Micrel

MIC2587/MIC2587R

October 2004

11

M9999-102204

(408) 955-1690

)

threshold, at which time another start cycle is initiated. This

will continue until either of the following occurs: a) the fault

condition is removed, b) the input supply voltage power is

removed/recycled, or c) the ON pin is toggled LOW then

HIGH. The duty cycle of the auto-restart function is

therefore fixed at 5% and the period of the auto-restart

cycle is given by:

FLT_AUTO

RESTARt

AUTO

t

20

t

×

=

−

() (

TIMERUP

TIMERL

TIMERH

TIMER

RESTART

-

AUTO

I

V

V

C

20

t

−

×

×

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

=

F

ms

250

C

t

TIMER

RESTART

-

AUTO

µ

(7)

The auto-restart period for the example above where the

Input Undervoltage Lockout

The MIC2587/MIC2587R have an internal undervoltage

lockout circuit that inhibits operation of the controller’s

internal circuitry unless the power supply voltage is stable

and within an acceptable tolerance. If the supply voltage to

the controller, with respect to ground is greater than the

internal circuits are enabled and the controller is then ready

for normal operation pending the state of the ON pin

voltage. Once in steady-state operation, the controller’s

internal circuits remain active so long as the supply voltage

with respect to ground is higher than the controller’s

Power-is-Good Output Signals

For the MIC2587-1 and MIC2587R-1, the power-good

output signal (PWRGD) will be high impedance when the

pull down to GND when the FB pin voltage is lower than

the power-good output signal (/PWRGD) will pull down to

threshold and will be high impedance when the FB pin

parts have an active-HIGH PWRGD signal and the (-2)

parts have an active-LOW /PWRGD output. PWRGD (or

/PWRGD) may be used as an enable signal for one or

more following 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 (RC) load at the Power-is-Good pin.

The Power-is-Good output pin is connected to an open-

drain, N-channel transistor implemented with high-voltage

structures. These transistors are capable of operating with

pull-up resistors to supply voltages as high as 100V.