MIC2587 Ver la hoja de datos (PDF) - Micrel

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Fabricante

MIC2587

Single-Channel, Positive High-Voltage Hot Swap Controller

Micrel 

Micrel

MIC2587/MIC2587R

the current-limit condition goes away before the TIMER pin

voltage rises above the VTIMERH threshold, then steady-

state operation resumes. To prevent excessive power

dissipation in the external MOSFET under load current

fault conditions, the FB pin voltage is used as the control

element in a circuit that lowers the current limit as a

function of the output voltage. When the load current

increases to the point where the output voltage at the load

approaches 0V (likewise, the MIC2587/MIC2587R’s FB pin

voltage also approaches 0V), the result is a proportionate

decrease in the maximum current allowed into the load.

This foldback current limit subcircuit’s transfer

characteristic is shown in Figure 1. Under excessive load

conditions (output and FB voltage equals 0V), the foldback

current limiting circuit controls the MIC2587/MIC2587R’s

GATE drive to force a constant 12mV (typical) voltage drop

across the external sense resistor.

Circuit Breaker Operation

The MIC2587/MIC2587R employ an electronic circuit

breaker that protects the external N-channel power

MOSFET and other system components against large-

scale output current faults, both during initial card insertion

or during steady-state operation. The current limit threshold

is set via an external resistor, RSENSE, connected between

the circuit’s VCC pin and SENSE pin. For the

MIC2587/MIC2587R, a fault current timing circuit is set via

an external capacitor (CTIMER) that determines the length of

the time delay (tFLT) for which the controller remains in

current limit before the circuit breaker is tripped.

Programming the response time of the overcurrent detector

helps to prevent nuisance tripping of the circuit breaker

because of high inrush currents charging bulk and

distributed capacitive loads. The nominal overcurrent

response time is calculated using the following equation:

tFLT

=

CFILTER × VTIMERH

ITIMERUP

tFLT (ms) = 20 × CFILTER (µF)

(4)

Whenever the voltage across RSENSE exceeds the

MIC2587/MIC2587R’s nominal circuit breaker threshold

voltage of 47mV during steady-state operation, two things

occur:

1. A constant-current regulation loop will engage

within 1µs after an overcurrent condition is

detected by RSENSE, and the control loop is

designed to hold the voltage across RSENSE equal

to 47mV. This feature protects both the load and

the MIC2587/MIC2587R circuits from excessively

high currents.

2. Capacitor CTIMER is then charged up to the VTIMERH

threshold (1.313V) by an internal 65µA current

source (ITIMERUP). If the excessive current persists

such that the voltage across CTIMER crosses the

VTIMERH threshold, the circuit breaker trips and the

GATE pin is immediately pulled low by a 30mA

(minimum) internal current sink. This operation

turns off the MOSFET quickly and disconnects the

input from the load. The value of CTIMER should be

selected to allow the circuit's minimum regulated

output current (IOUT) to equal ILIMIT for somewhat

longer than the time it takes to charge the total

load capacitance.

An initial value for CTIMER is found by calculating the time it

will take for the MIC2587/MIC2587R to completely charge

up the output capacitive load. Assuming the load is

enabled by the PWRGD (or /PWRGD) signal of the IC, the

turn-on delay time is derived from the following expression,

I = C × (dV/dt):

t TURN -ON

=

CLOAD × VCC(MAX)

ILIMIT

(5)

Using parametric values for the MIC2587/MIC2587R, an

expression relating a worse-case design value for CTIMER,

using the MIC2587/MIC2587R specification limits, to the

circuit's turn-on delay time is:

CTIMER(MAX)

=

tTURN-ON × ITIMERUP(MAX)

VTIMERH(MIN)

CTIMER(MAX)

=

t TURN - ON

× ⎜⎛ 120µA

⎝ 1.280V

⎟⎞

⎠

CTIMER (MAX)

=

t

TURN-ON

×

⎜⎛

⎝

94

×

10-6

µF

sec

⎟⎞

⎠

(6)

For example, in a system with a CLOAD = 1000µF, a

maximum VCC = +72V, and a maximum load current on a

nominal +48V buss of 1.65A, the nominal circuit design

equations steps are:

1. Choose ILIMIT = IHOT_SWAP(nom) = 2A (1.65A + 20%);

2. Select an RSENSE (Closest 1% standard value is

19.6mΩ);

3. Using ICHARGE = ILIMIT = 2A, the application circuit

turn-on time is calculated using Equation 5:

(1000µF × 72V)

tTURN-ON =

2A

= 36ms

Allowing for capacitor tolerances and a nominal 36ms turn-

on time, an initial worse-case value for CTIMER is:

CTIMER(MAX)

=

0.036s × ⎜⎛ 94 ×10-6

⎝

µF

sec

⎟⎞

⎠

=

3.38 µF

The closest standard ±5% tolerance capacitor value is

3.3µF and would be a good initial starting value for

prototyping.

Whenever the MIC2587 is not in current limit, CTIMER is

discharged to GND by an internal 3.5µA current sink

(ITIMERDN).

For the MIC2587R, the circuit breaker automatically resets

after (20) tFLT_AUTO time constants. If the fault condition still

exists, capacitor CTIMER will begin to charge up to the

October 2004

10

M9999-102204

(408) 955-1690

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