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I was planning to use the DCR022405 in a design, and made a 2 layer prototype board using the circuit in figure 4 of the datasheet (with only one converter). I input 28V, and regulated it down to 24V using an LM7824. There was no load. When I applied power, it smoked.
The only differences I can see between the datasheet recommendations and this prototype are that there is no ground plane for heatsinking, and that the caps used were not low ESR. Has anyone else had a similar experience?
The DCR022405 is not recommended for any application when the input voltage is absolute maximum limit is exceeded 29V or transient for any duration. The input power source during ramp-likely had a transient overshoot which exceeded the maximum voltage. The input capacitor must be a 2.2uF ceramic low ESR or the converter will oscillate and not function . Either condition will force the converter to be thermally overstressed. The in-rush current can be > 500mA from the LM7824. The input voltage wire if longer with add impedance and increased transient voltage If the LM7824 is hiccuping during power -up transition, this can also increase the power dissipation with thermal overstress and subsequent package failure.
Thanks for the quick reply. So you're saying that what most likely happened (if it wasn't strictly the fact that the capacitor was not low ESR) is that the regulator's output transient exceeded 29V for some period? If this is the case, how can I prevent it from happening in the future? Is it just a matter of increasing the capacitance on the output or adding a TVS, or should I just not use a linear regulator altogether?
Thanks for the e-mail. There are several concerns or reasons for this type of failure.
1. The IR losses associated with long wires or small power wires plus input power sources driven into current limit and oscillating can overstress the converter. This power source isolation oscillation can droop the 28V to lower voltages in the 18-13V range.
2. The 28V input power supply must decreased to an average of 24V to prevent voltage overstress on the converter. The absolute maximum voltage on the 24V converter is 29V. Use the input ceramic capacitors , as recommended
3. Add a soft start circuit on the input sync. pin , pin 18, which is also a on-off control pin.
Add a soft start control circuit which will delay the start-up of the DCR022405 for 7-10 msec or until the 24V bus is actively high.This delay will prevent the activation of the 24V bus current limit . Additional input capacitance 10-100uf may be required to hold-up the 24V source bus during the DCR012405 active switching transition .
See the attachement for delay and control of the on-off pin.
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