MIC2584 Datasheet, PDF (16/28 Pages) Micrel Semiconductor – Dual-Channel Hot Swap Controller/Sequencer

MIC2584/2585
and the resulting t_OCSLOW needed to achieve a 12V output is
approximately 2.5ms. (See "Power-On Reset, Overcurrent
Timer, and Sequenced Output Delays" section to calculate
t_OCSLOW).
GATE Capacitance Dominated Start-Up
In this case, the value of the load capacitance relative to the
GATE capacitance is small enough such that during start-up
the output current never exceeds the current limit threshold
as determined by Equation 3. The minimum value of C_GATE
that will ensure that the current limit is never exceeded is
given by the equation below:
C_GATE(Min) =
I_GATE
I_LIMIT
× C_LOAD
Where C_GATE is the summation of the MOSFET input capaci-
tance (C_ISS) specification and the value of the capacitor
connected to the GATE pin of the MIC2584/85 (and MOSFET)
to ground. Once C_GATE is determined, use the following
equation to determine the output slew rate
dV_OUT/dt for gate capacitance dominated start-up:
dV_OUT /dt
=
I_GATE
C_GATE
Table 1 depicts the output slew rate for various values of C_GATE.
C_GATE
0.001µF
I_GATE = 14µA
dV_OUT/dt
14V/ms
0.01µF
1.4V/ms
0.1µF
0.14V/ms
1µF
0.014V/ms
Table 1. Output Slew Rate Selection for GATE
Capacitance Dominated Start-Up
Current Limiting and Dual-Level Circuit Breaker
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 of each channel
sets the nominal current limit value for each channel of the
MIC2584/85 and the current limit is calculated using
Equation 2.
The MIC2584/85 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 MIC2584/85, once the
voltage sensed across these two pins exceeds 50mV on
either channel, the overcurrent timer, its duration set by
capacitor C_FILTER, starts to ramp the voltage at CFILTER
using a 2.5µA constant current source. If the voltage at
CFILTER reaches the overcurrent timer threshold (V_TMR) of
1.235V, then CFILTER immediately returns to ground as the
circuit breaker trips and both GATE outputs are immediately
shut down. For the second level, if the voltage sensed across
VCC and SENSE of either channel exceeds 100mV
(–J option) at any time, the circuit breaker trips and both
Micrel
GATE outputs shut down immediately, bypassing the
overcurrent timer period. To disable current limit and circuit
breaker operation, tie each channel’s SENSE and VCC pins
together and the CFILTER pin to ground.
Output Undervoltage Detection
The MIC2584/85 employ output undervoltage detection by
monitoring the output voltage through a resistive divider
connected at the FB pins. During turn on, while the voltage at
either FB pin is below its threshold (V_FB), the /POR pin is
asserted low. Once both FB pin voltages cross their respec-
tive threshold (V_FB), a 2.5µA current source charges capaci-
tor C_POR. Once the CPOR pin voltage reaches 1.235V, the
time period t_POR elapses as pin CPOR is pulled to ground and
the /POR pin goes HIGH. If the voltage at either FB drops
below V_FB for more than 10µs, the /POR pin resets for at least
one timing cycle defined by t_POR (See "Applications Informa-
tion" for an example).
Input/Output Overvoltage Protection
The MIC2585 monitors and detects overvoltage conditions in
the event of excessive supply transients at the MIC2585
input(s)/output(s). Whenever the voltage threshold is ex-
ceeded at either OV1 or OV2 of the MIC2585, the circuit
breaker is tripped and both GATE outputs are immediately
brought low.
Power-On Reset, Overcurrent Timer, and Sequenced
Output Delays
The Power-On Reset delay, t_POR, is the time period for the
/POR pin to go HIGH once the lagging voltage exceeds the
power-good threshold (V_FB) monitored at the FB pin. A
capacitor connected to CPOR sets the interval and is deter-
mined by using Equation 1 with V_POR substituted for V_START.
The resulting equation becomes:
t_POR
= C_POR
× ^VPOR
I_CPOR
≅ 0.5 × C_POR(µF)
(7)
where the Power-On Reset threshold (V_POR) and timer
current (I_CPOR) are typically 1.235V and 2.5µA, respectively.
For the MIC2584/85, a capacitor connected to CFILTER is
used to set the timer which activates the circuit breaker during
overcurrent conditions. When the voltage across either sense
resistor exceeds the slow trip current limit threshold of 50mV,
the overcurrent timer begins to charge for a period, t_OCSLOW,
determined by C_FILTER. If t_OCSLOW elapses, then the circuit
breaker is activated and both GATE outputs are immediately
pulled to ground. The following equation is used to determine
the overcurrent timer period, t_OCSLOW.
t_OCSLOW
= C_FILTER
×
V_TMR
I_TMR
≅ 0.5 × C_FILTER(µF)
(8)
where V_TMR, the overcurrent timer threshold, is 1.235V and
I_TMR, the overcurrent timer current, is 2.5µA. If no capacitor
for CFILTER is used, then t_OCSLOW defaults to 20µs.
MIC2584/2585
16
March 2005