MIC2582_14 Datasheet, PDF (15/27 Pages) Micrel Semiconductor – Single-Channel Hot Swap Controllers

Micrel, Inc.
MIC2582/MIC5283
until the output voltage rises to its final value. The output
slew rate and equivalent GATE voltage slew rate is
computed by the following equation:
Output voltage slew rate: dV_OUT/dt = ^ILIM
C_LOAD
Eq. 3
Table 1. Output Slew Rate Selection for Gate Capacitance-
Dominated Start-Up
CGATE
0.001µF
IGATE = 17µA
dVOUT/dt
17V/ms
0.01µF
1.7V/ms
0.1µF
0.17V/ms
where I_LIM is the programmed current-limit value.
Consequently, the value of C_FILTER must be selected to
ensure that the overcurrent response time, t_OCSLOW,
exceeds the time needed for the output to reach its final
value. For example, given a MOSFET with an input
capacitance C_ISS = C_GATE = 4700pF, C_LOAD is 2200µF, and
I_LIM is set to 6A with a 12V input, then the load
capacitance dominates as determined by the calculated
INRUSH > I_LIM. Therefore, the output voltage slew rate
determined from Equation 3 is:
1µF
0.017V/ms
Current Limiting and Dual-Level Circuit Breaking
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 1.
Output voltage slew rate: dV_OUT/dt = ^6A _{ 2.73} ^V Eq. 4
2200F
ms
and the resulting t_OCSLOW needed to achieve a 12V output
is approximately 4.5ms. (See Power-on-Reset and
Overcurrent Timer 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 the load
current during start-up never exceeds the current-limit
threshold as determined by Equation 1. The minimum
value of C_GATE that will ensure that the current limit is
never exceeded is given by the equation below:
dV_OUT/dt =
I_GATE
C_GATE
Eq. 5
The MIC2582/83 also features a dual-level circuit breaker
triggered via the 50mV and 100mV current-limit
thresholds which are 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
overcurrent timer, its duration set by capacitor C_FILTER,
starts to ramp the voltage at C_FILTER using a 6.5µA
constant current source. If the voltage at C_FILTER reaches
the overcurrent timer threshold (V_TH) of 1.24V, then
C_FILTER immediately returns to 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, the circuit breaker trips and the GATE shuts down
immediately, bypassing the overcurrent time period. The
MIC2582-MYM option is equipped with only a single
circuit breaker threshold (50mV). To disable current-limit
and circuit breaker operation, tie the SENSE and VCC
pins together and the C_FILTER (MIC2583/83R) pin to
ground.
Table 1 depicts the output slew rate for various values of
C_GATE.
Output Undervoltage Detection
The MIC2582/83 employ output undervoltage detection
by monitoring the output voltage through a resistive
divider connected at the FB pin. During turn-on, while the
voltage at the FB pin is below the threshold (V_FB), the
/POR pin is asserted low.
Once the FB pin voltage crosses V_FB, a 2.5µA current
source charges capacitor C_POR. Once the CPOR pin
voltage reaches 1.24V, the time period t_POR elapses as
the CPOR pin is pulled to ground and the /POR pin goes
HIGH. If the voltage at FB drops below V_FB for more than
10µs, the /POR pin resets for at least one timing cycle
May 23, 2014
15
Revision 5.0