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High side driver (7V-9V) with 3.3V microcontroller pin to switch it questions...
alank2:
I'm thinking about using 4 NiMH cells for a 3.3V project. It might use up to 300mA maxmimum, probably half of that or less normally.
I'm probably going to use a TR05S3V3 dc-to-dc to convert the 4.4V to 5.8V battery voltage to the 3.3V.
My question is what can be done to charge from a 7V adapter. Something else in this same system uses the 7V adapter, so that is the voltage the adapter needs to be.
I looked at some NiMH charge controller IC's, but they all seemed very pricy, like $8+ for some reason.
This is for a retrocomputer and it has a 4 pack NiCd that it has its own charging method for. It basically uses a resistor to trickle charge it at one rate and then monitors the battery voltage to decide whether to bypass a resistor and kick up the charging current. It basically has an always inline resistor of a smaller value (fast charging) and then uses a resistor to switch past another resistor which when added to the smaller value resistor lowers the current to the slow/trickle method.
My question is - what kind of driver is best to switch this? I've been looking up transistor/mosfet high side drivers, but there seems to be an issue trying to turn on a p channel mosfet when you only have a 3.3V logic level unless you use a BJT to drive the gate. Is there an easier way or solution? Basically I want to switch by the larger value resistor from a 3.3V logic level to allow a higher voltage (7V, but potentially 9V) to charge a battery pack.
David Hess:
--- Quote from: alank2 on July 25, 2019, 09:01:29 pm ---I've been looking up transistor/mosfet high side drivers, but there seems to be an issue trying to turn on a p channel mosfet when you only have a 3.3V logic level unless you use a BJT to drive the gate. Is there an easier way or solution? Basically I want to switch by the larger value resistor from a 3.3V logic level to allow a higher voltage (7V, but potentially 9V) to charge a battery pack.
--- End quote ---
How much simpler for level shifting than a single bipolar transistor or MOSFET do you want?
If you have a logic level n-channel MOSFET, connect its gate to the 3.3 volt positive supply and its source to the logic output. When the logic output is low, it will pull the drain down to the same low logic level. When the logic output is high, the drain can be pulled up to the breakdown voltage of the MOSFET.
An NPN transistor can be used the same way with a resistor in series with the base as long as the transistor is allowed to saturate. Or if the resistor is moved to the emitter, then the bipolar transistor becomes a switchable current sink which might be advantageous in some applications but the collector cannot fall lower than the logic supply.
iMo:
You may use an optocoupler, the led diode driven off the MCU 3.3V level via a 1k resistor, and the output transistor bypassing the larger value resistor. I think this is the simplest solution.
David Hess:
An optocoupler has the virtue of galvanic isolation which prevents ground loops.
mark03:
The single transistor is nice if open collector/drain is adequate, but what's the easiest solution when you need a high-voltage totem pole (low output impedance in both the high and low states) with logic-level input? I ended up using a FAN3268 but was not sure if simpler options were available.
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