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| 型号： | MIC2587R-1YM TR |
| 厂商： | Micrel Inc |
| 文件页数： | 14/22页 |
| 文件大小： | 972K |
| 描述： | IC CTRLR HOT SWAP POS HV 8-SOIC |
| 标准包装： | 2,500 |
| 类型： | 热交换控制器 |
| 应用： | 通用 |
| 内部开关： | 无 |
| 电源电压： | 10 V ~ 80 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 8-SOIC |
| 包装： | 带卷 (TR) |
| 其它名称： | MIC2587R-1YMTR MIC2587R-1YMTR-ND |
icrel, Inc.
MIC2587/MIC2587R
anuary 24, 2013
14
Revision 2.0
Overcurrent Protection
The MIC2587 and the MIC2587R use an external, low-
value resistor in series with the drain of the external
MOSFET to measure the current flowing into the load.
The VCC connection (Pin 8) and the SENSE connection
(Pin 7) are the (+) and () inputs, respectively, of the
device's internal current sensing circuits. Kelvin sense
connections are strongly recommended for sensing the
voltage across these pins. See the Applications
Information section for further details.
The nominal current limit is determined by Equation 3:
EN E
TRIP(TYP)
LIM
R
V
=
Eq. 3
where V
TRIP(TYP)
is the typical current-limit threshold
specified in the datasheet and R
SENSE
is the value of the
selected sense resistor. The controllers employ a
constant-current regulation scheme while in current limit.
The internal charge pumps output voltage, seen at the
GATE pin, is adjusted so that the voltage across the
external sense resistor is held equal to V
TRIP
while the
capacitor connected to the TIMER pin is being charged. If
the current-limit condition goes away before the TIMER
pin voltage rises above the V
TIMERH
threshold, then
steady-state operation resumes. To prevent excessive
power dissipation in the external MOSFET under load
current fault conditions, the FB pin voltage is used as an
input to a circuit that lowers the current limit as a function
of the FB pin voltage. When the load current increases to
the point where the output voltage at the load approaches
0V (likewise, the MIC2587/MIC2587Rs FB pin voltage
also approaches 0V), the result is a proportionate
decrease in the maximum current allowed into the load.
The transfer characteristic of this foldback current limit
subcircuit is shown in Figure 1. When V
OUT
= V
FB
= 0V,
the foldback current-limiting circuit controls the
MIC2587/MIC2587Rs GATE drive to force a constant
12mV (typical) voltage drop across the external sense
resistor.
Circuit Breaker Operation
The MIC2587/MIC2587R is equipped with an electronic
circuit breaker that protects the external N-channel power
MOSFET and other system components against large-
scale output current faults, both during initial card
insertion or during steady-state operation. The current
limit threshold is set via an external resistor, R
SENSE
,
connected between the controllers VCC (Pin 8) and
SENSE (Pin 7) pins. For the MIC2587/MIC2587R, a fault
current timing circuit is set via an external capacitor
C
TIMER
that determines the length of the time delay for
which the controller remains in current limit before the
circuit breaker is tripped.
Programming the response time of the overcurrent
detector helps to prevent nuisance tripping of the circuit
breaker attributed to transient current surges (e.g., inrush
current charging bulk load capacitance). The nominal
overcurrent response time (t
FLT
) is approximated by
Figure 4:
TIMERUP
TIMERH
TIMER
FLT
I
V
C
?/DIV>
=
?/DIV>
E 20
(ms)
FLT
TIMER
(礔)
Eq. 4
The typical overcurrent filter delay time for several
standard value capacitors is listed in Table 1.
Table 1. Overcurrent Filter Delay Time for Several Standard
Capacitor Values
C
TIMER
(礔)
t
FLT
(ms)
0.1
2
0.22
4.4
0.33
6.6
0.47
9.4
1.0
20
2.2
44
3.3
66
Whenever the voltage across R
SENSE
exceeds the
MIC2587/MIC2587Rs circuit-breaker threshold voltage
(47mV typical) during steady-state operation, the
following two events occur:
A constant-current regulation loop engages within 1祍
after an overcurrent condition is detected by the input
sense pins monitoring the voltage across R
SENSE
.
An internal 65礎 current source (I
TIMERUP
) begins to
charge C
TIMER
. If the excessive current persists such that
the voltage across C
TIMER
crosses the V
TIMERH
threshold
(1.313V, typically), the circuit breaker trips and the GATE
pin is immediately pulled low by a 80mA (typical) internal
current sink while the TIMER pin is discharged to ground
by a 3.5礎 current sink (I
TIMERDN)
.
An initial value for C
TIMER
is found by calculating the time
it will take for the MIC2587/MIC2587R to completely
charge the output capacitive load during startup. The
turn-on delay time is derived from the expression, I = C ?
(dV/dt):
LIMIT
CC(MAX)
LOAD
ON
-
TURN
I
V
C
?/DIV>
=
Eq. 5
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