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Analog Integrated Circuit Device Data

Freescale Semiconductor

34653

TYPICAL APPLICATIONS

rchived by Freescale Semiconductor, Inc., 2008

OVERCURRENT LIMIT

When in normal operation mode, the 34653 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 約, the

overcurrent signal is asserted, the gate of the Power

MOSFET is discharged fast (in less than 10 約) to try to

regulate the current, and the 34653 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 34653

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

34653 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

ILIM

) between the ILIM and VIN pins, as shown in

Figure 19

, page 17

. This way the 34653 device is adaptable

to different requirements and operating environments. The

overcurrent value ranges between 0.15 A and 1.25 A, with a

default value of 1.0 A. Table 8

lists examples of R

ILIM

for

different values of I

LIM

and Figure 22

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:

LIM

(A) = 129 / [ R

ILIM

(k? + 1.4 k?]

ILIM

(k? = [ 129 / I

LIM

(A)] - 1.4 k?/DIV>

Table 8. R

ILIM

Values for Some Desired I

LIM

Values

Figure 22. External Resistor (R

ILIM

) Value Versus

Current Limit Value (I

LIM

)

SHORT CIRCUIT DETECTION

If the current going through the load becomes > 5.0 A, the

Power MOSFET is discharged very fast (in less than 10 約)

to try to regulate the current, and the 34653 is in the

overcurrent mode for 3.0 ms. Then it follows the pattern

outlined in the Overcurrent Limit

paragraph above.

LIM

(A) R

ILIM

(k?/SPAN>)

0.15

859.71

0.2

644.43

0.3

429.15

0.4

321.52

0.5

256.93

0.6

213.88

0.7

183.12

0.8

160.06

0.9

142.12

1.0

127.77

1.1

116.02

1.2

106.24

1.25

101.93

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

ILIM (A)

LIM

(A)

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