MIC2586 datasheet(13/17 Pages) MICREL

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Micrel

MIC2586/MIC2586R

October 2004

M9999-102204

(408) 955-1690

Applications Information

External ON/OFF Control

The MIC2586/MIC2586R have an ON pin input that is used

to enable the controller to commence a start-up sequence

upon card insertion or to disable controller operation upon

card removal. In addition, the ON pin can be used to reset

the MIC2586/MIC2586R’s internal electronic circuit breaker

in the event of a load current fault. To reset the electronic

circuit breaker, the ON pin is toggled LOW then HIGH. The

ON pin is internally connected to an analog comparator with

80mV of hysteresis. When the ON pin voltage falls below its

internal VONL threshold, the GATE pin is immediately pulled

low. The GATE pin will be held low until the ON pin voltage is

above its internal VONH threshold. The external circuit's ON

threshold voltage level is programmed using a resistor divider

(R1 & R2) as shown in the "Typical Application” circuit. The

equations to set the trip points are shown below. For the

following example, the external circuit's ON threshold is set

to VONH(EX) = +37V, a value commonly used in +48V Central

Office power distribution applications.

ONH(EX) = VONH ×

+R2

⎛

⎝

⎜

⎞

⎠

⎟

(10)

Given VONH and R2, a value for R1 can be determined. A

suggested

value

for

that

which

will

provide

approximately 100

µA of current through the voltage divider

chain at VCC = VONH. This yields the following as a starting

point:

ONH(TYP)

100

µA

1.313V

100

µA

= 13.13kΩ

The closest standard 1% value for R2 is 13k

Ω. Now, solving

for R1 yields:

= R2 ×

ONH(EX)

ONH(TYP)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟−1

⎡

⎣

⎢

⎤

⎦

⎥

= 13kΩ×

37V

1.313V

⎛

⎝

⎜

⎞

⎠

⎟−1

⎡

⎣

⎢

⎤

⎦

⎥ = 353.3kΩ

The closest standard 1% value for R1 is 357k

Ω.

Using standard 1% resistor values, the external circuit's

nominal ON and OFF thresholds are:

VON(EX) = +36V

VOFF(EX) = +34V

In solving for VOFF(EX), replace VONH with VONL in Equation 10.

Output Voltage Power-is-Good Detection

The MIC2586/86R includes an analog comparator used to

monitor the output voltage of the controller through an

external resistor divider as shown in the "Typical Application"

circuit. The FB input pin is connected to the non-inverting

input and is compared against an internal reference voltage.

The analog comparator exhibits a hysteresis of 80mV.

Setting the "Power-is-Good" threshold for the circuit follows a

similar approach as setting the circuit's ON/OFF input

voltage. The equations to set the trip points are shown

below. For the following +48V telecom application, power-is-

good output signal PWRGD1 (or /PWRGD1) is to be de-

asserted when the output supply voltage is lower than +48V-

10% (+43.2V).

OUT(NOT GOOD) =VFBL ×

R5 + R6

⎛

⎝

⎜

⎞

⎠

⎟

(11)

Given VFBL and R6, a value for R5 can be determined. A

suggested

value

for

that

which

will

provide

approximately 100

µA of current through the voltage divider

chain at VOUT(NOT GOOD) = VFBL. This yields the following

equation as a starting point:

FBL(TYP)

100

µA

1.233V

100

µA

= 12.33kΩ

The closest standard 1% value for R6 is 12.4k

Ω. Now,

solving for R5 yields:

= R6 ×

OUT(NOT GOOD)

FBL(TYP)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟−1

⎡

⎣

⎢

⎤

⎦

⎥

= 12.4kΩ×

43.2V

1.233V

⎛

⎝

⎜

⎞

⎠

⎟−1

⎡

⎣

⎢

⎤

⎦

⎥ = 422kΩ

The closest standard 1% value for R5 is 422k

Ω.

Using standard 1% resistor values, the external circuit's

nominal "power-is-good" and "power-is-not-good" output

voltages are:

VOUT(GOOD) = +46V

VOUT(NOT GOOD) = +43.2V

In solving for VOUT(GOOD), substitute VFBH for VFBL in Equation

11.

Sense Resistor Selection

The sense resistor is nominally valued at:

OM)

HOT_SWAP(N

TRIP(TYP)

SENSE

(12)

where VTRIP(TYP) is the nominal circuit breaker threshold

voltage (47mV) and IHOT_SWAP(NOM) is the nominal inrush load

current level to trip the internal circuit breaker.

To accommodate worse-case tolerances in the sense

resistor (for a ±1% initial tolerance, allow ±3% tolerance for

variations over time and temperature) and circuit breaker

threshold voltages, a slightly more detailed calculation must

be used to determine the minimum and maximum hot swap

load currents.

The MIC2586/MIC2586R has a minimum current limit

threshold voltage of 39mV, thus the minimum hot swap load

current is determined where the sense resistor is 3% high:

HOT_SWAP(MIN) =

39mV

1.03

×R

SENSE(NOM)

()

37.9mV

SENSE(NOM)

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