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MIC2085-LBQS Datasheet

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MIC2085/2086
The second breakdown voltage criterion that must be met is
a bit subtler than simple drain-source breakdown voltage. In
MIC2085/86 applications, the gate of the external MOSFET
is driven up to a maximum of 21V by the internal output
MOSFET. At the same time, if the output of the external
MOSFET (its source) is suddenly subjected to a short, the
gate-source voltage will go to (21V 鈥?0V) = 21V. Since most
power MOSFETs generally have a maximum gate-source
breakdown of 20V or less, the use of a Zener clamp is
recommended in applications with V
CC
鈮?/div>
8V. A Zener diode
with 10V to 12V rating is recommended as shown in Figure
11. At the present time, most power MOSFETs with a 20V
gate-source voltage rating have a 30V drain-source break-
down rating or higher. As a general tip, choose surface-mount
devices with a drain-source rating of 30V or more as a starting
point.
Finally, the external gate drive of the MIC2085/86 requires a
low-voltage logic level MOSFET when operating at voltages
lower than 3V. There are 2.5V logic level MOSFETs avail-
able. Please see Table 4,
鈥淢OSFET and Sense Resistor
Vendors鈥?/div>
for suggested manufacturers.
MOSFET Steady-State Thermal Issues
The selection of a MOSFET to meet the maximum continuous
current is a fairly straightforward exercise. First, arm yourself
with the following data:
鈥?The value of I
LOAD(CONT, MAX.)
for the output in
question (see
鈥淪ense Resistor Selection鈥?/div>
).
鈥?The manufacturer鈥檚 data sheet for the candidate
MOSFET.
鈥?The maximum ambient temperature in which the
device will be required to operate.
Micrel
鈥?Any knowledge you can get about the heat
sinking available to the device (e.g., can heat be
dissipated into the ground plane or power plane,
if using a surface-mount part? Is any airflow
available?).
The data sheet will almost always give a value of on resis-
tance given for the MOSFET at a gate-source voltage of 4.5V,
and another value at a gate-source voltage of 10V. As a first
approximation, add the two values together and divide by two
to get the on-resistance of the part with 8V of enhancement.
Call this value R
ON
. Since a heavily enhanced MOSFET acts
as an ohmic (resistive) device, almost all that鈥檚 required to
determine steady-state power dissipation is to calculate I
2
R.
The one addendum to this is that MOSFETs have a slight
increase in R
ON
with increasing die temperature. A good
approximation for this value is 0.5% increase in R
ON
per
掳C
rise in junction temperature above the point at which R
ON
was
initially specified by the manufacturer. For instance, if the
selected MOSFET has a calculated R
ON
of 10m鈩?at a
T
J
= 25掳C, and the actual junction temperature ends up
at 110掳C, a good first cut at the operating value for R
ON
would be:
R
ON
鈮?/div>
10m鈩1 + (110 - 25)(0.005)]
鈮?/div>
14.3m鈩?/div>
The final step is to make sure that the heat sinking available
to the MOSFET is capable of dissipating at least as much
power (rated in
掳C/W)
as that with which the MOSFET鈥檚
performance was specified by the manufacturer. Here are a
few practical tips:
1. The heat from a surface-mount device such as
an SO-8 MOSFET flows almost entirely out of
the drain leads. If the drain leads can be sol-
dered down to one square inch or more, the
copper will act as the heat sink for the part. This
copper must be on the same layer of the board
as the MOSFET drain.
Q1
IRF7822
(SO-8)
4
V
IN
12V
3
R
SENSE
0.007鈩?/div>
1
2%
2
C1
1碌F
R1
47k鈩?/div>
*D1
1N5240B
10V
C
LOAD
220碌F
V
OUT
12V@5A
16
15
VCC
SENSE
GATE
R3
10鈩?/div>
14
R4
100k鈩?/div>
1%
4
C2
0.01碌F
ON
FB
MIC2085
7
R2
33k鈩?/div>
CPOR
3
R5
13.3k鈩?/div>
1%
/FAULT
/POR
6
5
Downstream
Signals
GND
8
C3
0.1碌F
Undervoltage (Output) = 11.0V
POR/START-UP Delay = 60ms
*Recommended for MOSFETs with gate-source
breakdown of 20V or less (IRF7822 VGS(MAX) = 12V)
for catastrophic output short circuit protection.
Additional pins omitted for clarity.
Figure 11. Zener Clamped MOSFET GATE
January 2004
23
M0235-121903

MIC2085-LBQS 产品属性

  • 196

  • 集成电路 (IC)

  • PMIC - 热交换

  • -

  • 热交换控制器

  • 通用型 Infiniband?

  • 无

  • -

  • 2.3 V ~ 16.5 V

  • -40°C ~ 85°C

  • 表面贴装

  • 16-SSOP（0.154"，3.90mm 宽）

  • 16-QSOP

  • 管件

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