MIC2582 Datasheet, PDF (18/22 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

MIC2582/MIC2583
MOSFET Selection
Selecting the proper external MOSFET for use with the
MIC2582/83 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
issues).
• Verify the selected part’s ability to withstand any
peak currents (transient thermal issues).
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_IN(MAX). For instance, a 12V input may
reasonably 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 value will be
20V.
The second breakdown voltage criterion that must be met is
a bit subtler than simple drain-source breakdown voltage, but
Micrel
is not hard to meet. In MIC2582/83 applications, the gate of
the external MOSFET is driven up to approximately 19.5V by
the internal output MOSFET (again, assuming 12V opera-
tion). 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 (19.5V – 0V) = 19.5V. 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 at or
above 13V, the 20V gate-source maximum will likely be
exceeded. As a result, an external Zener diode clamp should
be used to prevent breakdown of the external MOSFET when
operating at voltages above 8V. A Zener diode with 10V
rating is recommended as shown in Figure 9. At the present
time, most power MOSFETs with a 20V gate-source voltage
rating have a 30V drain-source breakdown rating or higher.
As a general tip, choose surface-mount devices with a drain-
source rating of 30V as a starting point.
Finally, the external gate drive of the MIC2582/83 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 MOSFET and Sense Resistor
Vendors for suggested manufacturers.
V_IN
12V
D1
(18V)
R_SENSE
0.006Ω
1 ^5% 2
3
4
C1
1µF
Q1
IRF7822
(SO-8)
*D2
1N5240B
10V
R1
33kΩ
8
7
R3
VCC SENSE
10Ω
GATE ⁶
2 _ON
C2
0.01µF
R2
33kΩ
MIC2582
CPOR
3
C3
0.05µF
GND
4
FB ⁵
V_IN
1
/POR
R6
47kΩ
DOWNSTREAM
SIGNAL
Undervoltage (Output) = 11.0V
/POR Delay = 25ms
START-UP Delay = 6ms
*Recommended for MOSFETs with gate-source
breakdown of 20V or less for catastrophic output
short circuit protection. (IRF7822 VGS(MAX) = 12V)
Figure 9. Zener Clamped MOSFET Gate
R4
100kΩ
1%
R5
13.3kΩ
1%
V_OUT
12V@6A
C_LOAD
220µF
MIC2582/MIC2583
18
April 2003