[LTC2974] About multi-LTC2974 connectivity.

Question

Hi ADI support team. There are some questions about multi-LTC2974 application. Could you help give some suggestion. 
 1. For multi-LTC2974 usage, Can VDD33 of the all LTC2974s be tied together as followed figure? 
 
 2. I don&rsquo;t understand the Figure 38. DC1613 Controller Connections When VPWR Is Used in the LTC2974 datasheet. There is a note of REPEAT OUTLINED CIRCUIT FOR EVERY LTC2974 in this picture, what does this note mean? Could you help clarify it? There must be two NTS4173P and the resister dividers for each LTC2974 ??? I wonder how I can connect the second LTC2974&rsquo;s VDD33 to the DC1613 header?

mpeters · Accepted Answer

Hi patsburg, 
 1. Do not tie VDD33 pins of different devices together. Each VDD33 is an LDO output and they may fight each other. 
 2. When 12V supply is powered down, you may power multiple 2974s from an external 3.3V supply. Figure 38 shows the example of the DC1613 USB dongle which has a 3.3V supply on the ribbon cable. Each 2974 must have its own PFET. Each PFET drain is tied to the dongle, each PFET source is separately tied to a 2974's VDD33. The resistor divider is only needed if the PFET's Vgs is exceeded. There are many FETs that have an 8V Vgs limit, so we added a divider to the FET's gate to reduce the gate voltage when 12V is applied. You only need one divider to drive all PFETs. Our diagram needs updating. The DC1613's 3.3V supply can source up to 100mA, which limits the number of devices to about 7. 
 The NTS4173P has a +/-12V Vgs max spec. You might use a divider that has two 100k resistors to limit the gate voltage to 6V. 
 Mike

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[LTC2974] About multi-LTC2974 connectivity.

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