DRV110: impossible to regulate HOLD current in a solenoid valve driver

Hi Davide,

I have notified our expert regarding this topic. Please expect a response by 5/29/20.

Thanks,

Aaron

HI 

I'm stuck on this problem and need support to understand why I'm not able to regulate HOLD current

just to add some  details on the solenoid I'm using I try to characterize the current using a 230-V AC, 50-Hz supply and measuring voltage across 2.2 ohm shunt resistor

PEAK voltage is 3.56v

so the peak current is around 0.8 A

solenoid static measurements: 160ohm and 204mH

what are the correct settings for limiting resistor Rs and shunt resistor Rsense ?

Why  I can't regulate current with the EVAL board (duty always minimum 7-8%)

thanks

Davide

Thanks Costin for clarification, very useful indeed

few more questions just to be sure

Costin Cazana said:

Eval board Rs=1ohm, this will limit the max Ihold to 150mA. In hold mode the slew rate of the load current is fast due to high load supply.

I guess You refer to Rsense not to Rs (limiting resistor for Vin) right ?

I don't find in any application note /  design document a formula or indication on how to size the Rsense from the rated solenoid AC current / voltage

if Rsense= 1ohm  limit to 150maA hold current means that the internal Vref threshold is 150mv ??

In this condition Vsense will reach the internal comparator threshold faster than the min PWM duty cycle.

You can slow down Ihold slew rate by adding a series resistor to lower the voltage across solenoid inductance

OK I will try to add a resistor i series,  any suggested value  ??

is any difference if I place on drain or source mosfet side ?

If Ipeak needed is 0.8A, usually the hold current needed is 1/4 to 1/3 of the Ipeak: 0.2A to 0.25 A.

My suggestion is to replace the 1ohm eval board Rsense with 0.5ohm( solder 1ohm in parallel.). With Rsense=0.5ohm you have a wider range of both Ipeak and Ihold for your application

OK understood ...should I apply this modification together the previous one (add resistor in series) so both? or is an alternative?

Thanks again for the support

br

Davide

Hi Costin

thanks for the additional clarification but I'm still in the fog

I read several times data sheet and application notes but still don't understand how to size passive components for my application...for sure my limit and I kindly ask for your patience

there is also some misunderstanding on the terminology maybe...let's make a summary

here is the reference schema

Rs = limiting resistor ==> is dimensioned to regulate Vin voltage to 15v

the formula is the following

so the value is totally independent from the the solenoid characteristics 

In the EVAL board you can chose 200K or 300K 

Rosc ==> define the PWM frequency, clear no doubt , simple formula, I select a value to have 20Kzh

Ckeep ==> define the keep time, clear no doubt, value in the provided table

Rhold Rkeep ==> control the hold and keep current , formulas are clear enouh and depends on the Ipeak and Ihold current 

now let's go to not clear part

Rsense ==> current that flows in the coil cause a voltage drop on Rsense  (Vsense) that is than compared with itnernal Vref and at each clock cycle. 

The voltage at the output node (VOUT) becomes low when VSENSE ≥ VREF. The duty cycle (D) of the output voltage varies from 8% to 100%. In

I don't find any rule / formula to size the Rsense, in previous answer you told me that 1 ohm is too much and Vsense too high for the slew rate of my solenoid so the PWM duty is always 7%

any suggestion to size Rsense ?  I guess it depends on the Ihold current and Vref (??) but not clear to me how to calculate

Rlimit ==> is not part of the schema I understand the suggestion to use it in order reduce the voltage across the solenoid

Rlimit=( Vdc-Vsolenoid_target)/Ipeak

Not clear to me how you define Vsolenoid_target

The insertion point is the following ?

and finally should I use both chenages: reduce Rsense to 0.5ohm AND insert Rlimit ?

thanks

Davide