MIC2582_09 Datasheet, PDF (16/25 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
Application Information
Design Consideration for Output Undervoltage
Detection
For output undervoltage detection, the first consideration
is to establish the output voltage level that indicates
“power is good.” For this example, the output value for
which a 12V supply will signal “good” is 11V. Next,
consider the tolerances of the input supply and FB
threshold (V_FB). For this example, the 12V supply varies
±5%, thus the resulting output voltage may be as low as
11.4V and as high as 12.6V. Additionally, the FB
threshold has ±50mV tolerance and may be as low as
1.19V and as high as 1.29V. Thus, to determine the
values of the resistive divider network (R5 and R6) at the
FB pin, shown in the typical application circuit on page 1,
use the following iterative design procedure.
1) Choose R6 to allow 100µA or more in the FB
resistive divider branch.
R6 = ^VFB(MAX) = ^1.29V = 12.9kΩ
100μA 100μA
R6 is chosen as 12.4kΩ ±1%.
2) Next, determine R5 using the output “good”
voltage of 11V and the following equation:
( ) _VOUT(Good)
= V_FB
⎡ R5 + R6
⎢⎣ _R6
⎤
⎥⎦
(9)
Using some basic algebra and simplifying Equation 9 to
isolate R5, yields:
MIC2582/MIC2583
R5
=
R6
⎢⎢⎣⎡⎜⎜⎝⎛
V_OUT(Good)
V_FB(MAX)
⎟⎞
⎟⎠
⎤
− 1⎥
⎥⎦
(9.1)
where V_FB(MAX) = 1.29V, VOUT(Good) = 11V, and R6 is
12.4kΩ. Substituting these values into Equation 9.1 now
yields R5 = 93.33kΩ. A standard 93.1kΩ ±% is selected.
Now, consider the 11.4V minimum output voltage, the
lower tolerance for R6 and higher tolerance for R5,
12.28kΩ and 94.03kΩ, respectively. With only 11.4V
available, the voltage sensed at the FB pin exceeds
V_FB(MAX), thus the /POR and PWRGD (MIC2583/83R)
signals will transition from LOW to HIGH, indicating
“power is good” given the worse case tolerances of this
example. Lastly, in giving consideration to the leakage
current associated with the FB input, it is recommended
to either: 1) provide ample design margin (20mV to
30mV) to allow for loss in the potential (∆V) at the FB
pin, or 2) allow >>100µA to flow in the FB resistor
network.
PCB Connection Sense
There are several configuration options for the
MIC2582/83’s ON pin to detect if the PCB has been fully
seated in the backplane before initiating a start-up cycle.
In the typical applications circuit, the MIC2582/83 is
mounted on the PCB with a resistive divider network
connected to the ON pin. R2 is connected to a short pin
on the PCB edge connector. Until the connectors mate,
the ON pin is held low which keeps the GATE output
charge pump off. Once the connectors mate, the resistor
network is pulled up to the input supply,
April 2009
Figure 6. PCB Connection Sense with ON/OFF Control
16
M9999-043009-C