UC1825A: Output stage dropout and maximal operating frequency

christian fallouh

Part Number: UC1825A

Hello,

I have additional questions related to UC1825A. Considering the output drives high impedances (it is buffered by other dedicated drivers) :

- What is the expected high voltage level dropout ? I understand from the datasheet that it might be up to 2.9V @ 20mA, what is the value at ~0mA ?

- What is limiting maximal switching frequency ? The datasheet mentions practical operation up to 1MHz, but rise time and fall time are specified at maximum 45ns (typ 20ns) on 1nF load and oscillator seems to be characterised up to 3MHz.

- Does "practical operation up to 1MHz" refer to oscillator frequency or to the switching frequency on each output (I guess there is a ratio of two between both frequencies).

Best regards,

Christian

over 3 years ago

0 Hong Huang over 3 years ago

TI__Guru 50415 points

Hi,

If at 0 mA, OUTA or OUTB will be about Vc - Vce where Vce should be about 0.5V.

It is 1 MHz. There are limiting factors inside IC.

1 MHz (switching frequency) is on each output, the oscillator frequency can be about 2 MHz (Figure 2).

0 christian fallouh over 3 years ago in reply to Hong Huang

Prodigy 120 points

Hi,

Thank you for your kind reply.

Is it possible to know what is the limiting factor inside the IC ? I would like to guess what kind of limitation I may face around 1MHz.

Best regards,

Christian

0 Hong Huang over 3 years ago in reply to christian fallouh

TI__Guru 50415 points

Hi,

The recommendation is to have Dmax > 70% on page 6 based on the IC internal design. With 70% Dmax, the oscillator frequency is at about 2MHz, so switching frequency is up to 1MHz.

0 christian fallouh over 3 years ago in reply to Hong Huang

Prodigy 120 points

Hi,

By using Rt = 3 kOhm and Ct = 220 pF, you have 2MHz oscillator frequency and maximal duty cycle = 90%.

I don't see the issue with this.

Christian

0 Hong Huang over 3 years ago in reply to christian fallouh

TI__Guru 50415 points

HI,

Yes, but that still gives you 1MHz switching frequency, so still in line with up to 1MHz claim.

0 Hong Huang over 3 years ago in reply to Hong Huang

TI__Guru 50415 points

Also, note some design needs smaller Dmax instead of 90%. The smallest suggests 70% which will give you 2MHz for oscillator and 1MHz for switching frequency.

0 christian fallouh over 3 years ago in reply to Hong Huang

Prodigy 120 points

Ok but 90% is far from 70%, you can decrease resistance to 2kOhm for example which gives 3MHz oscillator frequency and Dmax = 86% which is still over 70%.

So the only limitation is on duty cycle ?

0 Hong Huang over 3 years ago in reply to christian fallouh

TI__Guru 50415 points

Hi,

Yes, to claim a practical frequency with 70% Dmax., we would have to say up to 1MHz.

Figure 2 provides typical range of frequency setup, you may refer to that for your application.

Please also note, your MOSFETs turn on/off time may be varied longer than the datasheet assumed 45ns + 45ns, rise and fall. when you consider the maximum switching frequency and Dmax

Power management

Power management forum

UC1825A: Output stage dropout and maximal operating frequency