MIC2588_05 Datasheet, PDF (14/21 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controllers

MIC2588/MIC2594
Applications Information
Optional External Circuits for Added Protection/Perfor-
mance
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 ap-
pear 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 de-
scribed 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:
56V  0.7 s _{ 7.8A}
5H
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
V_BD(D-S) of the MOSFET. The drain of the transistor
Micrel
is connected to the DRAIN pin of the MIC2588/
MIC2594, and the resulting transient does have
enough voltage and energy to damage this, or any,
high-voltage hot swap controller.
2. If the load’s bypass capacitance (for example,
the input filter capacitors for DC-DC converter
module(s)) are on a board from which the board
with the MIC2588/MIC2594 and the MOSFET can
be unplugged, the same type of inductive transient
damage can occur to the MIC2588/MIC2594.
For many applications, the use of additional circuit compo-
nents can be implemented for optimum system performance
and/or protection. The circuit, shown in Figure 6, 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.
It is not mandatory that these techniques be utilized, how-
ever, the application environment will dictate suitability. For
protection against sudden on-card load dumps at the DRAIN
pin of the MIC2588/MIC2594 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 tran-
sient suppressing diode clamps is highly recommended for
maximizing the life of the hot swap controller(s).
-48V RTN
-48V RTN
*D1
100V
System
Reset
C1
0.47µF
R3
12.4kΩ
1%
*R7
10kΩ
R1
698kΩ
1%
*R5
47kΩ
R2
11.8kΩ
1%
1
/PWRGD
8
VDD
2
OV
3
UV
7
DRAIN
MIC2588-2BM
6
GATE
4
VEE
*M2
*D2
5
SENSE
68V
C2
0.22uF
*C6
0.47uF
*R6
2.7kΩ
C3
0.33uF
R4
10Ω
C5
47uF
RFDBK
10kΩ
CFDBK
10nF
100V
C4
0.1uF
-48VIN
RSENSE
0.01Ω
5%
* Optional components (See Applications Information for more details)
An SOT-363 is recommended for M2.
D2 is a 68V, 1W Zener diode.
M1
SUM110N10-09
-48VOUT
Figure 6 Optional Components for Added Performance/Protection
September 2005
14
M9999-083005