENSE must be higher than a certain ADC threshold. This can be expressed by the following equation:
(eq. 3)
Where VSENSEMAX is the maximum voltage that the ADC has to read at the maximum monitored load current. This value can be below or equal 5V which normally is the maximum operating range of the ADC.
In fault conditions (overload, short-circuit to GND that cause Power Limitation or Thermal Shutdown and in Open Load/Short to Battery in OFF state), in order to be able to differentiate a normal operating condition from a Fault condition, the Current sense pin must be capable to develop a voltage above the VSENSEH (Value given in the datasheet, VSENSEH = 8V typically). Therefore the following condition must be fulfilled: (eq. 4)
In conclusion the RSENSE value must fulfill two opposite conditions for having linearity in normal operating condition: one is avoiding Current sense pin current saturation (increase RSENSE) and the other is avoiding Current sense pin voltage saturation (decrease of RSENSE). Moreover, the RSENSE value has to be dimensioned in order to distinguish a normal operating condition (linear mode, VSENSE proportional to load current) from a Fault condition (Constant Voltage Generator developing VSENSE_H across the RSENSE).
An example about RSENSE value definition is shown:
Example:
Let’s consider the VND9025AJ (25mW HSD) with a nominal load current IN = 2A which corresponds to an intended VSENSE = 2V and typical K2 = 5000 (from datasheet). Let us apply above equation:
Let us suppose that the maximum load current the ADC has to monitor in linearity is 2 times the nominal current so to say 4A at VSENSEMAX = 4V.
So, this means that neither VSENSE_SAT nor ISENSE_SAT must be reached and in fault conditions a voltage above 4.8V must be issued.
Let us verify that RSENSE value chosen is the correct one by applying above equations (2), (3) and (4):
