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# ISOW7842: About output short circuit current

Part Number: ISOW7842

Hello support team,

I have some questions about "Output short circuit current" in ISOW7842.
I want to know how much current flows from VISO to GND when VISO is shorted to GND.
This question assumes in case that another device connected to the secondary side of ISOW7842 has failed and it's power supply has been short-circuited to GND.
Off course I understand that the primary current of ISOW7842 is ICCSC = 137 mA (TYP).

We experimented with a sample on hand.
When VCC = 5 V, VISO = 0 V, SEL = ‘H ' : ICCSC = 119 mA, IISO (OUT) SC = 175 mA
When VCC = 5 V, VISO = 0 V, SEL = ‘L’ : ICCSC = 119 mA, IISO (OUT) SC = 175 mA
(IISO(OUT)SC : the current on VISO pin when VISO and GND are short-circuited)

1. Is IISO (OUT) SC the current of 175 mA appropriate?
2. Is this current determined by the drivability of the secondary circuit?

Best regards,
M. Tachibana

• Hi Tachibana-san,

Thank you for posting to E2E! Please share a schematic of the setup used to capture these measurements: we do not expect Iiso_(out)_SC, the current from Viso when Viso and GND2 are shorted, to be greater than Icc_sc, the supply current into Vcc, at the same time.

Respectfully,
Manuel Chavez

• In reply to Manuel Chavez:

Hi Chavez-san,

The circuit diagram I experimented with is shown below.
The sample I had was only ISOW7841, so I measured using it.

I guess ISOW7842 will have the same result.

Sincerely,
M. Tachibana

• In reply to Masanori Tachibana:

Hi Tachibana-san,

You're welcome! These measurement values are not expected because Icc needs to be greater than Iiso due to the DC/DC transformer efficiency.

If they are switched, where Icc_sc = 175mA and Iiso(out)_sc = 119mA, then these values would be more appropriate.

Current output on Viso is limited during short circuit conditions, so the output current would be determined by the limited current value and DC/DC converter efficiency.

Thank you,
Manuel Chavez

• In reply to Manuel Chavez:

Hi Chavez-san,

The measured values of ICC_SC (= 119 mA) and IISO(OUT)_SC (= 175 mA) weren't replaced each other. Validated by a few of our members.

I think that the transfer from the primary side to the secondary side should be considered by power.
The power input to the primary side is 5 V x 119 mA = 595 mW.
Since VISO = 0 V, the secondary circuit inside the IC would be lower than 5 V.
Therefore, I guess IISO(OUT)_SC would be larger than 119mA.
However, I don't know the power efficiency when VISO = 0V, so I asked this question.

Sincerely,
M. Tachibana

• In reply to Masanori Tachibana:

Hi Chavez-san,

I roughly measured the relationship between IISO(OUT) and ICC or VISO using ISOW7841 on hand.
The circuit I measured is as below.

The measurement result is as follows,

1. VCC = 5 V, VISO = 5 V

When IISO(OUT) was less than approximately 240 mA, VISO kept about 5 V, and the efficiency seemed to be 50 ~ 60%.
When IISO(OUT) exceeded about 240mA, VISO droped sharply, and IISO(OUT) was limited to 175mA. ICC also droped to 119mA.

2. VCC = 5 V, VISO = 3.3 V

Sincerely,
M. Tachibana

• In reply to Masanori Tachibana:

Hi Tachibana-san,

Thank you for sharing these details! Our design team and lab data confirm the measurements and behavior you've shared are expected during short-circuit protection. I apologize for the confusion on my end.

Respectfully,
Manuel Chavez

• In reply to Manuel Chavez:

Hi Chavez-san,

Thank you for confirming with your design team.
And I appreciate your always kind support.

Sincerely,
M. Tachibana

• In reply to Masanori Tachibana:

Hi Chavez-san,

Excuse me, I'll ask you again just in case.
In conclusion, can I understand that IISO(OUT)_SC we measured is reasonable?
And is it difficult to estimate the efficiency of the internal transformer when VISO is short-circuited to GND? Because the secondary side is below the UVLO voltage and it is not operating properly.

Sincerely,
M. Tachibana

• In reply to Masanori Tachibana:

Hi Tachibana-san,

You're welcome! The statements in your last post are correct.

Measurements I captured under the same conditions for ISOW7841 are Icc_sc = ~112mA and Iiso(out)_sc = ~156mA.

Thank you,
Manuel Chavez

• In reply to Manuel Chavez:

Hi Chavez-san,

Thank you for measuring for the express purpose.
I understand that IISO(OUT)_SC is a current approximately our measured value, and that it is difficult to analyze in detail.