December 2003

11

M9999-122303

MIC2588/MIC2594

Micrel

Power-Good (PWRGD or /PWRGD) Output

For the MIC2588-1 and the MIC2594-1, the Power-Good

output signal (PWRGD) will be high impedance when V

DRAIN

drops below V

V

MIC2594-2, /PWRGD will pull down to the potential of the

V

impedance when V

have an active-high PWRGD signal and the -2 parts have an

active-low /PWRGD output. Either PWRGD or /PWRGD may

be used as an enable signal for one or more subsequent

DC/DC converter modules or for other system uses as

desired. When used as an enable signal, the time necessary

for the PWRGD (or /PWRGD) signal to pull-up (when in high

impedance state) will depend upon the load (RC) that is

present on this output.

Circuit Breaker Function

The MIC2588 and the MIC2594 employ an electronic circuit

breaker that protects the MOSFET and other system compo-

nents against faults such as short circuits. The current limit

threshold is set via an external resistor, R

between the V

limits the length of time (t

current in excess of its programmed threshold before the

circuit breaker is tripped. This short delay prevents nuisance

tripping of the circuit breaker due to system transients while

providing rapid protection against large-scale transient faults.

Whenever the voltage across R

things happen:

1. A constant-current regulation loop is engaged de-

signed to hold the voltage across R

50mV. This protects both the load and the MIC2588

circuit from excessively high currents. This loop will

engage in less than 1

µs from the time at which the

overvoltage condition on R

2. The internal 400

µs timer is started. If the 400µs

timeout period is exceeded, the circuit breaker trips

and the GATE pin is immediately pulled low by an

internal current pull-down. This operation turns off

the MOSFET quickly and disconnects the input from

the load.

Current Sensing

As mentioned before, the MIC2588 and the MIC2594 employ

an external low-value resistor in series with the source of the

external MOSFET to measure the current flowing into the

load. The V

is also one input to the part’s internal current sensing circuits

and the SENSE input is the other input.

Sense Resistor Selection

The sense resistor is nominally valued at:

R

(nom)

V

(typ)

I

(nom)

SENSE

TRIP

HOT_SWAP

=

where V

swap load current level to trip the internal circuit breaker in the

application.

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

currents.

As the MIC2588/94’s minimum current limit threshold voltage

is 40mV, the minimum hot swap load current is determined

where the sense resistor is 3% high:

I

(min)

40mV

1.03 R

(nom)

38.8mV

R

(nom)

HOT_SWAP

SENSE

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

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

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

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

V

I

(max)

60mV

0.97 R

(nom)

61.9mV

R

(nom)

HOT_SWAP

SENSE

SENSE

=

×

()

=

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

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

example, if an MIC2594 circuit must pass a minimum hot

swap load current of 4A without nuisance trips, R

SENSE

should be set to

38.8mV

4A

9.7m

standard value is 9.76m

Ω. At the other tolerance extremes,

I

I

(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

× R. Here, the I is

(0.97)(R

maximum power dissipation is:

P

2

× (9.47mΩ) = 0.376W

A 0.5

Ω sense resistor is a good choice in this application.

Undervoltage/Overvoltage Detection—MIC2588

The MIC2588 has “UV” and “OV” input pins. These pins 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 turn-off prevents damage to sensitive

circuit components should the input voltage exceed normal

operational limits. Each of these pins is internally connected to

an analog comparator with 20mV of hysteresis. When the UV

pin falls below its V

threshold, the GATE pin is immediately pulled low. The GATE

pin will be held low until UV exceeds its V

drops below its V

threshold trip points are programmed using the resistor divider