MIC2589 datasheet(13/17 Pages) MICREL

13 / 17 page

March 2004

M9999-031504

MIC2589/2595

Micrel

To accommodate worst-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 cur-

rents.

As the MIC2589/95’s minimum current-limit threshold volt-

age is 40mV, the minimum hot swap load current is deter-

mined where the sense resistor is 3% high:

(min)

40mV

1.03 R

(nom)

38.8mV

(nom)

HOT_SWAP

SENSE

Keep in mind that the minimum hot swap load current should

be greater than the application circuit’s upper steady-state

load current boundary. Once the lower value of R

SENSE has

been calculated, it is good practice to check the maximum hot

swap load current (IHOT_SWAP(max)) which the circuit may let

pass in the case of tolerance build-up in the opposite direc-

tion. Here, the worst-case maximum is found using a

TRIP(max) of 60mV and a sense resistor 3% low in value:

(max)

60mV

0.97 R

(nom)

61.9mV

(nom)

HOT_SWAP

SENSE

In this case, the application circuit must be sturdy enough to

operate over an ~1.6-to-1 range in hot swap load currents.

For example, if an MIC2595 circuit must pass a minimum hot

swap load current of 4A without nuisance trips, R

SENSE

should be set to

38.8mV

9.7m

=Ω , and the nearest 1%

standard value is 9.76m

Ω. At the other tolerance extremes,

IHOT_SWAP(max) for the circuit in question is then simply:

(max)

61.9mV

9.76m

6.3A

HOT_SWAP

Ω

With a knowledge of the application circuit’s maximum hot

swap load current, the power dissipation rating of the sense

resistor can be determined using P = I2

× R. Here, The I is

IHOT_SWAP(max) = 6.3A and the R is RSENSE(min) =

(0.97)(R

SENSE(nom)) = 9.47mΩ. Thus, the sense resistor’s

maximum power dissipation is:

MAX = (6.3A)

× (9.47mΩ) = 0.376W

A 0.5

Ω sense resistor is a good choice in this application.

No-Load Detection

For those applications in which a minimum load current will

always be present, the no-load detect capability of the

MIC2589/89R/95/95R family offers system designer the abil-

ity to perform a shutdown operation on such fault conditions,

such as an unscheduled or unexpected removal of PC boards

from the system or on-board fuse failure.

As long as the minimum current drawn by the load is at least

20% of the maximum output current (defined by

TRIP

SENSE

the output of the hot swap controllers will remain enabled. If

the output current falls below 12% of the maximum output

current, the controller’s no-load detection loop is enabled. In

this loop, an internal current source, I

CNLD, will charge an

external capacitor C

NLD. An expression for the controller’s

no-load time-out delay is given by:

NLD

CNLD

NLD

CNLD

=×









where V

CNLD = 1.24V (typ); ICNLD = 25µA (typ); and CNLD is

an external capacitor connected from Pin 6 to V

EE. Once the

voltage on C

NLD reaches its no-load threshold voltage, VCNLD,

the loop times out and the controller will shut down until it is

reset manually (MIC2589/95) or until it performs an auto-retry

operation (MIC2589R/95R).

Undervoltage/Overvoltage Detection (MIC2589 and

MIC2589R)

The MIC2589 and the MIC2589R have “UV” and “OV” input

pins that can be used to detect input supply rail undervoltage

and overvoltage conditions. Undervoltage lockout prevents

energizing the load until the supply input is stable and within

tolerance. In a similar fashion, overvoltage turnoff prevents

damage to sensitive circuit components should the input

voltage exceed normal operational limits. Each of these pins

is internally connected to analog comparators with 20mV of

hysteresis. When the UV pin falls below its V

UVL threshold or

the OV pin is above its V

OVH threshold, the GATE pin is

immediately pulled low. The GATE pin will be held low until

the UV pin is above its V

UVH threshold or the OV pin is below

its V

OVL threshold. The circuit’s UV and OV threshold voltage

levels are programmed using the resistor divider R1, R2, and

R3 as shown in the “Typical Application ” where the equations

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

example, the circuit’s UV threshold is set to V

UV = 37V and the

OV threshold is set at V

OV = 72V, values commonly used in

Central Office power distribution applications.

(typ)

R1 R2 R3

R2 R3

(typ)

R1 R2 R3

UVL

OVH

=×

()

=×

()

Given V

UV, VOV, and any one resistor value, the remaining

two resistor values can be determined. A suggested value for

R3 is that which will provide approximately 100

µA of current

through the voltage divider chain at V

DD = VUV. This yields the

following as a starting point:

R3 =

(typ)

100 A

1.223V

100 A

12.23k

OVH

=Ω

The closest standard 1% value for R3 = 12.4k

Ω. Solving for

R2 and R1 yields:

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