LM5069 datasheet(20/22 Pages) NSC

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PC Board Guidelines

The following guidelines should be followed when designing

the PC board for the LM5069:

•

Place the LM5069 close to the board’s input connector to

minimize trace inductance from the connector to the FET.

•

Place a small capacitor (1000 pF) directly adjacent to the

VIN and GND pins of the LM5069 to help minimize

transients which may occur on the input supply line.

Transients of several volts can easily occur when the

load current is shut off.

•

The sense resistor (R

S) should be close to the LM5069,

and connected to it using the Kelvin techniques shown in

Figure 8.

•

The high current path from the board’s input to the load

(via Q1), and the return path, should be parallel and close

to each other to minimize loop inductance.

•

The ground connection for the various components

around the LM5069 should be connected directly to each

other, and to the LM5069’s GND pin, and then connected

to the system ground at one point. Do not connect the

various component grounds to each other through the

high current ground line.

•

Provide adequate heat sinking for the series pass device

(Q1) to help reduce stresses during turn-on and turn-off.

•

The board’s edge connector can be designed to shut off

the LM5069 as the board is removed, before the supply

voltage is disconnected from the LM5069. In Figure 17

the voltage at the UVLO pin goes to ground before V

SYS

is removed from the LM5069 due to the shorter edge

connector pin. When the board is inserted into the edge

connector, the system voltage is applied to the LM5069’s

VIN pin before the UVLO voltage is taken high.

System Considerations

A) Continued proper operation of the LM5069 hot swap

circuit requires capacitance be present on the supply side of

the connector into which the hot swap circuit is plugged in,

as depicted in Figure 2. The capacitor in the “Live Back-

plane” section is necessary to absorb the transient gener-

ated whenever the hot swap circuit shuts off the load current.

If the capacitance is not present, inductance in the supply

lines will generate a voltage transient at shut-off which can

exceed the absolute maximum rating of the LM5069, result-

ing in its destruction.

B) If the load powered via the LM5069 hot swap circuit has

inductive characteristics, a diode is required across the

LM5069’s output. The diode provides a recirculating path for

the load’s current when the LM5069 shuts off that current.

Adding the diode prevents possible damage to the LM5069

as the OUT pin will be taken below ground by the inductive

load at shutoff. See Figure 18.

20197253

FIGURE 17. Recommended Board Connector Design

LM5069 Datasheet(PDF) 20 Page - National Semiconductor (TI)

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