A customer asked about some unexpected behavior they saw on their ADP1706-1.2 circuit. At no load, the customer noticed the glitch on VOUT that I show in the scope capture below. My team worked on a solution that we will post as a Reply. Below I show the schematic that was used to generate the above scope capture using the evaluation board. The 270uF capacitor, the 45mA current source and the CLOSED-TO-OPEN switch just generate a turn-off ramp on VIN that is similar to the customer's circuit behavior.

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ADP1706 Production
The ADP1706/ADP1707/ADP1708 are CMOS, low dropout linear regulators that operate from 2.5 V to 5.5 V and provide up to 1 A of output current. Using an...
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ADP1706 on Analog.com

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ADP1706-1.2 Brief High Output During Turn-Off at No Load

ARad

on May 13, 2024

Category: Hardware

Product Number: adp1706

A customer asked about some unexpected behavior they saw on their ADP1706-1.2 circuit. At no load, the customer noticed the glitch on VOUT that I show in the scope capture below. My team worked on a solution that we will post as a Reply.

LT1706-1.2 EVAL 3.3VIN to 1.2VOUT at no Load,VIN VOUT turn-off glitch_2.png

Below I show the schematic that was used to generate the above scope capture using the evaluation board. The 270uF capacitor, the 45mA current source and the CLOSED-TO-OPEN switch just generate a turn-off ramp on VIN that is similar to the customer's circuit behavior.

Edit Notes

Correct schematic.
[edited by: ARad at 3:16 PM (GMT -4) on 5 Jun 2024]

ADIApproved

on May 15, 2024 6:12 PM
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The results

The modification in the schematic below eliminated the glitch on the VOUT that is shown in the original scope capture above.
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BillQuach

on May 15, 2024 8:51 PM in reply to ADIApproved
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Description of the problem:

Note that in the original circuit, the EN pin was connected to the two IN pins. Therefore, the voltage on the EN pin is equals to the voltage on the IN pin (VEN = VIN).

When the switch is opened, both VEN and VIN are ramping down toward ground. When VEN and VIN dropped to near 1.5V, the input voltage VIN is below the minimum spec (2.5V), and VEN is in an undetermined state when VEN is between 0.4V and 1.8V. The spike occurs when the part is still enabled while VIN is dropped below VIN minimum.

Solution:

A single throw double poles (STDP) switch was added to separate VEN and VIN. Also, a 4.7KOhm pulldown resistor is connected to the EN pin. Note in the scope shot for the modified circuit, VEN (blue) was pulled down to ground immediately while VIN (red) is slowly ramping down, and no glitch appearing at VOUT (yellow).
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ARad

on Jun 5, 2024 5:56 PM
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The circuit below shows a practical way to prevent the unexpected behavior on VOUT. Just connect the enables together for the ADP1706-1.2 and its pre-regulator.
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