MIC2590B(2002) データシートの表示（PDF） - Micrel

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MIC2590B
(Rev.:2002)

Dual-Slot PCI Hot Plug Controller

Micrel 

MIC2590B

Application Information

Current Sensing

For the three power supplies switched with internal MOSFETs

(+12V, –12V, and VAUX), the MIC2590B provides all neces-

sary current sensing functions to protect the IC, the load, and

the power supply. For the remaining four supplies which the

part is designed to control, the high currents at which these

supplies typically operate makes sensing the current inside

the MIC2590B impractical. Therefore, each of these supplies

(3VA, 5VA, 3VB, and 5VB) requires an external current

sensing resistor. The VIN connection to the IC from each

supply (e.g., 5VINA) is connected to the positive terminal of

the slot’s current sense amplifier, and the corresponding

SENSE input (in this case, 5VSENSEA) is connected to the

negative terminal of the current sense amplifier.

Sense Resistor Selection

The MIC2590B uses low-value sense resistors to measure

the current flowing through the MOSFET switches to the

loads. These sense resistors are nominally valued at

50mΩ/ILOAD(CONT). To accommodate worst-case tolerances

for both the sense resistor, (allow ±3% over time and tem-

perature for a resistor with ±1% initial tolerance) and still

supply the maximum required steady-state load current, a

slightly more detailed calculation must be used.

The current limit threshold voltage (the “trip point”) for the

MIC2590B may be as low as 35mV, which would equate to a

sense resistor value of 35mΩ/ILOAD(CONT). Carrying the

numbers through for the case where the value of the sense

resistor is 3% high, this yields:

( )( ) RSENSE =

35mΩ

1.03 ILOAD(CONT)

= 34mΩ

ILOAD(CONT)

Once the value of RSENSE has been chosen in this manner,

it is good practice to check the maximum ILOAD(CONT) which

the circuit may let through in the case of tolerance build-up in

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

found using a 65mV trip voltage and a sense resistor which

is 3% low in value. The resulting current is:

ILOAD(CONT, MAX)

=

65mV

(0.97)(RSENSE(NOM) )

=

67mV

RSENSE(NOM)

As an example, if an output must carry a continuous 4.4A

without nuisance trips occurring, RSENSE for that output

should be 34mΩ/4.4A = 7.73mΩ. The nearest standard value

is 7.5mΩ, so a 7.5mΩ ±1% resistor would be a good choice.

At the other set of tolerance extremes,

ILOAD(CONT, MAX) for the output in question is then simply

67mV/7.5mΩ = 8.93A. Knowing this final datum, we can

determine the necessary wattage of the sense resistor, using

P = I2R. Here I will be ILOAD(CONT, MAX), and R will be

(0.97)(RSENSE(NOM)). These numbers yield the following:

PMAX = (8.93A)2(7.28mΩ) = 0.581W

A 1.0W sense resistor would work well in this application.

Micrel

Kelvin Sensing

Because of the low values of the sense resistors, special care

must be used to accurately measure the voltage drop across

them. Specifically, the voltage across each RSENSE must

employ Kelvin sensing. This is simply a means of making sure

that any voltage drops in the power traces connecting to the

resistors are not picked up in addition to the voltages across

the sense resistors themselves. If accuracy must be paid for,

it’s worth keeping.

Figure 9 illustrates how Kelvin sensing is performed. As can

be seen, all the high current in the circuit (let us say, from

+5VINA through RSENSE and then to the drain of the +5VA

output MOSFET) flows directly through the power PCB

traces and RSENSE. The voltage drop resulting across RSENSE

is sampled in such a way that the high currents through the

power traces will not introduce any extraneous IR drops.

Power Trace

From VCC

RSENSE

Power Trace

To MOSFET Drain

Signal Trace

to MIC2590B VCC

Signal Trace

to MIC2590B VSENSE

Figure 9. Kelvin Sensing Connections for RSENSE

MOSFET Selection

Selecting the proper MOSFET for use as current pass and

switching element for each of the 3V and 5V slots of the

MIC2590B involves four straightforward tasks:

1. Choice of a MOSFET which meets the minimum

voltage requirements.

2. Determination of maximum permissible on-state

resistance [RD-S(ON)].

3. Selection of a device to handle the maximum continu-

ous current (steady-state thermal issues).

4. Verification of the selected part’s ability to withstand

current peaks (transient thermal issues).

MOSFET Voltage Requirements

The first voltage requirement for each MOSFET is easily

stated: the drain-source breakdown voltage of the MOSFET

must be greater than VIN(MAX) for the slot in question. For

instance, the 5V input may reasonably be expected to see

high-frequency transients as high as 5.5V. Therefore, the

drain-source breakdown voltage of the MOSFET must be at

least 6V.

The second breakdown voltage criteria which must be met is

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

is not hard to meet. Low-voltage MOSFETs generally have

low breakdown voltage ratings from gate to source as well. In

MIC2590B applications, the gates of the external MOSFETs

are driven from the +12V input to the IC. That supply may well

be at 12V + (5% x 12V) = 12.6V. At the same time, if the output

of the MOSFET (its source) is suddenly shorted to ground,

the gate-source voltage will go to (12.6V – 0V) = 12.6V. This

MIC2590B

18

August 2002

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