MIC2584/2585
Micrel
Fast Output Discharge for Capacitive Loads
Output Turn-Off Sequencing - No Tracking
Inmanyapplicationswhereaswitchcontrolleristurnedoffby
either removing the PCB from the backplane or the ON pin is
reset, capacitive loading will cause the output to retain
voltage unless a ‘bleed’ (low impedance) path is in place in
ordertodischargethecapacitance.TheMIC2585isequipped
with an internal MOSFET that allows the discharging of any
load capacitance to ground through a 50Ω to 170Ω path. The
discharge feature is configured by wiring the DIS pin to the
output (source) of the external MOSFET and is enabled if the
TRK pin is below 0.3V after the controller has been disabled
by a logic low signal received at the ON pin of Figure 1. See
the "Typical Application" circuit of Figure 1. A series resistor
There are many applications where it is necessary or desir-
able for the supply rails to sequence during turn-on and turn-
off, as is the case with some microprocessor requirements.
The MIC2585 can be configured to allow one output to shut
off first, followed by the other output. Figure 8 illustrates an
examplecircuitthatsequencesOUT1andOUT2inafirston–
last off application. During start-up, capacitor C
allows for
DLY
V
to turn on followed by V
20ms later. Once the ON
OUT1
OUT2
pin receives a low signal by removing the PCB from the
backplane,orbyanexternalprocessorsignal,DIS1andDIS2
will assert low. The external crowbar circuit connected from
the DIS2 pin will immediately bring V
to ground while
OUT2
is required from DIS to V
so that the maximum current of
V
will discharge to ground through the 750Ω (680Ω
OUT
OUT1
25mA for the DIS pin is not exceeded.
external, 70Ω internal) series path.
VIN1
MIC2585
GATE1
OUT1
EN
ON
VOUT1
VIN2
ON
VOUT1/VOUT2
PG
/FAULT
Short Circuit
on VOUT1
GATE2
PG
OUT2
VOUT2
VIN3
/FAULT
MIC2585
GATE1
OUT1
ON
VOUT3
VIN4
VOUT3/VOUT4
System Timing
GATE2
/FAULT
OUT2
VOUT4
Figure 7. Supporting More Than Two Supplies
March 2005
19
MIC2584/2585
MICREL [ MICREL SEMICONDUCTOR ]
