MIC2594 MICREL [Micrel Semiconductor], MIC2594 Datasheet - Page 10

• Manufacturer Part Number: MIC2594
• Description: Single-Channel, Negative High-Voltage Hot Swap Power Controllers
• Manufacturer: MICREL [Micrel Semiconductor]

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
GATE pin of the IC applies a constant charging current
(I
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:
where I
I
transconductance values of power MOSFETs; and
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:
Transposing:
M9999-122303
GATE(–)
GATEON
ON
(MIC2594), a start cycle is initiated. At this time, the
I
I
I
| I
C
dv M1
GATE(–)
CHARGE
CHARGE
GATE(+)
FDBK
CHARGE
) to the gate of the external MOSFET (M1). C
dt
DRAIN
–I
= Gate Charging Current = I
C
GATE(+)
C
C
LOAD
C
|
FDBK
| I
LOAD
LOAD
CHARGE
C
I
GATE(–)
C
LOAD
, due to the extremely high
I
GATEON
FDBK
dv M1
–
dv M1
C
dt
I
FDBK
GATEON
C
|
I
dt
FDBK
GATEON
DRAIN
DRAIN
–
I
GATEON
C
FDBK
GATEON
(1)
;
FDBK
10
C
surges which would otherwise be caused by (C
C
gate of M1. An appropriate value for C
mined using the formula for a capacitive voltage divider:
Maximum voltage on C
V
V
V
While the value for R
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
R
75V for (V
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 10 to 33 .
For example, let us assume a hot swap controller is required
to maintain the inrush current into a 150 F load capacitance
at 1.7A maximum, and that this circuit may operate from
supply voltages as high as (V
to be used with the MIC2588/94 is an IRF540NS 100V
D
Calculating a value for C
Good engineering practice suggests the use of the worst-
case parameter values for I
Characteristics”
where the nearest standard 5% value is 5.6nF. Substituting
5.6nF into Equation 2 from above yields:
Finally, choosing R4 = 10
suitable, initial design for prototyping.
C
G-S(M1)
THRESHOLD
G-S(M1)
GATE
D-G(M1)
FDBK
2
GATE
PAK device which has a typical (C
1. For a standard 10V enhancement N-Channel
2. Choose 3.5V as a safe maximum voltage to safely
FDBK
C
C
C
MOSFET, V
avoid turn-on transients.
= 15k to 27k for systems with a maximum value of
and R
FDBK
FDBK
GATE
) coupling turn-on transients from the drain to the
C
C
GATE
DD
FDBK
is determined empirically, initial values between
[C
of M1:
GATE
– V
= [(V
FDBK
= [(V
150 F 45 A
150 F 60 A
5.6nF 250pF
EE
section:
C
DD
D G(Q1)
THRESHOLD
+ (C
DD
(min)) are appropriate.
1.7A
1.7A
prevent turn-on and hot swap current
– V
FDBK
– V
FDBK
EE
GATE
FBDK
EE
(min)) – V
is not critical, it should be chosen
GATEON
(min))
+ C
DD
and R
V
at turn-on must be less than
DD
using Equation 1 yields:
is about 4.25V.
– V
5.3nF
3.97nF
D-G(M1)
– V (min) – V
G-S(M1)
75V – 3.5V
FDBK
EE
EE
from the
(C
V
D-G
) = 75V. The MOSFET
3.5V
G-S(M1)
FDBK
] (C
GATE
)]
) of 250pF.
= 20k
December 2003
FDBK
+ C
may be deter-
“DC Electrical
G-S(M1)
+ C
D-G(M1)
0.12 F
will yield a
D-G(M1)
FDBK
Micrel
(2)
)]
)
+