MIC2588_05 Datasheet, PDF (11/21 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controllers

MIC2588/MIC2594
Functional Description
Hot Swap Insertion
When circuit boards are inserted into systems carrying live
supply voltages (“hot swapped”), high inrush currents often
result due to the charging of bulk capacitance that resides
across the circuit board’s supply pins. These current spikes
can cause the system’s supply voltages to temporarily go
out of regulation, causing data loss or system lock-up. In
more extreme cases, the transients occurring during a hot
swap event may cause permanent damage to connectors or
on-board components.
The MIC2588 and the MIC2594 are designed to address these
issues by limiting the magnitude of the transient or inrush cur-
rent during hot swap events. This is achieved by controlling
the rate at which power is applied to the circuit board (di/dt
and dv/dt management). Additionally, the MIC2588 and the
MIC2594 incorporate input voltage supervisory functions and
current limiting, thereby providing robust protection for both
the system and the circuit board.
Start-Up Cycle
When the input voltage to the controller is between the over-
voltage and undervoltage thresholds (MIC2588) or is greater
than V_ON (MIC2594), a start cycle is initiated to deliver power
to the load. At this time, the GATE pin of the controller ap-
plies a constant charging current (I_GATEON) to the gate of the
external MOSFET (M1). C_FDBK creates a Miller integrator
out of the MOSFET circuit, which limits the slew-rate of the
voltage at the drain of M1. The drain voltage rate-of-change
(dv/dt) of M1 is:
  ^{dv M1}_DRAIN
dt

 ^I_GATE(–) 
 C_FDBK 


–_
I_GATEON
C_FDBK


where I_GATE(+) = Gate Charging Current = I_GATEON;
I_GATE(–) ≅ –I_GATE(+), due to the extremely high transconduc-
tance values of power MOSFETs; and
  _{IGATE(–)}

C_FDBK

dv
M1_DRAIN
dt
Relating the above to the maximum transient (or inrush) current
charging the load capacitance upon hot swap or power-up
involves an extension of the same formula:
  _IINRUSH

C_LOAD

dv
dt
M1_DRAIN
I_INRUSH

C_LOAD


–_
I_GATEON
C_FDBK


| I_INRUSH |

C_LOAD
C_FDBK
 I_GATEON
(1)
The presence of C3 and R_FDBK prevent turn-on of the
external pass device by limiting the hot swap current
surges induced by AC coupled transients from the drain
to the gate of M1 (i.e., C_FDBK + C_GD(M1)). An appropriate
value for C3 may be determined using the formula for a
capacitive voltage divider.
Micrel
The maximum voltage on C3 at turn-on must be less than
V_THRESHOLD of M1.
1. For a standard 10V enhancement N-Channel
MOSFET, V_THRESHOLD is about 4.25V.
2. Choose 2V as the maximum voltage to avoid turn-
on transients.
    V_GS(M1) × [C3 + C_FDBK + C_GD(M1) ] = V_IN(max) × C_FDBK + C_GD(M1)
   V_GS(M1) × C3 = V_IN(max) – V_GS(M1) C_FDBK + C_GD(M1)
  C3 =
C_FDBK + C_GD(M1)
×
V_IN(max) – V_GS(M1)
V_GS(M1)
(2)
where V_IN(max) = V_DD – V_EE(min).
For example, we can determine appropriate capacitor values
given a hot swap controller that is required to maintain the
inrush current into a 220µF load capacitance at 2A maximum
and an input supply voltage as high as V_IN(max) = 75V. One
of the suggested MOSFETs to be used with the MIC2588/
MIC2594 is an SUM110N10-09,a 100V D²PAK device which
has a typical C_GD of 750pF.
Calculating a value for C_FBDK using Equation 1 yields:
C_FDBK

220F  45A
2A

4.95nF
Good engineering practice suggests the use of the worst-
case parameter values for I_GATEON from the “DC Electrical
Characteristics” section:
C_FDBK

220F  60A
2A

6.6nF
where the nearest standard 5% value is 6.8nF. Substituting
6.8nF into Equation 2 from above yields:
C3

6.8nF

750pF

75V – 2V
2V

 0.275F
For C3, the nearest standard 5% value is 0.22µF.
While the value for R_FDBK is not critical, it should be chosen
to allow a maximum of a few milliamperes to flow in the
gate-drain circuit of M1 during turn-on. While the final value
for R_FDBK is determined empirically, initial values between
R_FDBK = 15kΩ to 27kΩ for systems with a maximum value
of V_IN(max) = 75V are appropriate.
Resistor R4, in series with the MOSFET's gate, minimizes
the potential for parasitic high frequency oscillations from
occurring in M1. While the exact value of R4 is not critical,
commonly used values for R4 range from 10Ω to 33Ω.
Power-Good (PWRGD or /PWRGD) Output
For the MIC2588-1 and the MIC2594-1, the Power-Good
output signal (PWRGD) will be high impedance when
V_DRAIN drops below V_PGTH, and will pull down to V_DRAIN
when V_DRAIN is above V_PGTH. For the MIC2588-2 and the
MIC2594-2, /PWRGD will pull down to the potential of the
V_DRAIN pin when V_DRAIN drops below V_PGTH, and will be
high impedance when V_DRAIN is above V_PGTH. Hence, the
-1 parts have an active-high PWRGD signal and the -2 parts
have an active-low /PWRGD output. Either PWRGD or
/PWRGD may be used as an enable signal for one or more
M9999-083005
11
September 2005