Micrel
MIC2587/MIC2587R
Zener diode (18-V) connected from the source to the gate
as shown in the "Typical Applications" circuit is highly
recommended. A good choice for an 18-V Zener diode in
this application is the MMSZ5248B, available in a small
SOD123 package.
C
GATE
is used to adjust the GATE voltage slew rate while
R3 minimizes the potential for high-frequency parasitic
oscillations from occurring in M1. However, note that
resistance in this part of the circuit has a slight destabilizing
effect upon the MIC2587/MIC2587R's current regulation
loop. Compensation resistor R4 is necessary for
stabilization of the current regulation loop. The current
through the power transistor during initial inrush is given
by:
I
INRUSH
=
C
LOAD
×
I
GATEON
C
GATE
Functional Description
Hot Swap Insertion
When circuit boards are inserted into systems carrying live
supply voltages ("hot swapped"), high inrush currents often
result due to the charging of bulk capacitance that resides
across the circuit board's supply pins. These current spikes
can cause the system's supply voltages to temporarily go
out of regulation causing data loss or system lock-up. In
more extreme cases, the transients occurring during a hot
swap event may cause permanent damage to connectors
or on-board components.
The MIC2587/MIC2587R is designed to address these
issues by limiting the maximum current that is allowed to
flow during hot swap events. This is achieved by
implementing a constant-current control loop at turn-on. In
addition to inrush current control, the MIC2587 and
MIC2587R incorporate input voltage supervisory functions
and user-programmable overcurrent protection, thereby
providing robust protection for both the system and the
circuit board.
Input Supply Transient Suppression and Filtering
The MIC2587/MIC2587R is guaranteed to withstand
transient voltage spikes up to 100V. However, voltage
spikes in excess of 100V may cause damage to the
controller. In order to suppress transients caused by
parasitic inductances, wide (and short) power traces
should be utilized. Alternatively, a heavier trace plating will
help minimize inductive spikes that may arise during
events (e.g., short circuit loads) that can cause a large di/dt
to occur. External surge protection, such as a clamping
diode, is also recommended as an added safeguard for
device (and system) protection. Lastly, a 0.1µF decoupling
capacitor from the VCC pin to ground is recommended to
assist in noise rejection. Place this filter capacitor as close
as possible to the VCC pin of the controller.
Start-Up Cycle
When the power supply voltage to the MIC2587/MIC2587R
is higher than the V
UVH
and the V
ONH
threshold voltages, a
start cycle is initiated. When the controller is enabled, an
internal 16µA current source (I
GATEON
) is turned on and the
GATE pin voltage rises from 0V with respect to ground at a
rate equal to:
dV
GATE
I
GATEON
=
dt
C
GATE
(2)
The drain current of the MOSFET is monitored via an
external current sense resistor to ensure that it never
exceeds the programmed threshold, as described in the
"Circuit Breaker Operation"
section.
A capacitor connected to the controller’s TIMER pin sets
the value of overcurrent detector delay, t
FLT
, which is the
time for which an overcurrent event must last to signal a
fault condition and to cause an output latch-off. These
devices will be driving a capacitive load in most
applications, so a properly chosen value of C
TIMER
prevents
false-, or nuisance-, tripping at turn-on as well as providing
immunity to noise spikes after the start-up cycle is
complete. The procedure for selecting a value for C
TIMER
is
given in the
"Circuit Breaker Operation"
section.
Overcurrent Protection
The MIC2587 and the MIC2587R use an external, low-
value resistor in series with the drain of the external
MOSFET to measure the current flowing into the load. The
VCC connection (Pin 8) and the SENSE connection (Pin 7)
are the (+) and (-) inputs, respectively, of the device's
internal current sensing circuits. Kelvin sense connections
are strongly recommended for sensing the voltage across
these pins. See the “Applications Information” for further
details.
The nominal current limit is determined by the following
equation.
I
LIMIT
=
V
TRIP(TYP)
R
SENSE
(3)
(1)
The internal charge pump has sufficient output drive to fully
enhance commonly available power MOSFETs for the
lowest possible DC losses. The gate drive is guaranteed
to be between 7.5V and 18V over the entire supply voltage
operating range (10V to 80V), so 60V BV
DSS
and 30V
BV
DSS
N-channel power MOSFETs can be used for +48V
and +24V applications, respectively. However, an external
October 2004
9
where V
TRIP(TYP)
is the typical current limit threshold
specified in the datasheet and R
SENSE
is the value of the
selected sense resistor.
As the MIC2587 and the
MIC2587R employ a constant-current regulation scheme in
current limit, the charge pump’s output voltage at the
GATE pin is adjusted so that the voltage across the
external sense resistor is held equal to V
TRIP
while the
capacitor connected to the TIMER pin is being charged. If
M9999-102204
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
