MIC2587R datasheet(13/22 Pages) MICREL

13 / 22 page

Micrel, Inc.

MIC2587/MIC2587R

January 24, 2013

Revision 2.0

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 other on-board components.

The MIC2587/MIC2587R was 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, heavier trace plating will

help minimize inductive spikes that may arise during

events that cause a large di/dt to occur (e.g., short circuit

loads). External surge protection, such as a clamping

diode, is also recommended as an added safeguard for

device, and system protection. And 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

the

MIC2587/MIC2587R is higher than the VUVH and the VONH

threshold voltages, a start cycle is initiated. When the

controller is enabled, an internal 16µA current source

(IGATEON) is turned on and the GATE pin voltage rises

from 0V with respect to ground at a rate equal to

Equation 1:

GATE

GATEON

GATE

Eq. 1

where CGATE is the total capacitance seen at the GATE

output of the controller (external capacitor from GATE to

ground plus CGS of the external MOSFET). 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 BVDSS and 30V

BVDSS N-channel power MOSFETs with a maximum gate-

source voltage of 20V can be used for +48V and +24V

applications, respectively. However, due to the harsh

electrical environments of most backplanes and other

“live” power supplies, the use of 100V and 60V power

MOSFETs, respectively, is recommended to withstand

transient

spikes

caused

stray

inductances.

Additionally, an external Zener diode (18-V) connected

from the source to the gate as shown in the typical

applications circuit is also recommended. A good choice

for an 18-V Zener diode in this application is the

MMSZ5248B, available in a small SOD123 package.

C4 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:

INRUSH = CLOAD ×

GATEON

GATE

Eq. 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, tFLT, which is the

time for which an overcurrent event must last to signal a

fault condition and to cause the output to latch-off. The

MIC2587/MIC2587R controller is most often utilized in

applications with large capacitive loads, so a properly

chosen value of CTIMER 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 CTIMER is given in the "Circuit

Breaker Operation" section.

MIC2587R Datasheet(PDF) 13 Page - Micrel Semiconductor

MIC2587R Datasheet(HTML) 13 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL