MIC2582 Datasheet, PDF (20/22 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

MIC2582/MIC2583
two iterations, But it’s not a hard calculation to perform, and
tends to converge quickly.
Then the starting (steady-state)T_J is:
T_J ≅ T_A(MAX) + ∆T_J
≅ T_A(MAX) + [R_ON + (T_A(MAX) – T_A)(0.005/°C)(R_ON)]
x I² x R_θ(J-A)
T_J ≅ 55°C + [17mΩ + (55°C-25°C)(0.005)(17mΩ)]
x (2.5A)² x (50°C/W)
T_J ≅ (55°C + (0.122W)(50°C/W)
≅ 61.1°C
Iterate the calculation once to see if this value is within a few
percent of the expected final value. For this iteration we will
start with T_J equal to the already calculated value of 61.1°C:
T_J
≅
T_A
+ [17mΩ +
x (2.5A)² x
(61.1°C-25°C)(0.005)(17mΩ)]
(50°C/W)
T_J ≅ ( 55°C + (0.125W)(50°C/W) ≅ 61.27°C
Micrel
So our original approximation of 61.1°C was very close to the
correct value. We will use T_J = 61°C.
Finally, add (11.25W)(50°C/W)(0.08) = 45°C to the steady-
state T_J to get T_{J(TRANSIENT MAX.)} = 106°C. This is an accept-
able maximum junction temperature for this part.
PCB Layout Considerations
Because of the low values of the sense resistors used with the
MIC2582/83 controllers, special attention to the layout must
be used in order for the device’s circuit breaker function to
operate properly. Specifically, the use of a 4-wire Kelvin
connection to accurately measure the voltage across R_SENSE
is highly recommended. Kelvin sensing is simply a means of
making sure that any voltage drops in the power traces
connecting to the resistors does not get picked up by the
traces themselves. Additionally, these Kelvin connections
should be isolated from all other signal traces to avoid
introducing noise onto these sensitive nodes. Figure 11
illustrates a recommended, single layer layout for the R_SENSE,
2
1
Duty Cycle = 0.5
Normalized Thermal Transient Impedance, Junction-to-Ambient
0.2
0.1
0.1
0.05
0.02
0.01
10^–4
Notes:
P_DM
Single Pulse
t₁
t₂
1. Duty Cycle, D =
t₁
t₂
2. Per Unit Base = R_thJA = 50° C/W
3. T_JM – T_A = P_DMZ_thJA(t)
4. Surface Mounted
10^–3
10^–2
10^–1
1
Square Wave Pulse Duration (sec)
Figure 10. Transient Thermal Impedance
10
30
Current Flow
to the Load
W
*SENSE RESISTOR
(2512)
*POWER MOSFET
(SO-8)
D
G
D
S
D
S
D
S
Current Flow
to the Load
W
8
7
6
5
1
2
3
4
Current Flow ^**CPOR
to the Load
**R_GATE
93.1k
1%
12.4k
1%
**C_GATE
W
DRAWING IS NOT TO SCALE
*See Table 4 for part numbers and vendors.
**Optional components.
Trace width (W) guidelines given in "PCB Layout Recommendations" section of the datasheet.
Figure 11. Recommended PCB Layout for Sense Resistor, Power MOSFET, and Feedback Network
MIC2582/MIC2583
20
April 2003