LM5066 datasheet(21/63 Pages) TI

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LM5066

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SNVS655G – JUNE 2011 – REVISED FEBRUARY 2013

Refer to Programming Guide section: After all hardware design is complete, refer to the programming guide

for a step by step procedure regarding software.

MOSFET SELECTION

It is recommended that the external MOSFET (Q1) selection is based on the following criteria:

- The BVDSS rating should be greater than the maximum system voltage (VIN), plus ringing and transients which

can occur at VIN when the circuit card, or adjacent cards, are inserted or removed.

- The maximum continuous current rating should be based on the current limit threshold (for example, 26 mV/RS

for CL = VDD), not the maximum load current, since the circuit can operate near the current limit threshold

continuously.

- The Pulsed Drain Current spec (IDM) must be greater than the current threshold for the circuit breaker function

(48/96/193 mV/RS, depending on CL and CB configuration).

- The SOA (Safe Operating Area) chart of the device, and the thermal properties, should be used to determine

the maximum power dissipation threshold set by the RPWR resistor. The programmed maximum power dissipation

should have a reasonable margin from the maximum power defined by the MOSFET’s SOA curve. If the device

is set to infinitely retry, the MOSFET will be repeatedly stressed during fault restart cycles. The MOSFET

manufacturer should be consulted for guidelines.

- RDS(on) should be sufficiently low such that the power dissipation at maximum load current (ILIM

2 x R

DS(on)) does

not raise its junction temperature above the manufacturer’s recommendation.

- The gate-to-source voltage provided by the LM5066 can be as high as 16.5V, limited by the internal zener

diode from GATE to OUT. Q1 must be able to tolerate this voltage for its VGS rating. An additional zener diode

can be added from GATE to OUT to lower this voltage and limit the peak VGS. The zener diode’s forward current

rating must be at least 110 mA to conduct the GATE pull-down current when a circuit breaker condition is

detected.

CURRENT LIMIT (RS)

The LM5066 monitors the current in the external MOSFET Q1 by measuring the voltage across the sense

resistor (RS), connected from VIN to SENSE. The required resistor value is calculated from:

(1)

where ILIM is the desired current limit threshold. If the voltage across RS reaches VCL, the current limit circuit

modulates the gate of Q1 to regulate the current at ILIM. While the current limiting circuit is active, the fault timer is

active as described in the Fault Timer & Restart section. For proper operation, RS must be less than 200 mΩ.

VCL can be set to either 26 mV or 50 mV via hardware and/or software. This setting defaults to use of CL pin

which, when connected to VDD is 26 mV, or grounded is 50 mV. The value, when powered, can be set via the

PMBus with the MFR_SPECIFIC_DEVICE_SETUP command, which defaults to the 26 mV setting.

Once the desired setting is known, calculate the shunt based on that input voltage and maximum current. While

the maximum load current in normal operation can be used to determine the required power rating for resistor

RS, basing it on the current limit value provides a more reliable design since the circuit can operate near the

current limit threshold continuously. The resistor’s surge capability must also be considered since the circuit

breaker threshold is 1.94 or 3.87 times the current limit threshold.

Connections from RS to the LM5066 should be made using Kelvin techniques. In the suggested layout of

Figure 7 the small pads at the lower corners of the sense resistor connect only to the sense resistor terminals,

and not to the traces carrying the high current. With this technique, only the voltage across the sense resistor is

applied to VIN_K and SENSE, eliminating the voltage drop across the high current solder connections.

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