MIC2588_05 Datasheet, PDF (15/21 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controllers

MIC2588/MIC2594
Micrel
For systems that experience known load current surges
exceeding the 400µs internal overcurrent filter (t_FLT), the
RC circuit consisting of R6 and C6 provides a means for
additional overcurrent filtering to eliminate false “tripping”
of the circuit breaker due to these transient load current
surges. It is highly recommended to limit the increase of
the overcurrent filter to approximately 2x the internal filter to
allow the MOSFET to operate within its thermal specifica-
tions and SOA. R6 and C6 act as a low-pass filter to reduce
the slew rate of the SENSE pin voltage. The SENSE pin
current is nominally 200nA, resulting in a slight voltage drop
across R6 that will combine in series with the voltage across
R_SENSE to produce an effective circuit breaker trip voltage of
V_TRIP – (R6 × I_SENSE). The following equation can be used
to select component values for a given overcurrent filter
delay.
┌
t_OCDLY  – (R × C) ln│1 –
└
V_TRIP – V(t_O)
V(t) – V(t_O)
(8)
where V_TRIP is the typical circuit breaker trip voltage specified
in the electrical specifications, V(t0) is the voltage drop across
the sense resister before the short or overcurrent condition
occurs, and V(t) is the voltage drop across the sense resis-
tor when the short or overcurrent is applied. The following
example sets an overcurrent delay of 1ms for a 7.5A load
current surge with a 2A steady-state load current and 5A
current limit (R_SENSE = 10mΩ).
V_TRIP = 50mV
V(t₀) = 2A × 10mΩ = 20mV
V(t) = 7.5A × 10mΩ = 75mV
Using Equation 8, for R6 = 2.7kΩ, C6 is 0.47µF.
The capacitor (C2) connected from UV to reference (V_EE)
is used as a glitch filter for the input undervoltage monitor.
C2 combines with the resistive network at the UV pin to
form an RC time constant to slow the UV pin voltage fall
time whenever the input voltage experiences a negative
(magnitude) transient. During start-up, the UV rise time will
also be affected by a longer RC time constant due to R1,
therefore, the output start cycle will be delayed until the UV
pin crosses its threshold.
The circuit in Figure 6 consisting of M2, R7, R8, and a digital
control signal, can be used to reset the controller after the
GATE (and output) turns off. Once the output has been
latched off, applying a low-high-low pulse on the GATE of M2
via the System Enable control can toggle the UV pin. System
Enable is a user defined signal referenced to V_EE.
Sense Resistor Selection
The sense resistor is nominally valued at:
R_SENSE(nom)

V_TRIP(typ)
I_{HOT_SWAP(nom)}
(9)
where V_TRIP(TYP) is the typical (or nominal) circuit breaker
threshold voltage (50mV) and I_{HOT_SWAP(NOM)} is the nomi-
nal load current level necessary to trip the internal circuit
breaker.
To accommodate worse-case tolerances in the sense re-
sistor (for a ±1% initial tolerance, allow ±3% tolerance for
variations over time and temperature) and circuit breaker
threshold voltages, a slightly more detailed calculation must
be used to determine the minimum and maximum hot swap
load currents.
As the MIC2588’s minimum current limit threshold voltage
is 40mV, the minimum hot swap load current is determined
where the sense resistor is 3% high:
  ^{IHOT_SWAP(min) }
40mV
1.03 × R_SENSE(nom)

38.8mV
R_SENSE(nom)
Keep in mind that the minimum hot swap load current
should be greater than the application circuit’s upper
steady-state load current boundary. Once the lower value
of R_SENSE has been calculated, it is good practice to check
the maximum hot swap load current (I_{HOT_SWAP(MAX)}) that
the circuit may let pass in the case of tolerance build-up in
the opposite direction. Here, the worse case maximum is
found using a V_TRIP(MAX) threshold of 60mV and a sense
resistor 3% low in value:
  ^{IHOT_SWAP(max) }
60mV
0.97 × R_SENSE(nom)

61.9mV
R_SENSE(nom)
In this case, the application circuit must be sturdy enough to
operate up to approximately 1.5x the steady-state hot swap
load current. For example, if an MIC2588 circuit must pass a
minimum hot swap load current of 4A without nuisance trips,
R_SENSE should be set to:
R_SENSE(nom) 
40mV
4A
 10mΩ
where the nearest 1% standard value is 10.0mΩ. At the
other tolerance extremes, I_{HOT_SWAP(MAX)} for the circuit in
question is then simply:
I_{HOT_SWAP(max)} 
61.9mV
10mΩ
 6.19A
With a knowledge of the application circuit’s maximum
hot swap load current, the power dissipation rating of the
sense resistor can be determined using P = I² × R. Here,
the current is I_{HOT_SWAP(max)} = 6.19A and the resistance
R_SENSE(max) = (1.03)(R_SENSE(nom)) = 10.3mΩ.
Thus, the sense resistor’s maximum power dissipation is:
P_MAX = (6.19A)² × (10.3mΩ) = 0.395W
A 0.5W sense resistor is a good choice in this application.
Power MOSFET Selection
Selecting the proper external MOSFET for use with the-
MIC2588/MIC2594 involves three straightforward tasks:
•Choice of a MOSFET which meets minimum voltage
requirements.
•Selection of a device to handle the maximum continuous
current (steady-state thermal issues).
•Verify the selected part’s ability to withstand any peak
currents (transient thermal issues).
Power MOSFET Operating Voltage Requirements
The first voltage requirement for the MOSFET is easily stated:
September 2005
15
M9999-083005