TPS61088: How to calculate passive component values for loop stability (at COMP pin) ?

saiteja manchukanti

Part Number: TPS61088

Hello,

I am using TPS61088 in an application. My application requirements are as shown below.

Input voltage (V)	3 - 4.2
Output voltage (V)	6.6
Max Output Current (A)	1.5

I have few queries in selecting passive components for loop stability (at COMP pin). Please tell the values to be selected for GEA(Amplifier’s transconductance) and D (switching duty cycle) to calculate for passive components value.

Please find the datasheet link for your reference (http://www.ti.com/lit/ds/symlink/tps61088.pdf)

Regards.

over 6 years ago

0 Helen chen over 6 years ago

TI__Mastermind 40080 points

Dear Sir,

Sorry for the confusion for the equation on the datasheet

Please use gm to replace GEA*REA .

And for this trans-conductance amplifier, the equation on the datasheet is not clear. Please use the following equation:

1) Control stage: Gc=gm*Zcomp à Zcomp is the impedance of the compensation network. as C13 is much smaller than C8, so finally :

, the formula in the datasheet directly use f_comz, f_comp1 , f_comp2

2) Power stage (Dm is duty, Rsense=0.08, Cototal is the effective output capacitance, Lm is the inductance)

3) Dividing network : Gdiv=Vref/Vo

Multiply equation (1), (2),(3) together, you can get the total loop gain.

So here, your interested formula is :

f_comz=1/(2ᴨR8C8),

f_comp1=1/(2ᴨR8C13),

f_comp1=1/(2ᴨReaC8),

the value of Rea is very big (5Mohm).

0 saiteja manchukanti over 6 years ago in reply to Helen chen

Intellectual 330 points

Dear Helen Chen,

Thanks for your prompt reply. I am a newbie to DC-DC boost converters. I don't completely understand about loop stability of Boost Converters.

Can you please give me the equations for calculating R5, C5 and C8 at COMP pin?

In R5 equation, please tell the values to be chosen to calculate D (switching duty cycle) and GEA (amplifier's transconductance).

Regards.

0 Helen chen over 6 years ago in reply to saiteja manchukanti

TI__Mastermind 40080 points

Dear Sir,

You can found the formulas from datasheet page17-18. GEA =0.00019 A/V.

0 saiteja manchukanti over 6 years ago in reply to Helen chen

Intellectual 330 points

Dear Helen Chen,

Thanks for your response. Also please tell the value of D (switching duty cycle) to be used in the formula.

0 Helen chen over 6 years ago in reply to saiteja manchukanti

TI__Mastermind 40080 points

Dear Sir,

D is determined by the Vin,Vo. Vo=Vin/(1-D), with this formula, you can calcuated out the D.

0 saiteja manchukanti over 6 years ago in reply to Helen chen

Intellectual 330 points

Dear Helen Chen,

As per above equation, D value changes as input voltage varies.

In our application input varies from 3V to 4.2V, Can you suggest the value of Vin to calculate D?

Regards.

0 Helen chen over 6 years ago in reply to saiteja manchukanti

TI__Mastermind 40080 points

Dear Sir,

You can use Vin=3.6V.

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TPS61088: How to calculate passive component values for loop stability (at COMP pin) ?