Current VINDC

Hi Ryan. Thanks for your answer

The constant-voltage source, the one I mentioned the previous post, can deliver up to 400mA. 

So, my question is, Will always the BQ25504 draw 200mA from the source? , taking into account that my source has a higer current capacity?

Also I  have other question .  Why the inductor value must be 20 uH? Is there any method to estimate this value ?

Hello Jorge,

If you look at the bottom of Pg. 3 at Note 1 it states that the Max input power is 300mW. This means that you will need to think about how this device will operate in your system as a function of the power that it processes.

The power that it draws is a function of the load that is pulling power from the BQ25505 minus any power drawn from storage attached. What is the power consumption of your system and the voltage of this constant-voltage source?

The inductor is discussed in the design section of the datasheet on Pg. 17 and I have copied it below.

The overall size of this inductor is not that large since the current it is expected to carry is not very much. In fact if you look at the EVM the capacitor that is attached to the battery rail actually appears to be larger than the boosting inductor.

http://www.ti.com/tool/bq25504evm-674

The bq255xx family employs a hysteretic DCM boost converter.  The boost's power FET is only turned on if the output voltage droops below the VBAT_OV setpoint and only until the inductor current reaches 200mA, 300mA max.  The family is designed to be powered from high output impedance sources and use the MPPT feature to regulate VIN_DC to a set fraction of the sources open circuit voltage, that is sampled every 16s.  The 22uH inductor is recommendeded to accomodate the wide range of VIN to VOUT differentials that the IC might encounter.  Although not fully tested, larger inductances could be used but smaller inductances give higher current rise times (V=L*di/dt) which may result in the current measurement circuit being unable to measure the inductor current properly.

If your source is not high impedance and/or can provide more than 200mA typical, you can still use the bq255xx but it will not use all of the input power with each boost pulse.  In addition, if the source is low output impedance then you don't need the MPPT feature.  The MPPT feature can be disabled on the bq25504.

Hi Jeff. 

Thank for your answer.

You said the FET will remain ON until the inductor current reaches 200mA.  But not every energy source could provide that amount of current, so What will happen in this case ?

Basically, the device connected at VIN_DC is a buck-coverter with an output voltage of 1.8 V, hence the MPPT circuit is disabled. How much current will the BQ25504 draw from the previous source ?. I understood that once the inductor reaches 200mA the power FET will open, so no more current will be drawn from the source.

The boost power FET is on until either 200mA peak inductor (300mA max) is reached per cycle OR the input voltage droops to the MPPT set point if MPPT is enabled.  If MPPT is not enabled then the IC will attempt to draw 200mA inductor current.  

Thanks for your answer

Regarding the Pulse Frecuency Modulations (PFM) switching mode which is used  in BQ25504, I took the following picture from the EVM, my question is, Which frecuency is modulated, the frecuency between each 'burst' or the burst frecuency ?

Also, the datasheet has a parameter descibed as 'Boost converter mode switching frequency ', which its values is 1 MHz, Which frecuency is it?

The part starts a switching cycle if VSTOR drops below VBAT_OV.  It turns on the switch until either VIN_DC drops to the MPPT point or the power FET hits the peak current (200mA typical/300mA max).  If the input voltage does not drop, the fastest it will switch is 1MHz.  It is does not modulate frequency.  

Sorry, this is the picture I mentioned before 

If the IC does not modulate frecuency, what does PFM mean ?

PFM is used as a synonym for hysteretic operation.  It simply means the switching frequency will vary depending on the output voltage and output current until to some maximum switching frequency, in this case 1MHz.  The alternative is pulse width modulation with fixed switching frequency.

So the BQ25504 just varies the switching frecuency or also can varies the pulse width ? 

Now I have the following situation:

The source connected at VIN_DC maximum can deliver 100mA, and also the MPPT circuit is disabled (VSTOR tied to VOC_REF and VREF_SAMP tied to ground). So the power FET will remain on , When will it close ?

Finally . Is this documente reliable ?

http://powersoc2012.org/session-5/5.6_Ramadass.pdf

Thanks !

The frequency varies because it is a hysteretic converter.  The pulse width varies only if the MPPT circuit is active.

If the source can only deliver 100mA then you need to use the MPPT circuit.  Otherwise the bq25504 will collapse the source.

Regarding the document, yes.  Yogesh was the designer of the bq25504.

Hi !

I read all the previos posts and I have a question.

If i dont use the MPPT circuit the inductor current rise to 200 mA(300 mA MAX). How is the power efficiency under this condition? 

Efficiency of a boost converter drops as the VIN to VOUT differential increases.  So, if your input source droops because the boost charger is pulling more than its 100mA max output current, the boost charger's efficiency will be lower.

Hi Jeff

I've been thinking about my design, and I'm planing to use a load switch between BQ25504's VIN_DC and my power source (which is a fully charged capacitor), so I will only conect my source when its voltage its aceptable, and desconect it when has drob some level.

My doubt is that load switches could have inrush current at its ouput, but it happend just when the load have a capacitive component, something that does not happen in BQ25504.  So How could the load switch afect the current at VIN_DC ?

I think, maybe I'm wrong, that  this inrush current could be controlled by BQ25504's current sensor, which measures the current through th inductor, so once the current is 200mA the power FET will open and the inductor current will ramp to zero.   

PD: I'm not using the MPPT feature

Thank

Even if you disable the MPPT circuit's 16-s sampling of the input voltage by tying VOC_SAMP to VSTOR, you can still take advantage of the input voltage regulation feature by tying a fixed voltage reference to VREF_SAMP.  Then the IC will only allow current to be drawn from the input source if the voltage is above the voltage at VREF_SAMP.   The reference will waste a bit of power, however.

Regarding the inrush current from your source capacitor through a FET to the 4.7uF input capacitor of the 504, you can compute the amount using I=C*dv/dt for the 4.7uF capacitor.  The 504 pulls current from the capacitors until VIN_DC is below 100mV if VREF_SAMP=0V (or whatever voltage to which VREF_SAMP is set) or 200mA inductor current is reached.  

Hi Jeff. Thanks so much

Now I'm designing my PCB. So I've just read the datasheet and I have the following doubt concerning layout considerations. What does this sentence mean ?  

"To layout the ground, it is recommended to use short traces as well, separated from the power ground traces"

In my design I have two BQ25504 conected in parallel, and one VSTOR pin conected to a LDO input. I have already read the LDO layout considerations

Ideally, the grounds for the resistors and the capacitor on VREF_SAMP (analog grounds) will be routed away from the grounds of boost power stage input capacitor and output capacitor and storage element (power stage grounds).  I recommend that you look at the layout recommendations in the bq25505 datasheet and the bq25505 EVM layout.  

From the BQ25504 adn BQ25505 datasheet I can guess than the analog ground (Threshold resistor, Vref Capacitor), must be conected to AVSS pin, and que power grounds (input and output capacitor), should be conected to VSS pin.  However those two ground plane shoulds be conected at one PCB's point . Where can I conect those planes ?

Fron the EVM's top layers figures I saw that VSS pins and AVSS pin are conected through the  Thermal pad (The one in the middle) . But the BQ25505 Layout considerations says '' The PowerPad should not be used as a power ground return path "  

Is PowerPAD the same that Thermal pad ?  If not , What does the previous consideration mean ?

The thermal pad is the power pad.  We recommend that you do use wide ground traces and/or pours that provide a less resistive path to the power ground pin instead of relying on the ground under the power pad.  If you use separate ground traces/pours for analog ground and power gnd, tie them to the respective pins and then connect those pins under the IC to the power pad with traces, you should be okay.