Micrel
MIC2587/MIC2587R
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 PWRGD (or /PWRGD) is to be de-
asserted when the output supply voltage is lower than
+48V-10% (+43.2V).
Applications Information
External ON/OFF Control
The MIC2587/MIC2587R 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 MIC2587/MIC2587R’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
VOUT(NOT GOOD) = VFBL
×
(9)
⎜
⎝
⎟
⎠
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:
VFBL(TYP)
1.233V
R6 =
=
= 12.33 kΩ
100
µ
A
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 = 422 kΩ
⎢⎜
⎟
⎠
⎥
⎜
VFBL(TYP)
1.233V
⎝
⎣
⎢
⎥
⎦
⎦
⎝
⎠
⎣
The closest standard 1% value for R5 is 422kΩ.
⎛
⎞
R1+R2
R2
VONH(EX) = VONH
×
(8)
⎜
⎝
⎟
⎠
Using standard 1% resistor values, the external circuit's
nominal “power-is-good” and “power-is-not-good” output
voltages are:
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:
VOUT(GOOD) = +46V
VOUT(NOT GOOD) = +43.2V
In solving for VOUT(GOOD), substitute VFBH for VFBL in
Equation 9.
VONH(TYP)
1.313V
R2 =
=
= 13.13kΩ
100µA
100µA
Sense Resistor Selection
The sense resistor is nominally valued at:
VTRIP(TYP)
The closest standard 1% value for R2 is 13kΩ. Now,
solving for R1 yields:
RSENSE(NOM)
=
(10)
IHOT_SWAP(NOM)
⎡
⎤
⎛
⎞
⎡
⎤
⎛
⎞
VONH(EX)
37V
⎢
⎥
⎜
⎟
⎟
R1= R2 ×
−1 = 13kΩ ×
−1 = 353.3 kΩ
⎢⎜
⎟
⎠
⎥
⎜
where VTRIP(TYP) is the typical (or nominal) circuit breaker
threshold voltage (47mV) and IHOT_SWAP(NOM) is the nominal
inrush load current level to trip the internal circuit breaker.
VONH(TYP)
1.313V
⎝
⎢
⎥
⎦
⎣
⎦
⎝
⎠
⎣
The closest standard 1% value for R1 is 357kΩ.
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.
Using standard 1% resistor values, the external circuit's
nominal ON and OFF thresholds are:
V
ON(EX) = +36V
OFF(EX) = +34V
V
In solving for VOFF(EX), replace VONH with VONL in Equation 8.
As the MIC2587/MIC2587R's minimum current limit
threshold voltage is 39mV, the minimum hot swap load
current is determined where the sense resistor is 3% high:
Output Voltage Power-is-Good Detection
The MIC2587/MIC2587R 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.
39mV
1.03 ×R
37.9mV
IHOT_SWAP(MIN)
=
=
RSENSE(NOM)
(
)
SENSE(NOM)
Keep in mind that the minimum hot swap load current
should be greater than the application circuit's upper
steady-state load current boundary. Once the lower value
of RSENSE has been calculated, it is good practice to check
Setting the “Power-is-Good” threshold for the circuit follows
M9999-102204
(408) 955-1690
October 2004
12
MICREL [ MICREL SEMICONDUCTOR ]
