LM7705: Supply Voltage

Takashi Yamaguchi

Genius 13840 points

Part Number: LM7705

Hi team,

My customer is considering to use LM7705 + OPAMP

Could you answer for the question below?

Is it ok to apply separate power supply for VDD of LM7705 and +V of OPAMP?

For example, apply +5V to VDD of LM7705, and +13V to +V and -0.23V to -V of OPAMP.

Regards,

Yamaguchi

over 6 years ago

0 Dorian Brillet de Cande over 6 years ago

TI__Genius 14876 points

Hi Yamaguchi-san,

The LM7705 is designed to supply -0.23V with low ripple to the negative supply pin of an amplifier. If your Amplifier is rated to handle this Positive Voltage Supply then your customer should not have any particular concerns.

Thank you very much for reaching out to us.

Best Regards,

Dorian

0 Takashi Yamaguchi over 6 years ago in reply to Dorian Brillet de Cande

TI__Genius 13840 points

Hi Dorian-san,

I received additional questions from customer. Could you help with the following questions?

1. Customer is considering to use following circuit. Could you let me know if there is any problem?

/cfs-file/__key/communityserver-discussions-components-files/196/LM7705.ppt

2. Customer would like to know the input current for the LM7705 when there is load. Customer measured the supply current for the opamp(6pcs) and it was 11.5mA average. Therefore, LM7705 is sinking 11.5mA(ave) from Vout pin.

Is the following calculation of input current correct?

Pout would be -0.251V(min) x 11.5mA=2.9mW. According to the Fig.10 in the d/s, the efficiency would be more than 95%, so Pin would be 2.9mW/0.95=3.1mW.

Iin would be Pin/5V + Iq=3.1mW/5V + Iq=0.62mA + 240uA = 0.86mA.

Regards,

Yamaguchi

0 Dorian Brillet de Cande over 6 years ago in reply to Takashi Yamaguchi

TI__Genius 14876 points

Hi Yamaguchi-san,

I would recommend this approach for your system:

Vin being +13V and Iout/10 because in this configuration you have a ratio of 10 between input and output.

Using the Zener diode as protection only. Protection if Vin goes over 5.25V.

2. Going back to your example, Iin would be :

Iin=Pin/5V=3.1mW/5V=0.62mA

Do those information enough for your customer?

Thank you!

Regards,

Dorian

0 Dorian Brillet de Cande over 6 years ago in reply to Dorian Brillet de Cande

TI__Genius 14876 points

Hi Yamaguchi-san,

I haven’t heard back from you.
Do you have any follow up questions?
Otherwise I will assume I can close this thread. If not, just post a reply below .

Thank you,

Dorian

0 Takashi Yamaguchi over 6 years ago in reply to Dorian Brillet de Cande

TI__Genius 13840 points

Hi Dorian-san,

Thank you for your support. This issue has been resolved.

Regards,
Yamaguchi

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LM7705: Supply Voltage