Hi
I have a question regarding how to connect the CE, Mode and ISET2 inputs on the BQ24070 in the most correct and safe way, that I will very much appreciate your opinion about.
In my application all these inputs (CE, Mode and ISET2) should be permanently connected to logic high.
Originally I thought of connecting them all to the Vref pin (at 3.3V). However, the datasheet says the Vref output is only active when a voltage is present on the IN pin (from an AC-adapter or USB source), but that Vref is not active when a battery (connected to the BAT pin) is the only power source. If this is correctly understood, it will not be possible to use Vref as a logic high voltage source for the Mode and ISET2 inputs, since Vref will fall to 0V (or perhaps float) when no AC-adapter is connected. One may argue that it doens't matter what the states of Mode and ISET2 are when no AC-adapter is connected, since these signals are only used when an adapter is present and the battery is being charged, but I prefer not having the inputs float or be wrongly defined at any time (the BQ24070 quiescent current could perhaps also be increased if the inputs floats around ½Vcc where both the input transistors is turned on).
Instead I was thinking of connecting CE, Mode and ISET2 to the output of an 2.8V linear regulator (which is already used as supply for a microcontroller) for obtaining the logic high voltages. The regulator gets it's supply voltage from the OUT pin on BQ24070 (i.e. BQ24070 OUT is connected to the regulator input).
However, I'm somewhat unsure if this method will work properly in case no battery is connected, and only an AC-adapter is used. When the adapter is connected and power is applied to the IN pin, then there will only be an output voltage on the OUT pin if CE is set logic high (i.e. the Q1 mosfet will not connect OUT to IN if CE is logic low at power-on). However, as I see it, CE will never be set high, since the 2.8V regulator will never be turned on since the Q1 mosfet, and thereby the BQ24070 OUT, is never activated... the situation kind of bites itself in the tail. Or perhaps I have misuderstood how the BQ24070 acts when powered up?
Then instead I am thinking of connecting CE to the Vref pin, so that CE goes high when an AC-adapter is connected. I guess it doesn't matter that Vref, and thereby CE, is low when only a battery (but no AC-adapter) is connected, since the charging function doesn't need to be enabled when no adapter is present. Or do you think it is a wrong approach to connect CE to Vref?
(I guess CE could also be connected to the IN pin. However, the datasheet says the voltage on any logic input must not be higher than 7V. Since my application may sometimes be powered from a 12V car battery this could perhaps be a problem, unless some voltage limiting circuitry like a series resistor and voltage clamping transzorb is used to protect the CE input from overvoltages).
Best regards,
Peter
