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# TPS61175: Current Limit

Part Number: TPS61175

Dear TI Team

I have a question about the Current Iimit of TPS61175.
In the data sheet of TPS61175, the minimum value of N-Channel MOSFET current limit is described as 3A, but if you look at the conditions, it is described as "FSW = 1.2 MHz (Rfreq = 80 kΩ), VIN = 3.6 V, TA = –40°C to +85°C".

If this is the following condition, will the minimum value of the N-Channel MOSFET current limit be lower than 3A?

・ Vin ＝ 2.9V to 18V
・ Fsw ＝ 200kHz to 2.2MHz
・ Ta ＝ -40 ℃ to 85 ℃
・ D = Dmax or less

I would like to know the worst case of N-Channel MOSFET current limit, so I would be grateful if you could let me know.

Best Regards,
Y.Ottey

• Hi Ottey,

The duty cycle will have an effect on the switch current limit. The switch current limit vs. duty cycle curve is shown in figure 4.

The input voltage, switching frequency has little impact on the switch current limit.

However, our device won't test at full Vin range, duty cycle range because it costs too much effort.

Please leave enough margin in your design to avoid triggering current limit.

Hope that helps.

• Hi Zack

I understand that the value of the Duty cycle changes the current limit.
Can the N-Channel MOSFET current limit fall below 3A when the duty cycle changes?
(If possible, I would be grateful if you could tell me the minimum value of the Current limit when the Duty changes.)

Regards,
Y.Ottey

• Hi Ottey,

May I know your application's Vin, Vout, Iout condition? What's the reason that you'd like to know this spec?

• Hi Zack

The conditions that our customers are considering are as follows.
Vin = 6.3V
Vout = 11.4V

I would like to know if the current limit threshold does not fall below 3A even if the input / output voltage and duty ratio change from the above conditions due to the effects of inrush current at startup.

Regards,
Y.Ottey

• Hi Ottey,

Thank you for the information.

Will the apply the 1A load current during the startup? If not, I think you don't have to be worried about it.

TPS61175 has soft start function which significantly reduces the start-up current spike and output voltage overshoot. When the IC is enabled, the voltage at the SS capacitor clamps the output of the internal error amplifier that determines the duty cycle of PWM control, thereby the input inrush current is eliminated.

• Hi Zack

Sorry for the lack of information.
There is almost no load at startup, and a load of about 1A flows by turning on the connected load (motor) during operation.
(At that time, inrush current may flow and the input / output voltage (Duty ratio) may change.)

I think that the duty ratio changes and the current limit threshold changes, which may cause the value to fall below the current limit minimum value of 3A described in the data sheet.
Therefore, please tell me whether the minimum value of the current limit threshold does not fall below 3A even if the duty ratio is changed.

Best Regardes,

Y.Ottey

• Hi Ottey,

For your application, the input is 6.3V, output is 11.4V, Iout is 1A, if setting the switching frequency at 600kHz, L=6.8uH, the calculated inductor peak current is around 2.3A. So it has 700mA margin.

The duty cycle in steady state is ~48%. Per below switch current limit VS duty cycle curve, during load transient, the duty cycle will increase so current limit increases a little bit. You don't have to worry about it.

• Hi Zack

I understand what you have written.