MIC2588 Datasheet, PDF (10/14 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 current
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). In addition, to inrush current control, the
MIC2588 and the MIC2594 incorporate input voltage super-
visory 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 IC is between the overvoltage
and undervoltage thresholds (MIC2588) or is greater than
V_ON (MIC2594), a start cycle is initiated. At this time, the
GATE pin of the IC applies 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
transconductance values of power MOSFETs; and
( ) _{IGATE(–)}
=
C_FDBK
×
dv
M1_DRAIN
dt
Relating the above to the maximum transient current into the
load capacitance to be charged upon hot swap or power-up
involves a simple extension of the same formula:
( ) _ICHARGE
=
C_LOAD
×
dv
dt
M1_DRAIN
I_CHARGE
=
C_LOAD
×

–_
I_GATEON 
C_{FDBK }
| I_CHARGE |
= ^{CLOAD × IGATEON}
C_FDBK
Transposing:
C_FDBK
=
C_LOAD × I_GATEON
| I_CHARGE |
(1)
Micrel
C_GATE and R_FDBK prevent turn-on and hot swap current
surges which would otherwise be caused by (C_FDBK +
C_D-G(M1)) coupling turn-on transients from the drain to the
gate of M1. An appropriate value for C_GATE may be deter-
mined using the formula for a capacitive voltage divider:
Maximum voltage on C_GATE 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 3.5V as a safe maximum voltage to safely
avoid turn-on transients.
V_G-S(M1) × [C_GATE + (C_FDBK + C_D-G(M1))]
= [(V_DD – V_EE(min)) × (C_FDBK + C_D-G(M1))]
V_G-S(M1) × C_GATE = [(V_DD – V_EE(min)) – V_G-S(M1)] × (C_FDBK + C_D-G(M1))
( ) ^{( ) C}_GATE ⁼
C_FDBK + C_D−G(Q1)
×
V_DD – V_EE(min) – V_G-S(M1)
V_G-S(M1)
(2)
While the value for R_FDBK is not critical, it should be chosen
to allow a maximum of several 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
75V for (V_DD – V_EE(min)) are appropriate.
Resistor R4, in series with the MOSFETs gate, minimizes the
potential for parasitic high frequency oscillations from occur-
ring in M1. While the exact value of R4 is not critical,
commonly used values for R4 range from 10Ω to 33Ω.
For example, let us assume a hot swap controller is required
to maintain the inrush current into a 150µF load capacitance
at 1.7A maximum, and that this circuit may operate from
supply voltages as high as (V_DD – V_EE) = 75V. The MOSFET
to be used with the MIC2588/94 is an IRF540NS 100V
D²PAK device which has a typical (C_D-G) of 250pF.
Calculating a value for C_FBDK using Equation 1 yields:
C_FDBK
=
150µF × 45µA
1.7A
=
3.97nF
Good engineering practice suggests the use of the worst-
case parameter values for I_GATEON from the “DC Electrical
Characteristics” section:
C_FDBK
=
150µF × 60µA
1.7A
=
5.3nF
where the nearest standard 5% value is 5.6nF. Substituting
5.6nF into Equation 2 from above yields:
C_GATE
=
(5.6nF
+
250pF)
×
(75V – 3.5V)
3.5V
=
0.12µF
Finally, choosing R4 = 10Ω and R_FDBK = 20kΩ will yield a
suitable, initial design for prototyping.
M9999-122303
10
December 2003