34652 datasheet(22/26 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34652

TYPICAL APPLICATIONS

Figure 22. Charging Current External Capacitor (CICHG)

Versus Charging Current Rise Time (tICHGR)

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 ILIM. 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 ILIM minus

the hysteresis value, which is 12% of ILIM 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 RILIM Value

The user can change the current limit by adding a resistor

(RILIM) 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 RILIM for

different values of ILIM and Figure 23 shows a plot of RILIM

versus ILIM. 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 RILIM resistor we use the

following equations:

ILIM (A) = 129 / [RILIM(kΩ) + 1.4 kΩ]

RILIM (kΩ) = [129 / ILIM (A)] - 1.4 kΩ

Figure 23. External Resistor (RILIM) Value Versus

Current Limit Value (ILIM)

100

120

140

160

180

200

220

TICHGR (m s)

ICHG = 0.05 A

ICHG = 0.1 A

ICHG = 0.5 A

ICHG

tICHGR (ms)

Table 9. RILIM Values for Some Desired ILIM Values

ILIM(A)

RILIM(kΩ)

ILIM(A)

RILIM(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

—

100

150

200

250

300

350

400

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)

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