December 2003

13

M9999-122303

MIC2588/MIC2594

Micrel

Applications Information

4-Wire Kelvin Sensing

Because of the low value typically required for the sense

resistor, special care must be used to measure accurately the

voltage drop across it. Specifically, the measurement tech-

nique across each R

ing. This is simply a means of making sure that any voltage

drops in the power traces connecting 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 V

EE

through R

flows directly through the power PCB traces and R

voltage drop resulting across R

way that the high currents through the power traces will not

introduce any 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.

Power Trace

PCB Track Width:

0.03" per Ampere

using 1oz Cu

Power Trace

To MOSFET Source

Signal Trace

Signal Trace

to MIC2588/94 SENSE Pin

Note: Each SENSE lead trace shall be

balanced for best performance — equal

length/equal aspect ratio.

contact pads

Figure 6. 4-Wire Kelvin Sense Connections for R

SENSE

Protection Against Voltage Transients

In many telecom applications, it is very common for circuit

boards to encounter large-scale supply-voltage transients in

backplane environments. Because backplanes present a

complex impedance environment, these transients can be as

high as 2.5 times steady-state levels, or 120V in worst-case

situations. In addition, a sudden load dump anywhere on the

circuit card can generate a very high voltage spike at the drain

of the output MOSFET which, in turn, will appear at the

DRAIN pin of the MIC2588/MIC2594. In both cases, it is good

engineering practice to include protective measures to avoid

damaging sensitive ICs or the hot swap controller from these

large-scale transients. Two typical scenarios in which large-

scale transients occur are described below:

1. An output current load dump with no bypass (charge

bucket or bulk) capacitance to V

if L

resulting peak short-circuit current prior to the

MOSFET turning off would reach:

55V

0.7 s

5H

7.7A

×µ

()

µ

=

If there is no other path for this current to take when

the MOSFET turns off, it will avalanche the drain-

source junction of the MOSFET. Since the total

energy represented is small relative to the sturdi-

ness of modern power MOSFETs, it’s unlikely that

this will damage the transistor. However, the actual

avalanche voltage is unknown; all that can be

guaranteed is that it will be greater than the V

BD(D-

connected to the DRAIN pin of the MIC2588/94,

and the resulting transient does have enough

voltage and energy and can damage this, or any,

high-voltage hot swap controller.

2. If the load’s bypass capacitance (for example, the

input filter capacitors for a set of DC-DC converter

modules) are on a board from which the board with

the MIC2589/MIC2595 and the MOSFET can be

unplugged, the same type of inductive transient

damage can occur to the MIC2588/MIC2594.

Protecting the controller and the power MOSFET from dam-

age against these large-scale transients can take the forms

shown in Figure 7. It is not mandatory that these techniques

are used—the application environment will dictate suitability.

As protection against sudden on-card load dumps at the

DRAIN pin of the controller, a 2.2

µF or larger capacitor

directly from DRAIN to V

serve as a charge reservoir. Alternatively, a 68V, 1W, 5%

Zener diode clamp can be installed in a similar fashion. Note

that the clamp diode’s cathode is connected to the DRAIN pin

as shown in Figure 7. To protect the hot swap controller from

large-scale transients at the card input, a 100V clamp diode

(an SMAT70A or equivalent) can be used. In either case, the

lead lengths should be short and the layout compact to

prevent unwanted transients in the protection circuit.

[Circuit drawing under construction]

Figure 7. Using Large-Scale Transient Protection

Devices Around the MIC2588/94

Power buss inductance could easily result in localized high-

voltage transients during a turn-off event. The potential for

overstressing the part in such a case should be kept in check

with a suitable input capacitor and/or transient clamping

diode.

Power MOSFET Selection

[Section under construction]

Power MOSFET Operating Voltage Requirements

[Section under construction]

Power MOSFET Steady-State Thermal Issues

[Section under construction]

Power MOSFET Transient Thermal Issues

[Section under construction]

PCB Layout Considerations

[Section under construction]

Power MOSFET and Sense Resistor Vendors

[Section under construction]