MIC2593 datasheet(21/26 Pages) STMICROELECTRONICS

21 / 26 page

April 2004

M9999-042204

MIC2593

Micrel

Application Information

Current Sensing

For the three power supplies switched with internal MOS-

FETs (+12V, –12V, and V

AUX), the MIC2593 provides all

necessary 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 make sensing the

current inside the MIC2593 impractical. Therefore, each of

these supplies, 3V[A/B] and 5V[A/B], requires an external

current sensing resistor. The V

IN connection to the IC from

each supply (e.g., 5VINA) is connected to the positive termi-

nal of the slot’s current sense amplifier, and the correspond-

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

the negative terminal of the current sense amplifier.

Sense Resistor Selection

The MIC2593 uses low-value sense resistors to measure the

current flowing through the MOSFET switches to the loads.

These sense resistors are nominally valued at

50mV/I

LOAD(CONT). To accommodate worst-case tolerances

for the sense resistor (allow

±3% over time and temperature

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 (i.e., the “trip point”) for the

MIC2593 may be as low as 35mV, which would equate to a

sense resistor value of 35mV/I

LOAD(CONT). Carrying the

numbers through for the case where the value of the sense

resistor is 3% high yields this:

35mV

103 I

34mV

SENSE

LOAD CONT

= ( )(

)

()

Once the value of R

SENSE has been chosen in this manner,

it is good practice to check the maximum I

LOAD(CONT) which

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

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

rent is found using a 65mV trip voltage and a sense resistor

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

65mV

(0.97)(R

)

67mV

LOAD(CONT, MAX)

SENSE(NOM)

As an example, if an output must carry a continuous 4.5A

without nuisance trips occurring, R

SENSE for that output

should be 34m

Ω/4.5A = 7.55mΩ. The nearest standard value

is 7.5m

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

At the other set of tolerance extremes for the output in

question, I

LOAD(CONT,max) = 67mV/7.5mΩ = 8.93A. Knowing

this final datum, we can determine the necessary wattage of

the sense resistor, using P = I2R, where I is ILOAD(CONT, MAX),

and R is (0.97)(RSENSE(NOM)). These numbers yield the

following:

MAX = (8.93A)

2(7.28m

Ω) = 0.581W

A 1W sense resistor would work well in this application.

Kelvin Sensing

Because of the low values of the sense resistors, special

attention to the layout must be used in order for the MIC2593's

circuit breaker function to operate properly. Specifically, the

use of a 4-wire Kelvin connection to measure the voltage

across R

SENSE is highly recommended. Kelvin sensing is

simply a means of making sure that any voltage drops in the

power traces connecting to the resistors does not get picked

up by the traces themselves. The Kelvin connections should

be isolated from all other signal traces to avoid introducing

noise onto these sensitive nodes. Additionally, a high-fre-

quency noise filter across the sense inputs is highly recom-

mended to avoid nuisance tripping of the (overcurrent) circuit

breaker on the opposite slot to the slot that incurred an

overcurrent event. Due to the variation of each system's

susceptibility to noise, the exact value of this filter is experi-

mentally determined. A value between 10pF to 100pF is a

good starting point.

Figure 12 illustrates how Kelvin sensing is performed. All the

high current in the circuit (from the 5V supply through R

SENSE

and then to the drain of the 5V (Slot A) output MOSFET) flows

directly through the power PCB traces and R

SENSE. The

voltage drop resulting across R

SENSE is sampled in such a

way that the high currents through the power traces will not

introduce any extraneous IR drops.

RSENSE

Power Trace

From 5VIN

Power Trace

To MOSFET Drain

Signal Trace

to MIC2593 VIN

Signal Trace

to MIC2593 VSENSE

MIC2593

22pF

Figure 12. Kelvin Sense Connections for R

SENSE

(Applicable to 5V[A/B] and 3V[A/B])

MOSFET Selection

Selecting the proper MOSFET for use as a current pass and

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

MIC2593 primarily involves three straightforward tasks:

Choice of a MOSFET which meets the minimum voltage

requirements.

Selection of a device to handle the maximum continuous

current (steady-state thermal issues).

Verification that the selected part can withstand any

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

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