Micrel

MIC2587/MIC2587R

October 2004

9

M9999-102204

(408) 955-1690

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 0.1

µF 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

start cycle is initiated. When the controller is enabled, an

internal 16

µA current source (I

GATE pin voltage rises from 0V with respect to ground at a

rate equal to:

dV

GATE

dt

=

I

GATEON

C

GATE

(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

and +24V applications, respectively. However, an external

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.

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

I

GATEON

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

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

false-, or nuisance-, tripping at turn-on as well as providing

immunity to noise spikes after the start-up cycle 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.

SENSE

TRIP(TYP)

LIMIT

R

V

I

=

(3)

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

capacitor connected to the TIMER pin is being charged. If