Understanding UCC2813-X VCC Abs. Max Rating and UVLO
In the UCC2813-X datasheet, the VCC pin has an absolute maximum rating of 12V with a UVLO turn-on threshold of 12.5V for UCC2813-2 and UCC2813-4. The UVLO turn-on threshold can be higher than the maximum rating to protect the Zener diode. To power up the UCC2813-2/4 safely, the external voltage source needs to be above 13.5V with a current limiting resistor to protect the Zener breakdown current. There is no strict upper limit on the external voltage source as long as the Zener breakdown current is well below 30mA. This principle applies to UCCx802/4 ICs as well.
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Explain on UCC2813-X VCC Abs. Max Rating and UVLO Turn on Threshold 1
Background In UCC2813-X datasheet, the VCC pin Abs. max rating is 12V UCC2813-2 and UCC2813-4 has a turn on threshold of 12.5V Why the UVLO turn on threshold can be above Abs. max rating and how to deal with it 2
Internal Structure of VCC Pin Powering IC from low impedance source VCC 7 VCC 7 VCC OK + VCC OK + Vzener Vzener UVLO_ON/ UVLO_OFF UVLO_ON/ UVLO_OFF 5 GND 5 GND Tolerances on these parameters (UCC2813-2 as example) As soon as external voltage source is above Zener breakdown voltage, without current limit, zener diode will get damaged Abs. max rating on VCC is based on the minimum Zener breakdown voltage Powering IC from high impedance source Min Nom Max UVLO_ON 11.5V 12.5V 13.5V VCC Vzener 12V 13.5V 15V 7 VCC OK + Vzener UVLO_ON/ UVLO_OFF Vzener-UVLO_ON 0.5V 1V 5 GND Zener break down voltage is always above UVLO_ON level Zener diode can be safely breakdown as long as the current can be limited by the impedance 3
How to Power Up the UCC2813-2/4 To ensure IC starts up normally, the external voltage source should be higher than the maximum UVLO_ON threshold of 13.5V Using external current limiting resistor for limit the Zener breakdown current VCC 7 Rs VCC OK + Vzener UVLO_ON/ UVLO_OFF Vs 5 GND ??=????? 12? 20?? Choose Rs based on the equation Vs(max) is the Vs maximum voltage Make sure during start up, Vs follow this equation ??> 13.5? + ?? 0.23?? 4
Summary Abs. max rating on VDD is the minimum Zener breakdown voltage Zener breakdown voltage is always above UVLO_ON threshold The UCC2813-2/4 should be powered up using voltage source well above 13.5V with current limiting resistor There is no voltage high limit on the external voltage source, as long as the Zener breakdown current can be limited to well below 30mA (design margin is required) The same theory can be applied to UCCx802/4 5
