34652_07 Datasheet, PDF (22/26 Pages) Freescale Semiconductor, Inc – 2.0 A Negative Voltage Hot Swap Controller with Enhanced Programmability

TYPICAL APPLICATIONS
220
200
180
IIC_CH_HG_G = 0.05 A
160
140
120
100
ICI_CH_HG_G = 0.1 A
80
60
40
ICI_CH_HG_G = 0.5 A
20
0
0 1 2 3 4 5 6 7 8 9 10 11
T^tIICHHGGRR⁽(^mm^ss⁾)
Figure 22. Charging Current External Capacitor (C_ICHG)
Versus Charging Current Rise Time (t _ICHGR)
OVERCURRENT LIMIT
When in normal operation mode, the 34652 monitors the
load and compares (with a hysteresis) the current going
through a Sensor MOSFET with a reference current value
generated in reference to the current limit value I_LIM. If the
current going through the Sensor MOSFET becomes larger
than the reference current for more than 100 µs, the
overcurrent signal is asserted, the gate of the Power
MOSFET is discharged fast (in less than 10 µs) to try to
regulate the current, and the 34652 is in overcurrent mode for
3.0 ms. If after a 3.0 ms filter time the device is still in
overcurrent mode, the device turns off the Power MOSFET
and deactivates the power good output signals. The 34652
then initiates another start-up timer and goes back through
the enhancement process. If during the 3.0 ms timer the fault
was cleared where the load current was less than I_LIM minus
the hysteresis value, which is 12% of I_LIM value, then the
34652 goes back to the normal operation mode and the
power good output signals stay activated. This way the
device overcomes temporary overcurrent situations and at
the same time protects the load from more severe
overcurrent situations.
When the device passes the UVLO threshold, it checks if
there is any external resistor connected to the ILIM pin. If
there is, it determines the value of the overcurrent limit. If
there is not, it uses the default overcurrent limit value of 1.0 A.
It then uses the Sensor MOSFET to monitor the load for any
overcurrent conditions during operation as explained in the
previous paragraph.
Choosing the External Resistor R_ILIM Value
The user can change the current limit by adding a resistor
(R_ILIM) between the ILIM and VIN pins, as shown in
Figure 19, page 18. This way the 34652 device is adaptable
to different requirements and operating environments. The
overcurrent value ranges between 0.35 A and 2.25 A, with a
default value of 1.0 A. Table 9 lists examples of R_ILIM for
different values of I_LIM and Figure 23 shows a plot of R_ILIM
versus I_LIM. It is recommended that the closest 1% standard
resistor value to the actual value be chosen.
Note Accuracy requirements are application dependent.
To calculate the value of the R_ILIM resistor we use the
following equations:
I_LIM (A) = 129 / [R_ILIM(kΩ) + 1.4 kΩ]
R_ILIM (kΩ) = [ 129 / I_LIM (A)] - 1.4 kΩ
Table 9. R_ILIM Values for Some Desired I_LIM Values
I_LIM (A)
R_ILIM (kΩ)
I_LIM (A)
R_ILIM (kΩ)
0.35
367.65
1.5
84.71
0.4
321.52
1.6
79.33
0.5
256.93
1.7
74.58
0.6
213.88
1.8
70.36
0.7
183.12
1.9
66.58
0.8
160.06
2.0
63.18
0.9
142.12
2.1
60.11
1.0
127.77
2.2
57.31
1.1
116.02
2.25
56.01
1.2
106.24
—
—
1.3
97.96
—
—
1.4
90.86
—
—
400
350
300
250
200
150
100
50
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
ILIM (A)
Figure 23. External Resistor (R_ILIM) Value Versus
Current Limit Value (I_LIM)
34652
22
Analog Integrated Circuit Device Data
Freescale Semiconductor