Micrel
MIC2589/MIC2595
the MIC2589/MIC2595, and the resulting
transient does have enough voltage and
energy to damage this, or any, high-voltage
hot swap controller.
Application Information
Optional External Circuits for Added
Protection/Performance
2. If the load’s bypass capacitance (for
example, the input filter capacitors for DC-
DC converter module(s)) is 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 MIC2589/MIC2595.
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 that will
appear at the DRAIN pin of the MIC2589/MIC2595. 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:
For many applications, the use of additional circuit
components can be implemented for optimum system
performance and/or protection. The circuit, shown in
Figure 7, includes several components to address
some the following system (dynamic) responses
and/or functions: 1) suppression of transient voltage
spikes, 2) elimination of false “tripping” of the circuit
breaker due to undervoltage and overcurrent glitches,
and 3) the implementation of an external reset circuit.
1. An output current load dump with no bypass
(charge bucket or bulk) capacitance to VEE.
For example, if LLOAD = 5µH, VIN = 56V and
tOFF = 0.7µs, the resulting peak short-circuit
current prior to the MOSFET turning off
would reach:
It is not mandatory that these techniques be utilized,
however, the application environment will dictate
suitability. For protection against sudden on-card load
dumps at the DRAIN pin of the MIC2589/MIC2595
controller, a 68V, 1W, 5% Zener diode clamp (D2)
connected from the DRAIN to the VEE of the
controller can be implemented as shown. To protect
the controller from large-scale transients at the card
input, a 100V clamp diode (D1, SMAT70A or
equivalent) can be used. In either case, very short
lead lengths and compact layout design is strongly
recommended to prevent unwanted transients in the
protection circuitry. Power buss inductance often
produces localized (plug-in card) high-voltage
transients during a turn-off event. Managing these
repeated voltage stresses with sufficient input bulk
capacitance and/or transient suppressing diode
clamps is highly recommended for maximizing the life
of the hot swap controller(s).
(
56V × 0.7µs
)
= 7.8A
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
sturdiness 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
VBD(D-S) of the MOSFET. The drain of the
transistor is connected to the DRAIN pin of
20
M9999-120505
(408) 955-1690
December 2005
MICREL [ MICREL SEMICONDUCTOR ]
