LTC1155 查看數據表（PDF） - Linear Technology
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LTC1155 Datasheet PDF : 16 Pages
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LTC1155
APPLICATI S I FOR ATIO
For this example, we assume a worst-case scenario; i.e.,
that the power supply to the power MOSFET is “hard” and
provides a constant 5V regardless of the current. In this
case, the current is limited by the RDS(ON) of the MOSFET
and the drain sense resistance. Therefore:
IPEAK = VSUPPLY/0.08Ω
= 62.5A
The drop across the drain sense resistor under these
conditions is much larger than 100mV and is equal to the
drain current times the sense resistance:
VDROP = (IPEAK)(RSEN)
= 1.88V
By consulting the power MOSFET data sheet SOA graph,
we note that the IRLZ34 is capable of delivering 62.5A at
a drain-to-source voltage of 3.12V for approximately
10ms.
An RC time constant can now be calculated which satisfies
this requirement:
RC =
–t
In 1−
VSEN
RSEN • IMAX
RC =
– 0.01
In 1−
0.10
0.030 • 62.5
= – 0.01/– 0.054
= 182ms
This time constant should be viewed as a maximum safe
delay time and should be reduced if the competing
requirement of starting a high inrush current load is less
stringent; i.e., if the inrush time period is calculated at
20ms, the RC time constant should be set at roughly two
or three times this time period and not at the maximum of
182ms. A 60ms time constant would be produced with a
270k resistor and a 0.22µF capacitor (as shown in
Figure 1).
Graphical Approach to Selecting RDLY and CDLY
Figure 2 is a graph of normalized overcurrent shutdown
time versus normalized MOSFET current. This graph can
be used instead of the above equation to calculate the RC
time constant. The Y axis of the graph is normalized to one
RC time constant. The X axis is normalized to the set
current. (The set current is defined as the current required
to develop 100mV across the drain sense resistor).
10
1
0.1
0.01
1
2
5 10 20 50 100
MOSFET CURRENT (1 = SET CURRENT)
1155 F02
Figure 2. Shutdown Time vs MOSFET Current
Note that the shutdown time is shorter for increasing
levels of MOSFET current. This ensures that the total
energy dissipated by the MOSFET is always within the
bounds established by the MOSFET manufacturer for safe
operation.
In the example presented above, we established that the
power MOSFET should not be allowed to pass 62.5A for
more than 10ms. 62.5A is roughly 18 times the set current
of 3.3A. By drawing a line up from 18 and reflecting it off
the curve, we establish that the RC time constant should
be set at 10ms divided by 0.054, or 180ms. Both methods
result in the same conclusion.
Using a Speed Up Diode
A way to further reduce the amount of time that the power
MOSFET is in a short-circuit condition is to “bypass”the
delay resistor with a small signal diode as shown in Figure
3. The diode will engage when the drop across the drain
sense resistor exceeds 0.7V, providing a direct path to the
7
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