MIC2587 datasheet(13/16 Pages) MICREL

13 / 16 page

Micrel

MIC2587/MIC2587R

October 2004

M9999-102204

(408) 955-1690

the maximum hot swap load current (IHOT_SWAP(MAX)) which

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

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

found using a VTRIP(MAX) threshold of 55mV and a sense

resistor 3% low in value:

HOT_SWAP(MAX) =

55mV

0.97

×R

SENSE(NOM)

()

56.7mV

SENSE(NOM)

In this case, the application circuit must be sturdy enough

to operate over a ~1.5-to-1 range in hot swap load

currents. For example, if an MIC2587 circuit must pass a

minimum hot swap load current of 4A without nuisance

trips, RSENSE should be set to:

Ω

9.75m

39mV

SENSE(NOM)

where the nearest 1% standard value is 9.76m

Ω. At the

other tolerance extremes, IHOT_SWAP(MAX) for the circuit in

question is then simply:

HOT_SWAP(MAX) =

56.7mV

9.76m

Ω

= 5.8A

With a knowledge of the application circuit's maximum hot

swap load current, the power dissipation rating of the

sense resistor can be determined using P = I

× R. Here,

The current is IHOT_SWAP(MAX) = 5.8A and the resistance

RSENSE(MIN) = (0.97)(RSENSE(NOM)) = 9.47mΩ. Thus, the sense

resistor's maximum power dissipation is:

PMAX = (5.8A)

× (9.47mΩ) = 0.319W

A 0.5W sense resistor is a good choice in this application.

When the MIC2587/MIC2587R’s foldback current limiting

circuit is engaged in the above example, the current limit

would nominally fold back to 1.23A when the output is

shorted to ground.

PCB Layout Recommendations

4-Wire Kelvin Sensing

Because of the low value typically required for the sense

resistor, special care must be used to accurately measure

the voltage drop across it. Specifically, the measurement

technique across RSENSE must employ 4-wire Kelvin

sensing. This is simply a means of ensuring that any

voltage drops in the power traces connected to the

resistors are not picked up by the signal conductors

measuring the voltages across the sense resistors.

Figure 6 illustrates how to implement 4-wire Kelvin

sensing. As the figure shows, all the high current in the

circuit (from VCC through RSENSE and then to the drain of the

N-channel power MOSFET) flows directly through the

power PCB traces and through RSENSE. The voltage drop

across RSENSE is sampled in such a way that the high

currents through the power traces will not introduce

significant parasitic voltage drops in the sense leads. It is

recommended to connect the hot swap controller's sense

leads directly to the sense resistor's metalized contact

pads.

The

Kelvin

sense

signal

traces

should

symmetrical with equal length and width, kept as short as

possible and isolated from any noisy signals and planes.

Additionally, for designs that implement Kelvin sense

connections that exceed 1” in length and/or if the Kelvin

(signal) traces are vulnerable to noise possibly being

injected onto these signals, the example circuit shown in

Figure

can

implemented

combat

noisy

environments.

This circuit implements a 1.6 MHz

low-

pass filter to attenuate higher frequency disturbances on

the current sensing circuitry. However, individual system

analysis should be used to determine if filtering is

necessary and to select the appropriate cutoff frequency

for each specific application.

Other Layout Considerations

Figure 8 is a recommended PCB layout diagram for the

MIC2587-2BM.

Many hot swap applications will require

load currents of several amperes.

Therefore, the power

(VCC and Return) trace widths (W) need to be wide enough

to allow the current to flow while the rise in temperature for

a given copper plate (e.g., 1oz. or 2oz.) is kept to a

maximum of 10°C to 25°C. Also, these traces should be

as short as possible in order to minimize the IR drops

between the input and the load.

The feedback network

resistor values in Figure 8 are selected for a +24V

application. The resistors for the feedback (FB) and ON

pin networks should be placed close to the controller and

the associated traces should be as short as possible to

improve the circuit’s noise immunity. The input “clamping

diode” (D1) is referenced in the “Typical Application Circuit”

on Page 1.

If possible, use high-frequency PCB layout

techniques around the GATE circuitry (shown in the

“Typical Application Circuit”) and use a dummy resistor

(e.g., R3 = 0

Ω) during the prototype phase.

If R3 is

needed to eliminate high-frequency oscillations, common

values for R3 range between 4.7

Ω to 20Ω for various

power MOSFETs. Finally, the use of plated-through vias

will be needed to make circuit connection to the power and

ground planes when utilizing multi-layer PCBs.

MOSFET and Sense Resistor Vendors

Device types, part numbers, and manufacturer contacts for

power MOSFETs and sense resistors are provided in

Table 1.

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