MIC2582 Datasheet, PDF (15/22 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

MIC2582/MIC2583
Applications Information
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 so as to limit the current through the
divider to approximately 100µA or less.
R6
≅
V_FB(MAX)
100µA
≅
1.29V
100µA
≅ 12.9kΩ _.
R6 is chosen as 12.4kΩ ± 1%.
2) Next, determine R5 using the output “good”
voltage of 11V and the following equation:
V_OUT(Good)
=
V_FB



(R5 + R6)
R6



(9)
Using some basic algebra and simplifying Equation 9 to
isolate R5, yields:
Micrel
R5
=

R6_{}
V_OUT(Good)
V_FB(MAX)




– 1^

(9.1)
where V_FB(MAX) = 1.29V, V_OUT(Good) = 11V, and R6 is
12.4kΩ. Substituting these values into Equation 9.1 now
yields R5 = 93.33kΩ. A standard 93.1kΩ ± 1% 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.
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, 12V in this
example, and the ON pin voltage exceeds its threshold (V_ON)
of 1.24V and the MIC2582/83 initiates a start-up cycle. In
Figure 6, the connection sense consisting of a discrete
logic-level MOSFET and a few resistors allows for interrupt
Backplane PCB Edge
Connector Connector
Long
Pin
V_IN
5V
R_SENSE
0.010Ω
1 ^5% 2
C1
3
4
1F
/ON_OFF
/FAULT
GND
R5
20kΩ
Short
Pin
Medium or
Short Pin
R4
20kΩ
R1
33kΩ
R3
100Ω
R2
*Q2 ^33kΩ
16
VCC
3 _ON
15
SENSE
MIC2583
PCB Connection Sense
11
/FAULT
CPOR
4
C3
0.05 F
GND
7,8
Long
Pin
Undervoltage (Output) = 4.45V
/POR Delay = 25ms
START-UP Delay = 6ms
*Q2 is TN0201T (SOT-23)
**R8 is optional for noise filtering
Additional pins omitted for clarity.
Q1
Si7860DP
(PowerPAK“ SO-8)
GATE ¹⁴
**R8
10Ω
C2
0.01 F
DIS ¹³
FB ¹²
1
/POR
V_IN
R9
20Ω
Downstream
Signal
Figure 6. PCB Connection Sense with ON/OFF Control
V_OUT
5V@3A
C_LOAD
220 F
R6
27.4kΩ
1%
R7
10.5kΩ
1%
April 2003
15
MIC2582/MIC2583