MIC2589/2595
Micrel
thiswilldamagethetransistor.However,theactual
Applications Information
4-Wire Kelvin Sensing
avalanche voltage is unknown; all that can be
guaranteed is that it will be greater than the V
BD(D-
Because of the low value typically required for the sense
resistor,specialcaremustbeusedtomeasureaccuratelythe
voltage drop across it. Specifically, the measurement tech-
of the MOSFET. The drain of the transistor is
S)
connected to the DRAIN pin of the MIC2589/95,
and the resulting transient does have enough
voltage and energy and can damage this, or any,
high-voltage hot swap controller.
nique across R
must employ 4-wire Kelvin sensing.
SENSE
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.
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 MIC2589/95 and the MOSFET can be un-
plugged, thesametypeofinductivetransientdam-
age can occur to the MIC2589/95.
Figure 7 illustrates how to implement 4-wire Kelvin sensing.
Asthefigureshows,allthehighcurrentinthecircuit(fromV
EE
throughR
andthentothesourceoftheoutputMOSFET)
SENSE
flows directly through the power PCB traces and through
. The voltage drop across R is sampled in such
a 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.
Protecting the controller and the power MOSFET from dam-
age against these large-scale transients can take the forms
shown in Figure 8. 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 2.2µF or larger capacitor
directly from DRAIN to VEE of the controller can be used to
serve as a charge reservoir. Alternatively, a 68V, 1W, 5%
Zener diode clamp can be installed in a similar fashion. Note
thattheclampdiode’scathodeisconnectedtotheDRAINpin
as shown in Figure 8. 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.
R
SENSE
SENSE
RSENSE metalized
contact pads
Power Trace
From VEE
Power Trace
To MOSFET Source
RSENSE
PCB Track Width:
0.03" per Ampere
using 1oz. Cu
Signal Trace
to MIC2589/95 VEE Pin
Signal Trace
to MIC2589/95 SENSE Pin
Note: Each SENSE lead trace shall be
balanced for best performance — equal
length/equal aspect ratio.
[Circuit drawing under construction]
Figure 7. 4-Wire Kelvin Sense Connections for R
Protection Against Voltage Transients
SENSE
Figure 8. Using Large-Scale Transient Protection
Devices Around the MIC2589/95 and the
MIC2589R/95R
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
compleximpedanceenvironment, thesetransientscanbeas
high as 2.5 times steady-state levels, or 120V in worst-case
situations. In addition, a sudden load dump anywhere on the
circuitcardcangenerateaveryhighvoltagespikeatthedrain
of the output MOSFET which, in turn, will appear at the
DRAIN pin of the MIC2589/95. 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:
The same logic applies to the input of the MIC2589/95 circuit.
Power bus 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]
1. Anoutputcurrentloaddumpwithnobypass(charge
bucket or bulk) capacitance to V . For example,
EE
Power MOSFET Steady-State Thermal Issues
if L
= 5µH, V = 56V and t
= 0.7µs, the
LOAD
IN
OFF
[Section under construction]
resulting peak short-circuit current prior to the
MOSFET turning off would reach:
55V × 0.7µs
(
)
= 7.7A
Power MOSFET Transient Thermal Issues
5µH
[Section under construction]
Ifthereisnootherpathforthiscurrenttotakewhen
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
PCB Layout Considerations
[Section under construction]
March 2004
15
M9999-031504
MICREL [ MICREL SEMICONDUCTOR ]
