LMC7660: LMC7660 Schematic Review

Tom Tsao

Expert 1840 points

Part Number: LMC7660
Other Parts Discussed in Thread: TMUX7234, TL431

Tool/software:

Dear TI support team,

Please help me review the schematic of the LMC7660.

Thanks!

5 months ago

0 Clemens Ladisch 5 months ago

Guru 316440 points

This will not work because the voltage divider does not leave any current for the charge pump or for any other devices connected to the ±10 V supplies.

What is the actual problem you're trying to solve? Why do you need ±10 V, and how large is the load?

0 Aurora Tie 5 months ago

TI__Genius 16740 points

Hi Tom,

Pls double check the application with customer at first. Looking forward to your reply. Thanks.

Aurora

0 Tom Tsao 5 months ago in reply to Aurora Tie

Expert 1840 points

Hi Aurora and Clemens,

My application uses the LMC7660 to generate a -10V power supply for the TMUX7234, with a current of approximately 10mA.

Can I use a 10V Zener diode to clamp Vin at 10V to generate a -10V output? Or do you have any other recommended solutions?

Thanks!

+1 Clemens Ladisch 5 months ago in reply to Tom Tsao

Guru 316440 points

You can use a Zener, but then you have to remove R801. R800 must be sized to provide enough current for the Zener, the LMC7660, and the TMUX7234.

+1 Aurora Tie 5 months ago in reply to Tom Tsao

TI__Genius 16740 points

Hi Tom,

You can refer to Clemens' suggestion. Thanks.

Aurora

0 Tom Tsao 5 months ago in reply to Clemens Ladisch

Expert 1840 points

Hi Clemens,

The Zener diode I have selected has the following specifications:

Assuming the LMC requires an output current of 10mA,
Plus the Zener diode's current consumption of 5mA,
The resistor value is calculated as:

R = (14V - 10V) / 5mA = 267Ω

Is this suitable?

Thanks!

0 Clemens Ladisch 5 months ago in reply to Tom Tsao

Guru 316440 points

The LMC itself might need up to 400 µA. (14 V − 10 V) / (5 mA + 400 µA + 10 mA) ≈ 259 Ω.

Where do you get that 10 mA value from? The TMUX7234's supply current is much less than 1 mA, and independent from the current that flows through the switches.

(The Zener voltage will vary with current, so if you care about a stable output voltage or want to save power, better use the TL431.)

0 Aurora Tie 5 months ago in reply to Tom Tsao

TI__Genius 16740 points

Hi Tom,

Pls see below update. Thanks.

Aurora

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LMC7660: LMC7660 Schematic Review