MIC2593 datasheet(21/26 Pages) MICREL

21 / 26 page

Micrel, Inc.

MIC2593

September 2008

M9999-092208

Application Information

Current Sensing

For the three power supplies switched with internal MOS-

FETs (+12V, –12V, and VAUX), 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 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 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/ILOAD(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/ILOAD(CONT).

Carrying the numbers through for the case where the

value of the sense resistor is 3% high yields this:

()

LOAD(CONT)

SENSE

34mV

1.03

35mV

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 current is found using a 65mV trip

voltage and a sense resistor which is 3% low in value.

The resulting current is:

()

SENSE(NOM)

MAX)

LOAD(CONT,

67mV

0.97

65mV

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

without nuisance trips occurring, RSENSE for that output

should be 34mΩ/4.5A = 7.55mΩ. 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 for the

output in question, ILOAD(CONT,max) = 67mV/7.5mΩ = 8.93A.

Knowing this final datum, we can determine the

necessary wattage of the sense resistor, using P = I

2R,

where I is ILOAD(CONT, MAX), and R is (0.97)(RSENSE(NOM)).

These numbers yield the following:

PMAX = (8.93A)

2 (7.28mΩ) = 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

RSENSE

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-

frequency noise filter across the sense inputs is highly

recommended 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 experimentally 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

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

MOSFET) flows directly through the power PCB traces

and RSENSE. The voltage drop resulting across RSENSE is

sampled in such away 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 Sensing Connections for RSENSE

(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:

1. Choice of a MOSFET which meets the minimum

voltage requirements.

2. Selection of a device to handle the maximum

continuous current (steady-state thermal issues).

3. Verification that the selected part can withstand

any current peaks (transient thermal issues).

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