ATL431LI-Q1: application information

Hi Trailokya

I am the senior hardware engineer from KOSTAL China, and Ethan is our support FAE.

According to your question, let me explain more details about them.

• the expected output voltage  --- 5V
• if there is continious load current or user will be switching it --- We use it as LDO on board and should not switched off, the maximum load current 50mA
• if the short circuit only comes at the startup or there is any other short circuit condition? --- Not for startup beacuse no large bulk capacitor on the output, we just worried if output short to ground on board, the darlington transistor would be burned because there is no short current protection.

We use ATL431LI as LDO output due to its low quiesent current, 80uA maximum in cathode current. As a contrast, 1mA in cathode current for TL431.

If using the circuit that you mentioned, for example, 50mA output capability, in sleep mode, the quiesent current is still 50mA.

That's why we use the combination of ATA431LI and darlingtion transistor. It is better if there has darlingtion protecion in case short current happened. 

Hi Qilong 

Thanks for clarification. Can you please tell me the supply voltage. Is it a DC DC or battery.   I have couple of comments 

• The easiest sollution (with some performance limitation) will be to add a seriese resistor (R2 as shown in the figure) from supply 
• 

This method can be used only when  below condition satisfies

V(BAT)/ (MAX_IC_BJT ) < Rs < 80mA  

I have given 30 mA margin. You can tweek the variation. 

• Other solutions require couple of transistors and stability gets challanging in discreet implementation. 

Can I ask what is the reason you are not using a crude LDO at this stage ? Is it cost or any other reason? 

Regards

Trailokya 

Hi Trailokya,

Thanks for your serveral solutions.

As you mentioned, It is a battery supply voltage, if add a series resistor between supply and darlington, R2 power consumption is much higher in case Vout short to ground.

For example, VBAT is 9-16v, Vout is 5V and minmum 50mA output current, the saturation of dalington transistor is ~1.4V dropout, so maximum R2= (9-5-1.4)/50mA= 52ohm.

If Vout is short to ground, in the worst case VBAT is 16V, the power consumption R2= (16-1.4)*(16-1.4)/52= 4.1W.

It is hard for us to control the base current of darlington transistor accurately to limit the maximum output current beacuse the hfe gain of darlington transistor is at lease 6000~10000 typical.

The reason we use discret component beacase it is a backup power supply on board, and certainly the cost of crude LDO is expensive than ATL431LI. So the crude LDO is our alternative.

I am interested in your second comment, how to implemente the protection with couple of transistors?