TL783 Input voltage

Satya Someswara kaushik Y

Other Parts Discussed in Thread: TL783, TLV342, TL431

Hi,

I am designing a 5V to 100V dc/dc converter. I need a stable output of 100V with average load current being 0.1 mA. What is the typical dropout voltage of TL783?

Would there be any problem with power dissipation at higher dropout voltages in this case?

over 9 years ago

0 Ron Michallick over 9 years ago

TI__Guru*** 140136 points

Satya,

Typical drop out is figure 10 in data sheet, about 4V.
Minimum output current is figure 12 in data sheet, about 6mA (at low Vo-Vo)

Pd is (VIN-VOUT) * IOUT.

Does the 100V ground need to be isolated from the 5V ground?

0 Satya Someswara kaushik Y over 9 years ago in reply to Ron Michallick

Prodigy 100 points

What does the minimum current specification indicate?
1) current through resistors R1 and R2 + current through load or
2) only current through the load

Thanks

0 Ron Michallick over 9 years ago in reply to Satya Someswara kaushik Y

TI__Guru*** 140136 points

Satya,

In TL783 the output pin is also the 'ground pin' for all the internal bias currents. This bias current flows out of the output pin even with the main pass element (NMOS transistor) turned off. This minimum current can be consumed by the load or feedback resistors. If there is no where for this current to go, then the output will voltage will increase.

0 Satya Someswara kaushik Y over 9 years ago in reply to Ron Michallick

Prodigy 100 points

Ron,
My average load current is around 0.1mA. But the output required is 15 mA. What can be done for such low load currents?

0 Ron Michallick over 9 years ago in reply to Satya Someswara kaushik Y

TI__Guru*** 140136 points

Satya-san,

The feedback resistors can pass the required current.

For the resistor between output and adjust, use this formula.

R = 1.25V / 15mA = 83.33 ohms. Closest standard value is 82 ohms.

For resistor from adjust to ground, use this formula R = (100V/1.25V - 1) * 82 ohms = 6478. Standard value 6.49k
Power dissipation in resistor is 15mA^2 * 6490 = 1.46 watts!
You may use a power resistor or a series of low power resistors.

It seems the at efficiency is very low. What is your plan to boost the 5V to over 100V for the input to the TL783?
Could this system be regulated to provide 100V directly?

0 Satya Someswara kaushik Y over 9 years ago in reply to Ron Michallick

Prodigy 100 points

Yes, boost from 5V to 105V and then regulate the 105V to 100V using TL783. Is there any efficient option other than this? I didn't find any other regulators at this voltage.

0 Ron Michallick over 9 years ago in reply to Satya Someswara kaushik Y

TI__Guru*** 140136 points

Satya-san,

TL783 is only >100V regulator. We do have one 100V op-amp that could be used but input voltage is too high.

I threw together this TI Tina circuit. Basically the regulator is built using many parts.
The high voltage PNP and NPN isolate the low voltage integrated circuits from the high voltage input and output.

The transistor circuit needs a non-inverting error amplifier so I used TLV342 with a TL431 reference voltage.
Input current was 211uA for a 100uA output. This is much more efficient than 15mA input current.

I only lightly tested the circuit, power up transient and DC output voltage.

100V low current high voltage.TSC

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TL783 Input voltage