MIC2582/MIC2583
Micrel
GATE Capacitance Dominated Start-Up
charges capacitor C
. Once the CPOR pin voltage reaches
POR
1.24V, the time period t
to ground and the /POR pin goes HIGH. If the voltage at FB
elapses as the CPOR pin is pulled
In this case, the value of the load capacitance relative to the
GATEcapacitanceissmallenoughsuchthattheloadcurrent
during start-up never exceeds the current limit threshold as
POR
drops below V for more than 10µs, the /POR pin resets for at
FB
least one timing cycle defined by t
Information for an example).
(See Applications
determined by Equation 3. The minimum value of C
will ensure that the current limit is never exceeded is given by
the equation below:
that
POR
GATE
Power-On Reset and Overcurrent Timer Delays
The Power-On Reset delay, t , is the time period for the
IGATE
POR
CGATE(min) =
× CLOAD
(5)
/PORpintogoHIGHoncethevoltageattheFBpinexceedsthe
ILIM
power-good threshold (V ). A capacitor connected to CPOR
FB
where C
is the summation of the MOSFET input
GATE
setstheintervalandisdeterminedbyusingEquation1withV
TH
capacitance (C ) and the value of the external capacitor
ISS
substituted for V
. The resulting equation becomes:
START
connectedtotheGATEpinoftheMIC2582/83toground. Once
VTH
C
is determined, use the following equation to determine
tPOR = CPOR
×
0.5 × CPOR µF
(
)
GATE
(7)
ICPOR
the output slew rate for gate capacitance dominated start-up.
where the Power-On Reset threshold (V ) and timer current
I
TH
GATE
dV
/dt =
(I
) are typically 1.24V and 2.5µA, respectively.
OUT
(6)
CPOR
C
GATE
For the MIC2583/83R, a capacitor connected to CFILTER is
used to set the timer which activates the circuit breaker during
overcurrent conditions. When the voltage across the sense
resistor exceeds the slow trip current limit threshold of 50mV,
the overcurrent timer begins to charge for a time period
Table1depictstheoutputslewrateforvariousvaluesofC
.
GATE
IGATE = 17µA
CGATE
0.001µF
0.01µF
0.1µF
1µF
dVOUT/dt
17V/ms
(t
), determined by C
. When no capacitor is
OCSLOW
FILTER
1.7V/ms
connectedtoCFILTERandfortheMIC2582, t
defaults
OCSLOW
0.17V/ms
0.017V/ms
to 5µs. If t
elapses, then the circuit breaker is activated
OCSLOW
and the GATE output is immediately pulled to ground. For the
MIC2583/83R, the following equation is used to determine the
Table 1. Output Slew Rate Selection for GATE
Capacitance Dominated Start-Up
overcurrent timer period, t
.
OCSLOW
VTH
Current Limiting and Dual-Level Circuit Breaker
tOCSLOW = CFILTER
×
0.19 × CFILTER(µF)
(8)
ITIMER
Many applications will require that the inrush and steady state
supply current be limited at a specific value in order to protect
critical components within the system. Connecting a sense
resistor between the VCC and SENSE pins sets the nominal
current limit value of the MIC2582/83 and the current limit is
calculated using Equation 2.
where V , the CFILTER timer threshold, is 1.24V and I
,
TH
TIMER
the overcurrent timer current, is 6.5µA. Tables 2 and 3 provide
a quick reference for several timer calculations using select
standard value capacitors.
C
POR
t
t
POR
START
The MIC2582/83 also features a dual-level circuit breaker
triggered via 50mV and 100mV current limit thresholds sensed
across the VCC and SENSE pins. The first level of the circuit
breaker functions as follows. For the MIC2583/83R, once the
voltage sensed across these two pins exceeds 50mV, the
0.01µF
0.02µF
0.033µF
0.05µF
0.1µF
1.2ms
2.4ms
4ms
5ms
10ms
16.5ms
25ms
6ms
overcurrent timer, its duration set by capacitor C
, starts
FILTER
12ms
40ms
56ms
120ms
50ms
torampthevoltageatCFILTERusinga6.5µAconstantcurrent
source.IfthevoltageatCFILTERreachestheovercurrenttimer
0.33µF
0.47µF
1µF
165ms
235ms
500ms
threshold(V )of1.24V, thenCFILTERimmediatelyreturnsto
TH
ground as the circuit breaker trips and the GATE output is
immediately shut down. The default overcurrent time period for
the MIC2582/83 is 5µs. For the second level, if the voltage
sensed across VCC and SENSE exceeds 100mV at any time,
thecircuitbreakertripsandtheGATEshutsdownimmediately,
bypassing the overcurrent time period. To disable current limit
and circuit breaker operation, tie the SENSE and VCC pins
together and the CFILTER (MIC2583/83R) pin to ground.
Table 2. Selected Power-On Reset and Start-Up Delays
C
FILTER
t
OCSLOW
680pF
2200pF
4700pF
8200pF
0.033µF
0.1µF
130µs
420µs
900µs
1.5ms
6ms
Output Undervoltage Detection
The MIC2582/83 employ output undervoltage detection by
monitoring the output voltage through a resistive divider con-
nected at the FB pin. During turn on, while the voltage at the FB
19ms
42ms
90ms
0.22µF
0.47µF
pin is below the threshold (V ), the /POR pin is asserted low.
FB
Once the FB pin voltage crosses V , a 2.5µA current source
FB
Table 3. Selected Overcurrent Timer Delays
MIC2582/MIC2583
14
April 2003
MICREL [ MICREL SEMICONDUCTOR ]
