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LM5160: Any other factor impact switching frequency? What decides voltage start to switch?

Hung Ching Hsu
Expert 3086 points
Part Number: LM5160
Other Parts Discussed in Thread: TINA-TI

Hi Team 

My customer have two questions about LM5160. 

1. Any other factor impact switching frequency?

The customer thinks that only Ron and output voltage would have impact to switching frequency, but the deviation of switching frequency is larger than they

thought.

So the customer is wondering is there any other factor impact switching frequency?

2. What decides voltage start to switch?

The customer inputs voltage through enable pin, and the customer thought that the device will start to switch once the voltage exceeds Vth of enable pin.

But by confirmed on oscilloscope, it started to switch when input exceeds 10V. 

So the customer is wondering what decides voltage start to switch?

Thanks.

  • 0
    TI__Mastermind 29660 points

    Hello Hung,

    1. there are many first order and second order factors. Customer only has means to control Ron. Switching frequency should be effectively constant. Please share data if not so

    2. when device begins to switch is an AND function. When Vin rising threshold is exceeded and device En ising threshold is exceeded the device will begin to regulate..

  • 0 in reply to Marshall_Beck
    TI__Expert 3086 points

    Hi Marshall

    Thanks for the reply. 

    I've confirmed with the customer. 

    1. For SW frequency, the customer have parameter as shown. I think the SW frequency can be calculated by formula (1)

    shown in page 12 of datasheet, which is f=Vout/Ron. 

    So f should be 502kHz～517kHz, but after measured, the result was 524kHz～553kHz, higher than thought. 

    What may cause this happen? (BTW, Vin=24V±10% or 48V±15%)

    Vout :  min.16.79V，max.17.13V

    Ron：333kΩ±0.5%

    2. I think the description in 7.3.10 of datasheet is what you mentioned.

    "When the voltage at the VCC exceeds the VCC rising threshold, VCC(UV) = 3.98 V (typical), and the EN/UVLO voltage is greater than 1.24 V, normal switching operation begins."

    Regarding to the description above, the customer wants to know how "SW pin" will behave(which is in functional block

    diagram shown in 7.2.) when VCC(UV) < 3.98 V and EN/UVLO voltage<1.24V.

    He is asking this because the output voltage of LM5160 seems to drop dramatically when power is off.

    Does it work by turning low-side FET ON so output voltage is forced to drop quickly?

    And the customer is also wondering if there any measure can make output voltage decrease slowly?  

    Thanks.

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 29660 points

    Hello Hung this is curve fit equation...it won't be 100% accurate.

    IF power is removed or device disabled, the device will seize to switch. The output is not actively discharged.

    IF customer is interested in slow decay, add more output cap.

  • 0 in reply to Marshall_Beck
    TI__Expert 3086 points

    Hi Marshall

    Thanks for your reply. 

    As you said the formula " f=Vout/Ron" is curve fit equation...it won't be 100% accurate. 

    The customer is wondering if there is any way to calculate the frequency accurately? 

    I've checked quick start calculator for LM5160 but the formula there seems to be same as here. 

    And I am thinking if it is possible to get the value of frequency by simulation from TINA-TI, but I am not quite sure how to do it. 

    Could you kindly help with this if is possible? Thanks.

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 29660 points

    Hello,

    Accurate is relative.

    20kHz difference should not have much affect on avoid frequency band of interest.

    Alternatively, customer can characterize device to get more exact frequency.

    Lastly, customer can find device that is syncable to external clock to eliminate error of resistor.

    502kHz～517kHz, but after measured, the result was 524kHz～553kHz

  • 0 in reply to Marshall_Beck
    Prodigy 40 points

    Hi Marshall

    The switching frequency of this chip will change suddenly between 10V-30V.Why is this?

    Under normal circumstances, will only the duty cycle of the SW pin change?

    Thanks.

  • 0 in reply to qi zheng
    TI__Mastermind 29660 points

    To accommodate minimum on time and minimum off time of the device, frequency variation may be required. This frequency variation would occur, when max and min vin conditions are approached.