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LM20K Family PowerWise Synchronous Buck Regulator Slide 18

This circuit gives an example of sequentially sequencing the output voltage using the precision enable pin. When the input voltage is applied, Device1 will stay off until the input exceeds the turn-on voltage dictated by REN1 and REN2. Once Device1 is enabled, the output voltage will start to rise. When VOUT1 exceeds the turn on point set by REN3 and REN4, Device2 will be enabled and VOUT2 will start to rise. Since the turn-on threshold of the precision enable is approximately 1.25V +/- 4%, it is possible to have both devices turn on at a very precise voltage. It is very different from traditional enable pins that typically turn on at a FET threshold voltage, which can have a large variation and a strong temperature coefficient. Using the precision enable circuit to sequence the device as shown here can also remove the need to use an external voltage supervisor to control the sequencing. The initial turn-on is set by REN1 and REN2 while the delay time from when VIN is applied and VOUT2 starts can be adjusted using CSS.

PTM Published on: 2011-11-02