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# LM5160A: current limit operation

Part Number: LM5160A
Other Parts Discussed in Thread: LM5160

LM5160A has 3 current limit threshold.

I would like to know about each current limit operation.

1, High side current limit
I think high side FET turns off immediately and Low side FET turns on when high side FET peak current exceeds this threshold during high side ON period.
High side FET OFF and low side FET ON period is decided due to Vin and FB voltage.
Is this understand correct?

2, Low side sourcing current limit
Which timing current is checked? I think low side source current decreases with time. And low side source current peak point will be low side FET turns ON timing.
So I think low side source current does not exceed this threshold because there is high side current limit function.
How does this function operate?

3, Low side sinking current limit
I think both high side FET and low side FET turn off when low side sink current exceed this threshold.
Is this understand correct?
If it is yes, I would like to know how long is this off period. I think output voltage is higher than set point when sink current becomes large.
So does off period become the time until FB voltage falls below reference voltage level?

Best Regards,
Kohei Sasaki

• Sasaki-san,

See answers in bold font inserted below.

--

Tim

quote user="K. Sasaki14"]

Part Number: LM5160A

LM5160A has 3 current limit threshold.

I would like to know about each current limit operation.

1, High side current limit
I think high side FET turns off immediately and Low side FET turns on when high side FET peak current exceeds this threshold during high side ON period.
High side FET OFF and low side FET ON period is decided due to Vin and FB voltage.
Is this understand correct?

[TH] Yes, correct. 2.5A is the typical peak current limit threshold. See section 7.3.7 of the LM5160 datasheet for more detail.

.

2, Low side sourcing current limit
Which timing current is checked? I think low side source current decreases with time. And low side source current peak point will be low side FET turns ON timing.
So I think low side source current does not exceed this threshold because there is high side current limit function.
How does this function operate?

[TH] There isn't a sourcing current limit of the low-side FET per se. However, there is a zero-cross detection of low-side FET current that turns off this FET when the inductor current reaches zero. This is only applicable when FPWM is tied low to enable discontinuous conduction mode (DCM) at light loads. See sections 7.3.9 and 7.4.1 of the LM5160 datasheet for more detail.

3, Low side sinking current limit
I think both high side FET and low side FET turn off when low side sink current exceed this threshold.
Is this understand correct?
If it is yes, I would like to know how long is this off period. I think output voltage is higher than set point when sink current becomes large.
So does off period become the time until FB voltage falls below reference voltage level?

[TH] The negative current limit of 5.4A typical (i.e. current flowing into the SW pin from SW to GND) is generally intended for Fly-Buck converter operation, whereby positive current on the secondary output is reflected negatively by the transformer to the primary side. So, this negative current limit protects the secondary-side output in case of an overcurrent. The off-time subsequent to a current limit event is given by equation 5 in the LM5160 datasheet.

Best Regards,
Kohei Sasaki

[/quote]

• Tim-san,

About Low side sinking current limit,
Do both high and low side FET turn off when LM5160A detects this limit?

Best Regards,
Kohei Sasaki

• Sasaki-san,

Yes, the low-side FET turns off (the high-side FET was already off). The current transitions from the low-side FET to the high-side FET's body diode.

Regards,

Tim

• Tim-san,

I have one question about LM5160 operation.
I tested with attached file circuit for reproduction of device operation which is observed at our customer.

So I could observed waveform like DCM operation.
From waveform, I am thinking this is low side sink current limit. However off time is not match with your comment.

20190709_LM5160A operation.pptx

Best Regards,
Kohei Sasaki

• Sasaki-san,

Yes, it may be a fixed delay like what you have measured. I will confirm and get back to you.

Regards,

Tim

• Tim-san,

It may not be fixed delay because about 140ms off time was observed at our customer's board.

Best Regards,
Kohei Sasaki

• Tim-san,

Best Regards,
Kohei Sasaki

• Sasaki-san,

The converter should restart when the FB voltage reaches the threshold. This may be affected by the RCC ripple injection circuit, in particular the coupling capacitor time constant with the feedback resistors.

Regards,

Tim

• Tim-san,