MIC2588 Datasheet, PDF (13/14 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controllers

MIC2588/MIC2594
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_SENSE must employ 4-wire Kelvin sens-
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_SENSE, and then to the source of the output MOSFET)
flows directly through the power PCB traces and R_SENSE. The
voltage drop resulting across R_SENSE 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.
PCB Track Width:
0.03" per Ampere
using 1oz Cu
Power Trace
From V_EE
R_SENSE metalized
contact pads
R_SENSE
Power Trace
To MOSFET Source
Signal Trace
to MIC2588/94 V_EE Pin
Signal Trace
to MIC2588/94 SENSE Pin
Note: Each SENSE lead trace shall be
balanced for best performance — equal
length/equal aspect ratio.
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_EE. For example,
if L_LOAD = 5µH, V_IN = 56V and t_OFF = 0.7µs, the
resulting peak short-circuit current prior to the
MOSFET turning off would reach:
(55V × 0.7µs) _{= 7.7A}
5µH
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
Micrel
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-
S) ^{of the MOSFET. The drain of the transistor is}
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 2.2µF or larger capacitor
directly from DRAIN to V_EE 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 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]
December 2003
13
M9999-122303