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I am so relieved to find someone else also thinking that......I have been staring at it and wondering from where it came....but the explanation there (just above equn 3) appears to say that the controller does indeed measure the duty cycle...and yes the zxld1370 does appear to adjust vsense in relation to duty cycle.Equation 3 seems to be pulled out of thin air
You will get nice rise and fall on the converter's input current, but not on the output current. But the mystery of the self-adjusting Vsense also makes me doubt any assumptions about its dynamics.The point of measuring the current like they do is that you can do it with hysteretic control which means no loop compensation needed....also, you get much less overshoot when flashing on and off, and also better response for pwm dimming of the leds. (greater dimming range)
Right, the datasheet presents many contradictions, and apparently several typos as well... generally I would avoid parts like this, unless they make a demo board that allows it behavior to be easily observed.I am now trying to also work out how the zxld1370 regulates the switching frequency to about 400KHz. (it says it does this) Surely, if the zxld1370 is monitoring the vsense voltage peak and troughs, then the inductor value determines the switching frequency, along with the input and output voltages?
....it surely will be the same for output current, I mean, the ZXLD1370 should operate wthout an output capacitor, as its hysteretic, and the sense resistor is meant to sense the full ripple in the inductor current , as it senses the peaks and troughs of the inductor current......thus surely it will be pretty devoid of output current overshoot, unlike what one might see using an alternative part such as LM3421...buckboost schematic on page 56 of LM3421 datasheetYou will get nice rise and fall on the converter's input current, but not on the output current
Thank you for contacting Diodes.
I have been passed your inquiry from Inquiries Europe.
Equation 4 shows that the LED current is independent of
input voltage and output voltage.
Equation 3 shows the Sense Resistor current IRS.
ILED is IRS*(1-D) so the (1-D) term in IRS is cancelled out
leaving ILED independent of D hence VIN and VOUT.
If VADJ=VREF then Equation 4 simplifies to
ILED=(VSENSE/RS)*GI_ADJ.
The input and output current are related by a RHPZ though. For example if you apply a negative step to the current setpoint and duty cycle, you will get overshoot on the output current. For example, if your converter is steadily operating at D=0.75, and you suddenly drop it to D=0, your input current will indeed slope down gracefully, but your output current will instantly quadruple before decaying down to zero. Hysteretic control doesn't make this phenomenon magically disappear. In practice, there is always a tradeoff between control bandwidth and overshoot.....it surely will be the same for output current, I mean, the ZXLD1370 should operate wthout an output capacitor, as its hysteretic, and the sense resistor is meant to sense the full ripple in the inductor current , as it senses the peaks and troughs of the inductor current......
The converter plant itself, when configured as a boost or buckboost, will have a RHPZ in the duty cycle to output current transfer function. That will never change.Surely Hysteretic converters don't have RHPZ's?
You would not get overshoot on the inductor current, since that is the current on which the hysteresis acts. But the dynamical relationship between the inductor current and LED/output current does not change. It's still the same old transfer function, RHPZ and all. I invite you to simulate it if you're skeptical.FvM's patent article tells that the ZXLD1370 regulates the peaks and troughs of the inductor current, such that it changes D to allow the ILED to stay the same when either vin or vout changes....as the datasheet etc says, the inductor current can never go above the level of [(average inductor current) + 15%]...so you can't get big overshoot , surely?
The gating function thing won't change whether the RHPZ is there or not (unless it happens to have a DSP built in). That function is just there to address steady state behavior, not dynamical behavior.sorry but I cannot simulate it because diodes.com don't give enough away about the "gating function" which they say does the control.
Of course there should be an output capacitor, every schematic in the datasheet even shows one. And they show it connected to ground, which seems like a terrible idea in the buckboost configuration... better to be put directly in parallel with the LEDs. And when we talk about overshoot, naturally we're referring to the average current per switching cycle, not instantaneous peak current.As you know, being hysteretic the ZXLD1370 based bukboost can have no output cap......in such case the peak led current can never go above the peak level of the inductor current.
I'm not even going to attempt to explain how their compensation works, given that the documentation simply doesn't give adequate information. But I don't think the controller will be unstable. I'm saying that the output current will have overshoot on it if you use the PWM dimming function in a boost or buckboost configuration.Regarding the compensation cap for the zxld1370, its just recommended to be 100p in all cases...so its obviously just for noise purposes...so realistically, there is no compensation network with an zxld1370 based buckboost, and surely a bukboost with an rhpz could never work without a significant compensation network?
The zxld1370 in buck mode definitely doesn't need an output capacitor, so why should the zxld1370 in buckboost mode need an output capacitor?......either way its just controlling the peak and trough points of the inductor current....and by the way, I think it does have a dsp (or similar) inside it.Of course there should be an output capacitor
Because sometimes datasheets say dumb and wrong things? LEDs don't like high peak:average current ratios.The zxld1370 in buck mode definitely doesn't need an output capacitor, so why should the zxld1370 in buckboost mode need an output capacitor?......either way its just controlling the peak and trough points of the inductor current....and by the way, I think it does have a dsp (or similar) inside it.
Regarding the output capacitor, page 16 says (under "application circuit design") that the output capacitor "if needed".....bla bla....so this proves that in bukboost mode the output cap is optional...otherwise, why would it say "output capacitor (if needed)"
The RHPZ isn't a function of the controller. There is literally nothing the controller can do to eliminate the RHPZ (except run the thing in DCM) in a CCM boost/buckboost converter.If the zxld1370'ssense resistor was always in the led current loop, then yes I would agree that it would suffer the rhpz...but it is not, it is in the same place as where it is when the zxld1370 is a buck converter...the zxld1370 doesn't know if its connected up as a buck or a bukboost, surely?
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