TPS61378-Q1: Consultation on TPS61378-Q1 design issues

Hi SHENG,

please refer to the calculation.

1. TPS61378 can meet the requirements.

A place to note that the frequency cannot be too high as the device may reach its minimum on time limit when Vin=12V and Vout=12.5V.

2. the load disconnect function keeps the output disconnect from input when EN is low. Also, the device can protect from damaging when output short happens.

The iso-fet will work as an LDO to ensure the device work like a 'buck' when output is lower than input, we call this down mode.

Please be careful that in down mode the device will decrease the current limit and easily get overheat.

3. yes. we recommend selecting a typical value of ripple current and calculate the inductance.

0245.TPS61378 Calculation Tool V1.1.xlsx

Best Regards

Fergus

HI.Fergus He.

Thank you for your reply.

1、Is the Ripple % parameter set by me? For example, I set it to 30% or 40%.

2、I read in the manual a description of the use of buck mode, and my use case is most of the time in the state of input 12V output 12.5V/250mA. If this is the case, the chip should work in LDO buck mode, I want to calculate the junction temperature at 85℃, P=UI=0.5V*0.25A=0.125W,TJ=85+0.125*46.2=90.775℃.
If the input voltage is 85% of the output voltage, P=UI=1.875V*0.25A=0.468W,TJ=85+0.468*46.2=106.65℃.
Am I calculating this correctly? Thanks.

Hi Sheng,

1. the ripple is preset. it is recommended to be 0.8A to 2A.

2. the calculation is not right.

If the device works in down mode (Vin>85% of Vo at 2.2Mhz), the efficiency will be low.

the OUT voltage will be 12+2V and output pin is 12.5V, so the voltage drop will be (14-12.5) V and 1.5V*0.25A=0.375W.

the loss on inductor and the two switches not considered.

 estimate the efficiency as 90%, and the loss will be 14V*0.25A*0.1=0.35W.

the total loss will be 0.725W.

If the device is out of down mode (Vin <85% of Vo at 2.2Mhz), you can estimate the efficiency as 90%, and the loss will be 12.5V*0.25A*0.1=0.3W.

then you can calculate the temperature through thermal resistance.

It is recommended to decrease the frequency to keep device be out of the down mode.

Best Regards

Fergus

HI.Fergus He.

Thank you for your reply.

1、What is the ripple preset if I output 250mA, 500mA,1A?

2、If you are operating in buck mode, do you still need to calculate the inductor and switching tube losses? At this time, the lower tube is working in the off state and the upper tube is working in the full conduction state, isn't this the case?

3、My product design requires the TPS61378 to operate in buck mode,Normally the input is 12V input, only in special cases it will be between 9~12V. if it operates in buck mode, what is the switching frequency set to? If I need to work in buck mode, what is the appropriate switching frequency setting?

Hi Sheng,

1. the ripple current is recommended to be 0.8A to 2A.

2. the upper mos is not full conducting, the out voltage is 2V higher than Vin, to ensure the lower mos does not work at minimum duty.

3. your input is 12V and output is 12.5V, and it is unnecessary to work in BUCK mode.

As it is mentioned before, the down mode threshold is defined by the minimum on time detecting,

that is, the device detects the on time reaches 70ns(0.15*1/2.2Mhz).

So, if you want the device work out of down mode, to ensure the on time larger than 70ns.

the duty can be calculated:1- Vin*eff/Vo=0.136, and when frequency is 1.9Mhz, the on time will be 70ns.

if you set the frequency higher than 1.9Mhz, the device will enter down mode at 12Vin and 12.5Vout.

contrarily, frequency below 1.9Mhz will keep the device out of down mode at the condition.

but please leave some margin as the calculation may differ from reality.

One more reminder, it is not recommended to work in down mode as the efficiency is low.

Best Regards

Fergus

HI.Fergus He.

1、My understanding of the third reply above is that if the input is 12V and the output is 12.5V, lowering the switching frequency appropriately will allow the chip to still operate in boost mode. Am I understanding this correctly?

2、 As long as the input voltage is not higher than the output voltage chip is able to work in boost mode by adjusting the operating frequency and other factors? Only when the input voltage is higher than the output voltage is it bound to enter buck mode?

Thank you very much for your support.