MIC2584/2585
Micrel
the opposite direction. Here, the worst-case maximum cur-
rent is found using a 57.5mV trip voltage and a sense resistor
that is 3% low in value. The resulting equation is:
MOSFET Voltage Requirements
The first voltage requirement for the MOSFET is easily stated:
the drain-source breakdown voltage of the MOSFET must be
greater than V
. For instance, a 12V input may reason-
57.5mV
59.3mV
IN(MAX)
ILOAD(CONT,MAX)
=
=
ably be expected to see high-frequency transients as high as
18V. Therefore, the drain-source breakdown voltage of the
MOSFET must be at least 19V. For ample safety margin and
standard availability, the closest minimum value will be 20V.
(12)
RSENSE(NOM)
0.97 R
(
)
(
)
SENSE(NOM)
As an example, if an output must carry a continuous 6A
without nuisance trips occurring, Equation 11
The second breakdown voltage criterion that must be met is a
bit subtler than simple drain-source breakdown voltage, but is
not hard to meet. In MIC2584/85 applications, the gate of the
external MOSFET is driven up to approximately 20V by the
internal output MOSFET (again, assuming 12V operation). At
thesametime,iftheoutputoftheexternalMOSFET(itssource)
issuddenlysubjectedtoashort, thegate-sourcevoltagewillgo
to (20V – 0V) = 20V. This means that the external MOSFET
must be chosen to have a gate-source breakdown voltage of
20V or more, which is an available standard maximum value.
However, if operation is above 12V, the 20V gate-source
maximumwilllikelybeexceeded.Asaresult,anexternalZener
diode clamp should be used to prevent breakdown of the
external MOSFET when operating at voltages above 10V. A
Zener diode with 10V rating is recommended as shown in
Figure 12. At the present time, most power MOSFETs with a
20Vgate-sourcevoltageratinghavea30Vdrain-sourcebreak-
down rating or higher. As a general tip, choose surface-mount
devices with a drain-source rating of 30V as a starting point.
41.3mV
R
=
= 6.88mΩ
. The next lowest
yields:
SENSE(MAX)
6A
standardvalueis6mΩ Attheothersetoftoleranceextremes
for the output in question,
.
59.3mV
I
=
= 9.88A
. Knowing this final da-
LOAD(CONT,MAX)
6.0mΩ
tum, we can determine the necessary wattage of the sense
2
I will be I
, and
resistor using P = I R, where
LOAD(CONT, MAX)
R will be (0.97)(R
).
These numbers yield the
= 0.568W. In this ex-
SENSE(NOM)
2
following: P
= (9.88A) (5.82mΩ)
MAX
ample, a 1W sense resistor is sufficient.
MOSFET Selection
Selecting the proper external MOSFET for use with the
MIC2584/85 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 is-
sues).
Finally, the external gate drive of the MIC2584/85 requires a
low-voltage logic level MOSFET when operating at voltages
lower than 3V. There are 2.5V logic level MOSFETs available.
See Table 5, "MOSFET and Sense Resistor Vendors" for
suggested manufacturers.
• Verify the selected part’s ability to withstand any
peak currents (transient thermal issues).
CORE SUPPLY
I/O SUPPLY
(VCC
)
(VDD)
VDD
VDD
Data In
OE
Data In
OE
I1
OUTPUT
DRIVER
CIRCUIT
BLOCK
OUTPUT
DRIVER
CIRCUIT
BLOCK
I2
Data Out
Data Out
External
Bus Control
TX_/RX
CORE
I/O
Dual Supply DSP
Peripheral
Figure 11. Bidirectional Port Bus Contention
March 2005
23
MIC2584/2585
MICREL [ MICREL SEMICONDUCTOR ]
