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LM5170-Q1: Circuit Breaker

ksma
Intellectual 290 points
Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

Hello,

Please tell me about the circuit breaker of LM5170.

The circuit diagram is shown in Figure 33 of data sheet p30.(48->14V : BUCK)

When the breaker is turned on, voltage is generated at LV_Port with no load (approximately 4V) even if the EN signal is turned off.

Is this normal behavior? (Effect of breaker MOS VGS voltage?)

At this time, the voltage between BRKG_BRKS is 12V. The voltage between BRKG_GND was 16V.

Another board generates a voltage of about 8V on the LV_Port.

Is this LV_Port voltage difference within the fluctuation range?

At this time, the voltage between BRKG_BRKS is 12V. The voltage between BRKG_GND was 20V.

The voltage between BRKG_BRKS is specified in the data sheet.

The voltage between BRKG_GND is not specified.

What is the normal voltage range between BRKG_GND?

Thank you very much.

  • 0
    TI__Guru* 78060 points

    Hi Ksma,

    Thank you for your interests in the LM5170.  According to the datasheet, the EN does not control the circuit breaker.  The breaker is controlled by UVLO pin.  EN just sets off switching.  

    When the breaker is turned on, BRKG to BRKS voltage, namely the breaker FET Vgs, will be VIN pin voltage minus the 12V-port voltage, which is internally clamped at about 12V (namely the maximum is ~12V). Because BRKS will basically at the 12V port voltage, so BRKG can move with the 12V port voltage.

    Hope this clarifies.

    Thanks,

    Youhao Xi, Applications Engineering. 

  • 0 in reply to Youhao Xi
    Intellectual 290 points

    Hello Youhao Xi.

    Thank you for your reply.

    Excuse me. The expression of the question was bad.

    Attach the schematic.

    The purpose of the question is to:

    Even if EN is off (that is, the main MOS is off), when the breaker starts（UVLO⇒H),

    Voltage is generated at LV_Port.

    Q1: Is this a normal operation?

    Q2: How is the voltage of BRKG determined when EN is off?

    Best regards

  • 0 in reply to ksma
    Intellectual 290 points
    Excuse me.
    The schematic is not attached and will be added.
  • 0 in reply to ksma
    TI__Guru* 78060 points

    I see you question now.  Sorry for the misunderstanding. 

    When you see 4V or 8V, they are all normal.  Please refer to the block diagram of the IC in the datasheet. There is 300uA current source at BRKG to turn on the breaker FETs.  There is also a 12V Zener between BRKG and BRKS.  When the Zener starts clamp the BRKG-BRKS voltage at 12V, current will flow through the Zener and get out of BRKS, which can pull out the 12V-port is it is an open circuit (no load).  The voltage built at the 12V-port depends on the external impedance, so you see different voltages. This is normal and if you add a load >300uA you will see that the 12V port voltage drops to ground.

    Hope this clarifies.

    Youhao