# TPS61088 - Design and testing

#### Dikxita

Joined Apr 22, 2024
27
I have used TPS61088 IC in one of my circuit design for boosting. This IC have a resistor divider circuit between Vout and FB pin as shown in the below circuit. Since I want an output of 5V, so with respect to below circuit using voltage divider formula in this case Vout =5V, R1= 769Kohms, Voltage at FB pin = 1.204(given in datasheet) i got R3= 243Kohms(using voltage divider formula that is Vout = (Vs*R2)/(R1+R2) by substituting Vout = 1.204V, Vs= 5V, R1=768Kohms as per below circuit). But when i practically measured after receiving designed board, Im getting 10V at Vout pin. Please help me to know why I'm getting10V instead of 5V.

#### Ian0

Joined Aug 7, 2020
10,270
Where do V1, V2 and V3 connect?

#### Dikxita

Joined Apr 22, 2024
27
Where do V1, V2 and V3 connect?
About V1, V2, V3 no information is mentioned in datasheet. Hence I have just connected a 2pin connector to measure voltage at that point. Also i guess, V1 ,V2 ,V3 are the voltages mentioned under output voltage/output current column in the below table. I'm not sure though.

#### ericgibbs

Joined Jan 29, 2010
19,091
Hi Dikx.
This App datasheet may help.
E
I would consider the V1,2,3 refer to the three suggested Vouts

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#### Ian0

Joined Aug 7, 2020
10,270
About V1, V2, V3 no information is mentioned in datasheet. Hence I have just connected a 2pin connector to measure voltage at that point. Also i guess, V1 ,V2 ,V3 are the voltages mentioned under output voltage/output current column in the below table. I'm not sure though.
View attachment 322438
What voltage do you measure on the feedback pin?
If you have Vref (1.204V), then the circuit is functioning properly, and your voltage divider is wrong.
if you have a different voltage, then the circuit is not running properly.

#### Dikxita

Joined Apr 22, 2024
27
What voltage do you measure on the feedback pin?
If you have Vref (1.204V), then the circuit is functioning properly, and your voltage divider is wrong.
if you have a different voltage, then the circuit is not running properly.
I measured voltage at Fb pin ,its 1.22V
Plz check the table below its from TPS61088 datasheet. I have made MODE pin short to ground, hence its in PWM mode. So I'm getting max Vref voltage with respect to below table. So the circuit is working fine right and the problem is with voltage divider?

I'm using programmable DC power supply. I have set Vin = 3V and Iin = .5A

#### Dikxita

Joined Apr 22, 2024
27
Hi Dikx.
This App datasheet may help.
E
View attachment 322442I would consider the V1,2,3 refer to the three suggested Vouts
Hey, can you please tell me why 120K pull down is required/used at FB pin in the datasheet u shared, will it not also affect in generating output voltage at Vout pin?

#### Ian0

Joined Aug 7, 2020
10,270
I measured voltage at Fb pin ,its 1.22V
Plz check the table below its from TPS61088 datasheet. I have made MODE pin short to ground, hence its in PWM mode. So I'm getting max Vref voltage with respect to below table. So the circuit is working fine right and the problem is with voltage divider?
View attachment 322455
I'm using programmable DC power supply. I have set Vin = 3V and Iin = .5A
So your divider is the problem.
Output will be:
Vref*(1+R1/R2)
which gives 9V.

#### Dikxita

Joined Apr 22, 2024
27
So your divider is the problem.
Output will be:
Vref*(1+R1/R2)
which gives 9V.
Could you please elaborate on how 9V will get?

#### ericgibbs

Joined Jan 29, 2010
19,091
hi D,
This shows the value of Vfb for the 3 combinations ie: 1.2V

E

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#### Ian0

Joined Aug 7, 2020
10,270
Could you please elaborate on how 9V will get?
Vref=1.2V
1.2*(1+768000/120000)= ??

#### ericgibbs

Joined Jan 29, 2010
19,091
hi Dik,
I was following the App example that has the C2, 1000pF across the top resistor
The combination of the 49.9R and the 1000pF form a simple bypass filter.
E

Added: BW plot for the C1 effect.

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#### Dikxita

Joined Apr 22, 2024
27
Vref=1.2V
1.2*(1+768000/120000)= ??
Yes, Understood. Thank you so much.
Currently i don't have 120K resistor with me. Can i use R1 as 715K and R2 as 110K?

#### Dikxita

Joined Apr 22, 2024
27
hi Dik,
I was following the App example that has the C2, 1000pF across the top resistor
The combination of the 49.9R and the 1000pF form a simple bypass filter.
E
View attachment 322467
Added: BW plot for the C1 effect.
View attachment 322469
Thanks Eric. I haven't made a provision for bypass filter. Is it important that I include? If i don't include will it affect my output and circuit?

#### Ian0

Joined Aug 7, 2020
10,270
Yes, Understood. Thank you so much.
Currently i don't have 120K resistor with me. Can i use R1 as 715K and R2 as 110K?
Yes, if you want 9Volts.
This is a good utility for calculating resistive dividers.

#### ericgibbs

Joined Jan 29, 2010
19,091
hi Dik,
Based on the Vfb you will set Vout at ~9.7V with a110K.
E

#### Dikxita

Joined Apr 22, 2024
27
Yes, if you want 9Volts.
This is a good utility for calculating resistive dividers.
Thanks Ian. I'm still stuck at V1,V2,V3 and those unknown resistor(R3,R4 and R5) in the circuit. What's the purpose or intension of including V1,V2,V3 and R3,R4 ,R5? Could please help me to understand?

#### ericgibbs

Joined Jan 29, 2010
19,091
I haven't made a provision for bypass filter. Is it important that I include? If i don't include will it affect my output and circuit?
Hi D,
By bypassing the 768K resistor with the 1000pF the response to high going Vout spikes, it will increase the Vfb response, thus pull down the Vout faster to the desired Vout.
Personally, I would not include the 49.9R and 1nF
E

#### Dikxita

Joined Apr 22, 2024
27
hi Dik,
Based on the Vfb you will set Vout at ~9.7V with a110K.
E
Yes understood Eric but I haven't made a provision for bypass filter. Is it important that I include? If i don't include will it affect my output and circuit?