A simple power supply sequencer from MOSFET/BJT? - Page 1
Forum section: electronics / projects

author: randsl
author_rank: Contributor
timestamp: September 18, 2017, 02:16:55 am
content: I have an RF amplifier which needs a positive 5V/250mA supply & a negative bias voltage (-2V).When switching on, the negative supply should be applied before the positive power supply, otherwise it draws too much current.Also in my circuit, I've only +5V rail which comes from a voltage regulator which I don't have the control of EN pin.Also I don't have the chance to use a microcontroller/fpga pin to switch a regulator/transistor with a software delay.I'm using a TPS60402 charge pump voltage inverter & a resistive divider to generate -2V from +5V supply.___________ +5V ___________+5V/250mA --------------|?? |------------------- | || | Delay cct. | | RF AMP || | 50 - 100 ms| | || ---------------- ---------------| ___________ -2V | -Ve bias|---------| charge |------------------- ----|| pump || voltage inv. |----------------Is there a way to delay the +5V supply by about 50 mS - 100 mS using a simple method by having a MOSFET/BJT + some caps & resisotors? Rather than using a power supply sequencer chip?Has anyone used a similar method before?cheers,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2017, 03:46:53 am
content: Seems like a USB power switch (typically 5V 500mA capacity) would be well used here. Quite cheap and plentiful. Logic level input.A transistor or two should be all that's needed. How much depends on the required threshold and timing precision/stability, and which polarity logic level is needed. It may be worthwhile using an IC instead, e.g., TLV431 (acts like a BJT with precision Vbe), any power monitor / reset generator (precision threshold, delay options available, downside: may be self-powered rather than with a separate input pin), or getting more general, any comparator and voltage reference device (or even just a comparator, in which case the decision will be ratiometric with +5V).Or, taking a step back, can you find a charge pump with PGOOD output signal?Tim

author: Someone
author_rank: Super Contributor
timestamp: September 18, 2017, 03:59:26 am
content: Quote from: randsl on September 18, 2017, 02:16:55 amIt usually starts with a circuit similar to the mosfet reverse polarity protection design and then adds some switching and RC delays, whether that is cheaper than a load switch depends on your constraints.

author: Ian.M
author_rank: Super Contributor
timestamp: September 18, 2017, 06:03:48 am
content: Even for something fairly crappy, it gets quite complex. This circuit uses the -ve bias rail regulator to trigger the +5V rail switch-on, so it cant come up till the bias rail is present. Once the 5V rail is up, the bias shunt regulator draws current through the gate pull-down MOSFET M2. You'll probably have to fiddle with component values to get the exact behaviour you want, as a lot of the timing is dependent on the charge pump output current.If the budget permits, Q1, the bias regulator could with advantage be replaced by a TLV431, and the voltage set potential divider (R4,R5) adjusted accordingly. Due to the much sharper switching action of the TLV431, C5 would no longer be required as a full -2V would already be present on the bias rail when the +5V comes up.