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參數(shù)資料
| 型號： | MIC2594-2BM TR |
| 廠商： | Micrel Inc |
| 文件頁數(shù)： | 15/21頁 |
| 文件大小： | 1079K |
| 描述： | IC CTRLR HOT SWAP NEG HV 8-SOIC |
| 標準包裝： | 2,500 |
| 類型： | 熱交換控制器 |
| 應(yīng)用： | 通用 |
| 內(nèi)部開關(guān)： | 無 |
| 電源電壓： | -19 V ~ -80 V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 8-SOIC（0.154"，3.90mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 8-SOIC |
| 包裝： | 帶卷 (TR) |
| 其它名稱： | MIC2594-2BMTR MIC2594-2BMTR-ND |
September 2005
15
M9999-083005
MIC2588/MIC2594
Micrel
For systems that experience known load current surges
exceeding the 400祍 internal overcurrent lter (t
FLT
), the
RC circuit consisting of R6 and C6 provides a means for
additional overcurrent ltering 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 lter to approximately 2x the internal lter to
allow the MOSFET to operate within its thermal specica-
tions and SOA. R6 and C6 act as a low-pass lter 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 lter
delay.
(R ?C) ln 1
V
TRIP
V(t
O
)
V(t) V(t
O
)
t
OCDLY
=
(8)
where V
TRIP
is the typical circuit breaker trip voltage specied
in the electrical specications, 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
= 10m&).
V
TRIP
= 50mV
V(t
0
) = 2A ?10m& = 20mV
V(t) = 7.5A ?10m& = 75mV
Using Equation 8, for R6 = 2.7k&, C6 is 0.47礔.
The capacitor (C2) connected from UV to reference (V
EE
)
is used as a glitch lter 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 dened signal referenced to V
EE
.
Sense Resistor Selection
The sense resistor is nominally valued at:
V
TRIP(typ)
I
HOT_SWAP(nom)
R
SENSE(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 ?% initial tolerance, allow ?% 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 MIC2588s minimum current limit threshold voltage
is 40mV, the minimum hot swap load current is determined
where the sense resistor is 3% high:
40mV
1.03 ?R
SENSE(nom)
38.8mV
R
SENSE(nom)
I
HOT_SWAP(min)
(
=
=
)
Keep in mind that the minimum hot swap load current
should be greater than the application circuits 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:
60mV
0.97 ?R
SENSE(nom)
61.9mV
R
SENSE(nom)
I
HOT_SWAP(max)
(
=
=
)
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:
40mV
4A
10m&
R
SENSE(nom)
=
=
where the nearest 1% standard value is 10.0m&. At the
other tolerance extremes, I
HOT_SWAP(MAX)
for the circuit in
question is then simply:
61.9mV
10m&
6.19A
I
HOT_SWAP(max)
=
=
With a knowledge of the application circuits maximum
hot swap load current, the power dissipation rating of the
sense resistor can be determined using P = I
2
?R. Here,
the current is I
HOT_SWAP(max)
= 6.19A and the resistance
R
SENSE(max)
= (1.03)(R
SENSE(nom)
) = 10.3m&.
Thus, the sense resistors maximum power dissipation is:
P
MAX
= (6.19A)
2
?(10.3m&) = 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 parts ability to withstand any peak
currents (transient thermal issues).
Power MOSFET Operating Voltage Requirements
The rst voltage requirement for the MOSFET is easily stated:
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