Buck Boost selection

Based on your low load currents, a boost converter is likely the best choice.  But what is the voltage range of your 3.6V batteries?  This voltage will be further reduced by your diodes and fuses, so the actual input voltage to our chip will be lower.

The TPS61220 is a good low power boost converter.  You may need some additional input capacitance (100uF maybe) after the fuse to provide the power for the switching converter to start.

Thanks Chris for the reply. It looks like it could be a solution without much modification to my circuit.

The batteries can be as low as 2V and as high as 3.64V. I've seen the voltage at the input to the TPS63000 go as low as 1.6V though. Also, Why do you suggest a boost, instead of a buck-boost?

The DS of TPS621220 (actually i'd like to use TPS621221 to get 3.3V constantly)  says that it can boost up from 0.7V. Why the 100u cap then?

I'm a bit concerned about its load transients. Usually the system operates below 20mA. It rises to about 50mA for wireless transmission once every second. I'm happy to have the ripple below 10mV.

As per the DS, it can boost from 0.7 to Vout, which in my case should be 3.3V. How does providing 3.6V from the battery affect the performance?

Forgot to mention. I also need a 75mA current limiting before the regulator, to prevent the fuses from blowing. Do you have a suggestion on how I could achieve it?

Many Thanks.

The input to the dc/dc converter will be your battery voltage minus a diode drop minus an IR drop across your fuses.  This will always give our chip a much lower input voltage than 3.64V.  So, you will always need a boost converter since your input voltage to our device will be lower than the 3.3V you want at the output.  A boost is a simpler solution than a buck boost.

You will likely need more input capacitance after your fuses for any power supply.  The reason is that a switching power supply will draw large peak currents to operate.  This would cause too high an IR drop across your fuse, so a capacitor is needed to lower the source impedance of your system.

Your load transients are rather small and should not be an issue for these devices.

I recommend ordering the EVM for whichever device you decide to go with and testing with your battery configuration.

For an input current limit, I recommend posting in the power interface forum.  Go ahead and restate your power requirements instead of referring to these threads only.  (You can still put these two thread's links in that new thread.)

Makes sense! Thanks!

For a load of let's say 60 Ohms, do you reckon this regulator will consume huge currents(more than 80mA)?

Yes.  Per a simple power calculation of Power in * efficiency = power out.

3.3V powering a 60 Ohm load is 182 mW.  At 2Vin, and 100% efficiency, this is 91 mA of input current.  Your efficiency will be less than 100%, so your fuse will blow before your batteries are fully discharged.

I'm basing this off of what I've seen using TPS63000. When it switches on, the input voltage drops from 3.6 to about 1.6V. If its the same with TPS61221, then this isn't a solution since the fuse will blow the moment I switch it on? Is it worth using the evaluation kit at all then?

You'll need to understand why Vin drops.  It is likely due to inrush current or too high of an output power.

If it from inrush current, then extra input capacitance after the fuse should solve it.

If it is too much output power, then there's not too much you can do about it as your input source cannot support your load's required power.

Just test with more input capacitance and see if the waveforms look different.

I've only got a tantalum capacitor 470u lying about here for now. That did improve the input a bit. But not enough. The waveforms look better. I suppose a ceramic is required to verify proper working. I'll test it with a ceramic and get back to u.

Also, the even with no load, the situation is the same. Shorting the fuse also solves the problem. So I don't suppose it due to high output power. It must be the inrush current. May be the inductor is drawing too much current to start up?

Yes, your tantalum may have too much ESR so that it is not providing good decoupling.

With a very high 8 Ohm source impedance, you will need more capacitance even at no load.

It may be worth looking at TPS61260 for this circuit. TPS61260 has been designed to handle high impedance sources properly. The adjustable output current limit may also be helpful in this design.

Hi Jeurgen,

This looks very apt for my application, especially the current limiting just using resistors. However, the startup current draw worries me (>80mA). A large capacitor (220uF or so) could help reduce the startup spike?

Chris

Yes, capacitance after your fuse will help the startup on any switching power supply.

Hi Chris

based on your requirment. Vin_min=2V, Vout=3.3V, Iout_max=65mA(or even higher), the max input current reach 134mA if consider 80% converter efficiency. so i don't think you can use 80mA fuser. thanks

Jasper li

AE of boost converter solution

Hi Jasper,

Well, my system operates at less than 20mA most of the time and spikes to about 45 once every second for a short burst of time. I could may be get away with adding a giant capacitor to provide that charge from close to the input of the regulator and not directly from the battery? That way the fuse may not blow?

You mean current burst about 45mA, right? So the maximum input current into the boost converter would be about 93mA if Vin of converter is 2V. if the input capacitor is very small. this current will go through the battery, diode and fuser. this no only damage the fuser, but generate high power loss and the would triiger converter UVLO if the battery is in low voltage. 

a large input cap can avoid this problem. the large the input capacitance, the smaller this burst current go through the battery. the ideal condition would be all burst current is from input cap, so only less than 20mA current flow through battery and fuser.

you can calculate the input capacitance based burst time and maximum battery current  or other requirements, and simulate and experiment on the EVM.  

Just my curiousity, why use a 80mA fuser? thanks

 Jasper li

I've just got the EVM to play with.

I've taken the jumper that connects the LED, off. And I've powered my system from the output pins of the EVM. I've got an LED on my system that shows if it is powered on. The light keeps flickering. Does the length of the wire to the input of the EVM board matter? I've used about 8cm wire between the fuse and the EVM board for testing purpose. The good news is that it doesn't blow the fuse at all.

The EVM is configured to be current limited to 100mA I suppose (through the 2K resistor). And since current is not drawn over 80mA continuously it is not enough to blow the fuse. There is an input capacitor of 150uF on the EVM. Do you think increasing that to a 220uF or so solve the problem?

The 80mA fuse needs to be there as per our certifying body for intrinsic safety. There are two reasons for this:
1) 8 Ohm resistance will current limit
2) The system must not draw more than 80mA (a short circuit for example).

Chris

Update:

I just disconnected the switching regulator and connected a 2K resistor as load to see the voltage drop across each of the "set in stone" components. It looks like its negligible! The voltage i supplied with the battery was 3.6V. Across the diode it drops to 3.45V, across the 250mA fuse it drops to 3.45V still and across the big 80mA fuse is 3.44V.

I don't understand why the buck boost or the boost is still struggling to provide 3.3V for such a light load. Any thoughts would be appreciated.

Reminder of the circuit:

     |-- 3.6V Li Battery -------Diode----250mA Fuse-----|
     |                                                                                      |
     |-- 3.6V Li Battery -------Diode----250mA Fuse-----|-----80mA fuse----Switching regulator --Load
     |                                                                                      |                                                                          |
     |--  3.6V Li Battery -------Diode----250mA Fuse----|                                                                          |
     |                                                                                                                                                              GND
GND

Hi Chris

what is part number of the diode used? and what the minimum load of your system and your output load. could you measure the input votlage and output votlage waveform when abnormal phenomenon is observed. thanks

jasper li

The diode used for reverse charging is BAT760 from NXP. I'm not sure what the load is in Ohms. When it is running normally (say, from a 3.3V power source), it consumes a little less than 15mA and once a second a Zigbee transmission will pump the current drawn to about 45mA. The battery powers an MSP430, some FRAMs, a 433MHz transmitter (switches on every 2 seconds for about 10ms), 2.4GHz zigbee transmitter, few op amps and passive devices. I'm not sure how to decide the minimum load and output load from this.

If I use any of the boost or the buck boost converters, the voltage at the input pin is seen to be "ringing" - goes up to 3V and drops down to 1.7V and this keeps repeating. I may be able to attach a waveform later today.

Chris

this may cause by the voltage across input diode and fuser when zigbee transmission. what is frequency of the "ringing"? increasing the input cap may help to solve the problem? and  could you also measure the load current if possible. thanks

jasper li

Well, now there is no ringing. The voltage just drops down to 1.8V or so and does not power up my system. I have added a massive 150uF at the output and still doesn't help.

As mentioned earlier, the load current cant creep up to 45mA and as low as 10-15mA.

The bizarre part is that without the regulator and with all the fuses and diodes in place, the system powers up okay! Without the fuses and diodes, and with the regulator, system powers up okay as well. Both together doesn't!

Could you provide the scope waveform of battery voltage, input votlage of TPS61260, output voltage of TPS61260, and load current(using current probe) if possible. thanks

jasper li

Scope waveform of battery not connected to anything:

Battery connected through fuses and diodes to the load:

Input to TPS61261 with fuses and diodes and load:

Output of TPS61261 with fuses and diodes and load:

Input to TPS61261 without fuses and diodes but with load:

Output of TPS61261 without fuses and diodes but with load:

I do not have a current probe to provide u with the waveforms.

P.S: I use TPS61261 EVM, not TPS61260.

P.P.S: Detaching the Zigbee Modem makes the system work, as you suspected. But the TPS61261 can power up happily with the Zigbee modem and without the fuses and diodes. That does not make sense.

Chris.

i think the problem is clear. 

1. when TPS61261 without fuses and diodes but with load

the input voltage is 2.76V and stable, in the rang of operating voltage, so the device work well.

2. when TPS61261 with fuses and diodes and load

the input voltage of TPS61261 drop down to 0.5V periodically, out of the operating voltage, so the device stop working periodically. and the output voltage can't be stable at 3V to power your system. 

i think the problem is caused by high resistor of diode and fuser. you must make sure that the input voltage of TPS61261 is in the operating rang. 

by the way, your system seem to sink a high current every 200ms, not 1s as you said.

thanks

jasper li

What solutions can I look at, to stabilise the supply voltage? Add more capacitance to the input? It is 150uF at the input right now. May be add another 100uF? Any use adding more capacitance at the output?

The sinking of high current is less than 1s because that is the Zigbee trying to join the base station. But is unsuccessful since the circuitry switches off briefly and starts the process all over again.So, that is the node trying to join the network.

It is still unclear why the voltage drops to 0.5V.

This works

     |-- 3.6V Li Battery ------------|
     |                                               |
     |-- 3.6V Li Battery ------------|----- --Load
     |                                               |              |                                                             
     |--  3.6V Li Battery -----------|               |
     |                                                          GND
GND

This also works:

     |-- 3.6V Li Battery --------|
     |                                         |
     |-- 3.6V Li Battery ------- |-----Switching regulator --Load
     |                                         |                                               |
     |--  3.6V Li Battery -------|                                               |
     |                                                                                     GND
GND

This also works

     |-- 3.6V Li Battery -------Diode----250mA Fuse-----|
     |                                                                                      |
     |-- 3.6V Li Battery -------Diode----250mA Fuse-----|-----80mA fuse-----Load
     |                                                                                      |                                     |
     |--  3.6V Li Battery -------Diode----250mA Fuse----|                                     |
     |                                                                                                                        GND
GND

But this does not:

     |-- 3.6V Li Battery -------Diode----250mA Fuse-----|
     |                                                                                      |
     |-- 3.6V Li Battery -------Diode----250mA Fuse-----|-----80mA fuse----Switching regulator --Load
     |                                                                                      |                                                                          |
     |--  3.6V Li Battery -------Diode----250mA Fuse----|                                                                          |
     |                                                                                                                                                              GND
GND

That seems confusing to me. Why does adding resistance on the line via diodes and fuses throw the regulator off its normal functionality, coz clearly both works well independently?