MIC2584-XBTS datasheet(23/28 Pages) MICREL

23 / 28 page

March 2005

MIC2584/2585

Micrel

the opposite direction. Here, the worst-case maximum cur-

rent is found using a 57.5mV trip voltage and a sense resistor

that is 3% low in value. The resulting equation is:

57.5mV

0.97 R

59.3mV

LOAD(CONT,MAX)

SENSE(NOM)

()

(12)

As an example, if an output must carry a continuous 6A

without

nuisance

trips

occurring,

Equation

yields: R

41.3mV

SENSE(MAX) ==

Ω

688

. The next lowest

standard value is 6m

Ω. At the other set of tolerance extremes

for the output in question,

59.3mV

6.0m

LOAD(CONT,MAX) =

Ω

= 988

. Knowing this final da-

tum, we can determine the necessary wattage of the sense

resistor using P = I2R, where I will be ILOAD(CONT, MAX), and

R will be (0.97)(R

SENSE(NOM)). These numbers yield the

following: P

MAX = (9.88A)

2 (5.82m

Ω) = 0.568W. In this ex-

ample, a 1W sense resistor is sufficient.

MOSFET Selection

Selecting the proper external MOSFET for use with the

MIC2584/85 involves three straightforward tasks:

• Choice of a MOSFET which meets minimum

voltage requirements.

• Selection of a device to handle the maximum

continuous current (steady-state thermal is-

sues).

• Verify the selected part’s ability to withstand any

peak currents (transient thermal issues).

MOSFET Voltage Requirements

The first voltage requirement for the MOSFET is easily stated:

the drain-source breakdown voltage of the MOSFET must be

greater than V

IN(MAX). For instance, a 12V input may reason-

ably be expected to see high-frequency transients as high as

18V. Therefore, the drain-source breakdown voltage of the

MOSFET must be at least 19V. For ample safety margin and

standard availability, the closest minimum value will be 20V.

The second breakdown voltage criterion that must be met is a

bit subtler than simple drain-source breakdown voltage, but is

not hard to meet. In MIC2584/85 applications, the gate of the

external MOSFET is driven up to approximately 20V by the

internal output MOSFET (again, assuming 12V operation). At

the same time, if the output of the external MOSFET (its source)

is suddenly subjected to a short, the gate-source voltage will go

to (20V – 0V) = 20V. This means that the external MOSFET

must be chosen to have a gate-source breakdown voltage of

20V or more, which is an available standard maximum value.

However, if operation is above 12V, the 20V gate-source

maximum will likely be exceeded. As a result, an external Zener

diode clamp should be used to prevent breakdown of the

external MOSFET when operating at voltages above 10V. A

Zener diode with 10V rating is recommended as shown in

Figure 12. At the present time, most power MOSFETs with a

20V gate-source voltage rating have a 30V drain-source break-

down rating or higher. As a general tip, choose surface-mount

devices with a drain-source rating of 30V as a starting point.

Finally, the external gate drive of the MIC2584/85 requires a

low-voltage logic level MOSFET when operating at voltages

lower than 3V. There are 2.5V logic level MOSFETs available.

See Table 5, "

MOSFET and Sense Resistor Vendors" for

suggested manufacturers.

TX_/RX

VDD

CORE SUPPLY

(VCC)

OUTPUT

DRIVER

CIRCUIT

BLOCK

OUTPUT

DRIVER

CIRCUIT

BLOCK

Data In

Data Out

External

Bus Control

I/O

Dual Supply DSP

Peripheral

CORE

I/O SUPPLY

(VDD)

Data In

Data Out

Figure 11. Bidirectional Port Bus Contention

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