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參數資料
| 型號： | MIC2341-2YTQ |
| 廠商： | Micrel Inc |
| 文件頁數： | 28/32頁 |
| 文件大小： | 1277K |
| 描述： | IC HOT PLUG CTLR DUAL PCI 48TQFP |
| 標準包裝： | 250 |
| 類型： | 熱交換控制器 |
| 應用： | 通用型，PCI Express? |
| 內部開關： | 無 |
| 電源電壓： | 3.3V，12V |
| 工作溫度： | 0°C ~ 70°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 48-TQFP |
| 供應商設備封裝： | 48-TQFP（7x7） |
| 包裝： | 管件 |
| 產品目錄頁面： | 1085 (CN2011-ZH PDF) |
| 其它名稱： | 576-2995 |
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Micrel, Inc.
MIC2341/2341R
October 2007
28
M9999-102507-A
(408) 944-0800
Applications Information
Sense Resistor Selection
The 12V and the 3.3V supplies employ internal current
sensing circuitry to detect overcurrent conditions that
may trip the circuit breaker. An external sense resistor is
used to monitor the current that passes through the
external MOSFET for each slot of the 12V and 3.3V
rails. The sense resistor is nominally valued at:
LIMIT
THLIMIT
SENSE(NOM)
I
V
R
=
where V
THILIMIT
is the typical (or nominal) circuit breaker
threshold voltage (50mV) and I
LIMIT
is the nominal inrush
load current level to trip the internal circuit breaker.
To accommodate worse-case tolerances in the sense
resistor (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 MIC2341s minimum current limit threshold
voltage is 45mV, the minimum hot swap load current is
determined where the sense resistor is 3% high:
SENSE(NOM)
SENSE(NOM)
LIMIT(MIN)
R
43.7mV
)
R
(1.03
45mV
I
=
?/DIV>
=
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
LIMIT(MAX)
)
which 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
THILIMIT(MAX)
threshold of
55mV and a sense resistor 3% low in value:
SENSE(NOM)
SENSE(NOM)
LIMIT(MAX)
R
56.7mV
)
R
(0.97
55mV
I
=
?/DIV>
=
In this case, the application circuits must be sturdy
enough to operate up to approximately 1.25x the steady-
state hot swap load currents. For example, if one of the
12V slots of the MIC2341 circuit must pass a minimum
hot swap load current of 1.5A without nuisance trips,
R
SENSE
should be set to:
30m&
1.5A
45mV
R
SENSE(NOM)
=
=
where the nearest 1% standard value is 30.1m&. At the
other tolerance extremes, I
LIMIT(MAX)
for the circuit in
question is then simply:
1.88A
30.1m&
56.7mV
I
LIMIT(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
LIMIT(MAX)
= 1.88A and the resistance
R
SENSE(MAX)
= (1.03)(R
SENSE(NOM)
) = 31.00m&. Thus, the
sense resistors maximum power dissipation is:
P
MAX
= (1.88A)
2
X (31.00m&) = 0.110W
A 0.25W sense resistor is a good choice in this
application.
PCB Layout Suggestions and Hints
4-Wire Kelvin Sensing
Because of the low value required for the sense resistor,
special care must be used to accurately measure the
voltage drop across it. Specifically, the measurement
technique across R
SENSE
must employ 4-wire Kelvin
sensing. This is simply a means of ensuring that any
voltage drops in the power traces connected to the
resistors are not picked up by the signal conductors
measuring the voltages across the sense resistors.
Figure 12 illustrates how to implement 4-wire Kelvin
sensing. As the figure shows, all the high current in the
circuit (from V
IN
through R
SENSE
and then to the drain of
the N-channel power MOSFET) flows directly through
the power PCB traces and through R
SENSE
. The voltage
drop across R
SENSE
is sampled in such a way that the
high currents through the power traces will not introduce
significant parasitic voltage drops in the sense leads. It is
recommended to connect the hot swap controllers
sense leads directly to the sense resistors metalized
contact pads. The Kelvin sense signal traces should be
symmetrical with equal length and width, kept as short
as possible, and isolated from any noisy signals and
planes.
Additionally, for designs that implement Kelvin sense
connections that exceed 1" in length and/or if the Kelvin
(signal) traces are vulnerable to noise possibly being
injected onto these signals, the example circuit shown in
Figure 13 can be implemented to combat noisy
environments. This circuit implements a 1.6 MHz low-
pass filter to attenuate higher frequency disturbances on
the current sensing circuitry. However, individual system
analysis should be used to determine if filtering is
necessary and to select the appropriate cutoff frequency
for each specific application.
Other Layout Considerations
Figure 14 is a suggested PCB layout diagram for the
MIC2341 power traces, Kelvin sense connections, and
capacitor components. In this illustration, only the 12V
Slot B is shown but a similar approach is suggested for
both slots of each Main power rail (12V and 3.3V). Many
hot swap applications will require load currents of
several amperes. Therefore, the power (12VIN and
Return, 3VIN and Return) trace widths (W) need to be
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