Micrel
MIC2586/MIC2586R
Setting the "Power-is-Good" threshold for the circuit follows a
similar approach as setting the circuit's ON/OFF input
voltage. The equations to set the trip points are shown
below. For the following +48V telecom application, power-is-
good output signal PWRGD1 (or /PWRGD1) is to be de-
asserted when the output supply voltage is lower than +48V-
10% (+43.2V).
Applications Information
External ON/OFF Control
The MIC2586/MIC2586R have an ON pin input that is used
to enable the controller to commence a start-up sequence
upon card insertion or to disable controller operation upon
card removal. In addition, the ON pin can be used to reset
the MIC2586/MIC2586R’s internal electronic circuit breaker
in the event of a load current fault. To reset the electronic
circuit breaker, the ON pin is toggled LOW then HIGH. The
ON pin is internally connected to an analog comparator with
80mV of hysteresis. When the ON pin voltage falls below its
internal VONL threshold, the GATE pin is immediately pulled
low. The GATE pin will be held low until the ON pin voltage is
above its internal VONH threshold. The external circuit's ON
threshold voltage level is programmed using a resistor divider
(R1 & R2) as shown in the "Typical Application” circuit. The
equations to set the trip points are shown below. For the
following example, the external circuit's ON threshold is set
to VONH(EX) = +37V, a value commonly used in +48V Central
Office power distribution applications.
⎛
⎞
R5 +R6
R6
V
OUT(NOT GOOD) = V
×
(11)
⎜
⎝
⎟
⎠
FBL
Given VFBL and R6, a value for R5 can be determined. A
suggested value for R6 is that which will provide
approximately 100µA of current through the voltage divider
chain at VOUT(NOT
= VFBL. This yields the following
GOOD)
equation as a starting point:
V
1.233V
FBL(TYP)
R6 =
=
= 12.33kΩ
100µA 100µA
The closest standard 1% value for R6 is 12.4kΩ. Now,
solving for R5 yields:
⎡
⎤
⎛
⎞
⎡
⎤
VOUT(NOT GOOD)
⎛
⎞
⎟
43.2V
⎢
⎥
R5 =R6 × ⎜
⎟−1 =12.4kΩ ×
−1 = 422kΩ
⎜
⎢
⎥
⎜
⎟
⎛
⎞
R1+R2
R2
V
⎝1.233V⎠
⎣
⎦
⎢
⎣
⎥
⎦
⎝
FBL(TYP)
⎠
VONH(EX) = V
×
(10)
⎜
⎝
⎟
⎠
ONH
Given VONH and R2, a value for R1 can be determined. A
suggested value for R2 is that which will provide
approximately 100µA of current through the voltage divider
chain at VCC = VONH. This yields the following as a starting
point:
The closest standard 1% value for R5 is 422kΩ.
Using standard 1% resistor values, the external circuit's
nominal "power-is-good" and "power-is-not-good" output
voltages are:
V
V
OUT(GOOD) = +46V
V
1.313V
ONH(TYP)
OUT(NOT GOOD) = +43.2V
R2 =
=
=13.13kΩ
100µA 100µA
In solving for VOUT(GOOD), substitute VFBH for VFBL in Equation
11.
The closest standard 1% value for R2 is 13kΩ. Now, solving
for R1 yields:
Sense Resistor Selection
The sense resistor is nominally valued at:
VTRIP(TYP)
⎡
⎢
⎤
⎛
⎞
⎡
⎤
V
⎛
⎞
⎟
37V
ONH(EX)
⎥
R1= R2 ×
−1 = 13kΩ ×
−1 = 353.3kΩ
⎜
⎜
⎟
⎟
⎜
⎢
⎥
V
⎝1.313V⎠
⎣
⎦
⎢
⎝
⎣
⎥
⎦
ONH(TYP)
⎠
RSENSE
=
(12)
IHOT_SWAP(NOM)
The closest standard 1% value for R1 is 357kΩ.
Using standard 1% resistor values, the external circuit's
nominal ON and OFF thresholds are:
where VTRIP(TYP) is the nominal circuit breaker threshold
voltage (47mV) and IHOT_SWAP(NOM) is the nominal inrush load
current level to trip the internal circuit breaker.
To accommodate worse-case tolerances in the sense
resistor (for a ±1% initial tolerance, allow ±3% tolerance for
variations over time and temperature) and circuit breaker
threshold voltages, a slightly more detailed calculation must
be used to determine the minimum and maximum hot swap
load currents.
V
V
ON(EX) = +36V
OFF(EX) = +34V
In solving for VOFF(EX), replace VONH with VONL in Equation 10.
Output Voltage Power-is-Good Detection
The MIC2586/86R includes an analog comparator used to
monitor the output voltage of the controller through an
external resistor divider as shown in the "Typical Application"
circuit. The FB input pin is connected to the non-inverting
input and is compared against an internal reference voltage.
The analog comparator exhibits a hysteresis of 80mV.
The MIC2586/MIC2586R has a minimum current limit
threshold voltage of 39mV, thus the minimum hot swap load
current is determined where the sense resistor is 3% high:
39mV
1.03×R
37.9mV
IHOT_SWAP(MIN)
=
=
RSENSE(NOM)
(
)
SENSE(NOM)
M9999-102204
(408) 955-1690
October 2004
13
MICREL [ MICREL SEMICONDUCTOR ]
