DC to DC switcher help

- R
- Richard Sloan
  
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posted
19 years ago

Mon, May 24, 2004 5:16 PM

I am using the RN5RK series DC-DC from Ricoh, right now the device is putting out a ripple voltage at a frequency of around 40Khz, which interferes with one of the IC's in the cirucit that has a 38Khz reference. How do I move this 40Khz higher, the IC states 100Khz operation, but is this variable with voltage in? current out? The data sheets leave me with a lot of questions.

I have 2xAA in, and currently 5.0V out. I am drawing maybe 30mA

Here is a small blurn from Ricoh: The RN5RKxx1A/xx1B consist of an oscillator, a VFM control circuit, a driver transistor to have low ON resistance (Lx switch), a reference voltage unit, a high speed comparator, resistors for voltage detection, an Lx switch protection circuit and an internal chip enable circuit.

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I have another circuit with 2xAA in and 3.3V out and draws 2mA and it ripple freq is around 2500Hz!

Please post here in news group for all to learn!

Richard.

- R
- rickman
  
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posted
19 years ago

Mon, May 24, 2004 8:37 PM

Both of these units sound like they are older technology. But ripple noise is always a problem with switchers. If you only need 30 mA or so, you can use a voltage doubler followed by an LDO to give you very clean power. Check at TI and Maxim for switched capacitor power converter chips. I believe someone makes a doubler with a built in LDO, maybe Linear Tech.

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- R
- Raider of the Lost Electron
  
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posted
19 years ago

Mon, May 24, 2004 8:38 PM

this

voltage

switch

ripple

I hazard a guess that the use of "chopper" in this context has the same meaning as the "chopper" did in my textbook days of old (have I run into this - oh yes;-)

The RN5RK has a fixed dutycycle and switching frequency, and the only way to regulate the output is to turn the oscillator or power drive completely off. This means that the ripple frequency as seen due to the (guessed) hysteresis of the comparator will be dependent on the load current. One way to increase the frequency of ripple is to decrease the input voltage - then the chopper will need to be running at a higher frequency. Try with one AA only.

HTH

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Morten
Raider of the Lost Electron