MIC2592B-2YTQ Micrel Inc, MIC2592B-2YTQ Datasheet - Page 27

• Manufacturer Part Number: MIC2592B-2YTQ
• Description: IC CTRLR HOTPLUG PCI DUAL 48TQFP
• Manufacturer: Micrel Inc
• Type: Hot-Swap Controllerr

Specifications of MIC2592B-2YTQ

• Applications: General Purpose, PCI Express
• Internal Switch(s): No
• Voltage - Supply: 3.3V, 12V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 48-TQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 576-1099

Applications Information
Sense Resistor Selection
The 12V and the 3.3V supplies employ internal current sens-
ing 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:
where V
threshold voltage (50mV) and I
load current level 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 MIC2592B’s minimum current limit threshold voltage
is 45mV, the minimum hot swap load current is determined
where the sense resistor is 3% high:
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
been calculated, it is good practice to check the maximum
hot swap load current (I
let pass in the case of tolerance build-up in the opposite
let pass in the case of tolerance build-up in the opposite
direction. Here, the worse-case maximum is found using a
V
low in value:
low in value:
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
MIC2592B circuit must pass a minimum hot swap load current
of 1.5A without nuisance trips, R
where the nearest 1% standard value is 30.1mΩ. At the other
tolerance extremes, I
then simply:
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 = I
is I
(1.03)(R
(1.03)(R
maximum power dissipation is:
maximum power dissipation is:
P
March 2005
THILIMIT(MAX)
THILIMIT(MAX)
THILIMIT(MAX)
MAX
LIMIT(MAX)
LIMIT(MAX)
LIMIT(MAX)
= (1.88A)
R
R
I
I
LIMIT(MAX)
LIMIT(MIN)
I
SENSE(NOM)
SENSE(NOM)
SENSE(NOM)
LIMIT(MAX)
SENSE(NOM)
SENSE(NOM)
THILIMIT
THILIMIT
THILIMIT
= 1.88A and the resistance R
threshold of 55mV and a sense resistor 3%
=
2
=
=
X (31.00mΩ) = 0.110W
is the typical (or nominal) circuit breaker
is the typical (or nominal) circuit breaker
(1.03 × R
=
=
30.1mΩ
56.7mV
(0.97 × R
) = 31.00mΩ. Thus, the sense resistor’s
) = 31.00mΩ. Thus, the sense resistor’s
V
45mV
1.5A
LIMIT(MAX)
THILIMIT
I
LIMIT
45mV
55mV
LIMIT(MAX)
LIMIT(MAX)
LIMIT(MAX)
SENSE(NOM)
= 1.88A
SENSE(NOM)
= 30mΩ
for the circuit in question is
LIMIT
LIMIT
LIMIT
SENSE
)
) which the circuit may
)
=
is the nominal inrush
is the nominal inrush
=
2
R
should be set to:
R. Here, the current
R
43.7mV
SENSE(NOM)
56.7mV
SENSE(NOM)
SENSE(MAX)
SENSE(MAX)
SENSE(MAX)
SENSE
has
=
27
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 volt-
age drop across it. Specifi cally, the measurement technique
across R
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 13 illustrates how to implement 4-wire Kelvin sensing.
As the fi gure shows, all the high current in the circuit (from
V
power MOSFET) fl ows directly through the power PCB traces
and through R
sampled in such a way that the high currents through the
power traces will not introduce signifi cant parasitic voltage
drops in the sense leads. It is recommended to connect
the hot swap controller’s sense leads directly to the sense
resistor’s 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 con-
nections 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 14 can
be implemented to combat noisy environments. This circuit
implements a 1.6 MHz low-pass fi lter to attenuate higher
frequency disturbances on the current sensing circuitry.
However, individual system analysis should be used to de-
termine if fi ltering is necessary and to select the appropriate
cutoff frequency for each specifi c application.
Other Layout Considerations
Figure 15 is a suggested PCB layout diagram for the MIC2592B
power traces, Kelvin sense connections, and capacitor com-
ponents. 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 wide enough to allow the current to fl ow while
the rise in temperature for a given copper plate (e.g., 1oz.
or 2oz.) is kept to a maximum of 10°C to 25°C. The return
(or power ground) trace should be the same width as the
positive voltage power traces (input/load) and isolated from
any ground and signal planes so that the controller’s power
is common mode. Also, these traces should be as short as
possible in order to minimize the IR drops between the input
and the load. As indicated in the Pin Description section, an
external connection must be made that ties together both
channel inputs ((+) Kelvin sense) of each Main power rail (i.e.,
3VINA and 3VINB, 12VINA and 12VINB must be externally
connected). These connections should be implemented di-
rectly at the chip. Insure that the voltage drop between the
two (+) Kelvin sense inputs for each rail is no greater than
0.2mV by using a common power path for the two inputs
IN
through R
SENSE
SENSE
SENSE
must employ 4-wire Kelvin sensing. This is
and then to the drain of the N-channel
. The voltage drop across R
M9999-033105
SENSE
is