MIC2589 Micrel Semiconductor, MIC2589 Datasheet - Page 15

• Manufacturer Part Number: MIC2589
• Description: (MIC2589 / MIC2595) SINGLE-CHANNEL NEGATIVE HIGH VOLTAGE HOT SWAP POWER CONTROLLERS/SEQUENCERS
• Manufacturer: Micrel Semiconductor

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 R
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 7 illustrates how to implement 4-wire Kelvin sensing.
As the figure shows, all the high current in the circuit (from V
through R
flows directly through the power PCB traces and through
R
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.
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 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:
March 2004
MIC2589/2595
Figure 7. 4-Wire Kelvin Sense Connections for R
PCB Track Width:
0.03" per Ampere
SENSE
using 1oz. Cu
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:
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
. The voltage drop across R
SENSE
55V 0.7 s
LOAD
5 H
Power Trace
From V
= 5 H, V
SENSE
and then to the source of the output MOSFET)
to MIC2589/95 V
EE
must employ 4-wire Kelvin sensing.
= 7.7A
Signal Trace
IN
Note: Each SENSE lead trace shall be
balanced for best performance — equal
length/equal aspect ratio.
= 56V and t
EE
Pin
R
SENSE
contact pads
R
SENSE
metalized
SENSE
EE
OFF
Signal Trace
to MIC2589/95 SENSE Pin
. For example,
is sampled in such
= 0.7 s, the
To MOSFET Source
Power Trace
SENSE
EE
15
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 2.2 F 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
that the clamp diode’s cathode is connected to the DRAIN pin
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.
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]
Power MOSFET Steady-State Thermal Issues
[Section under construction]
Power MOSFET Transient Thermal Issues
[Section under construction]
PCB Layout Considerations
[Section under construction]
Figure 8. Using Large-Scale Transient Protection
2. If the load’s bypass capacitance (for example, the
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
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.
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, the same type of inductive transient dam-
age can occur to the MIC2589/95.
Devices Around the MIC2589/95 and the
of the MOSFET. The drain of the transistor is
[Circuit drawing under construction]
MIC2589R/95R
M9999-031504
BD(D-
Micrel