34652 datasheet(13/26 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34652

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

Figure 7. Power MOSFET Turn-On and the Gradual

Increase in the Charging Current from 0 A to ICHG

(2.0 ms in Example)

The ICHG current charges up the load capacitor relatively

slowly. When the load capacitor is fully charged, the Power

MOSFET reaches its full enhancement, which triggers the

current limit detection to change from ICHG to ILIM and the load

current to decrease (Point C in Figure 6, page 12). The

current spike at Point C in Figure 6 is better displayed in

Figure 8. We can see that when the

⎜V

OUT - VIN ⎜< 0.5 V, the

Power MOSFET fully turns on to reach its full enhancement,

charging the capacitor an additional 0.5 V with a higher

current value that quickly ramps down. This eliminates the

need for a current slew rate control because the hazard for a

voltage change is less than 0.5 V. The power good output

signals activate after a 20 ms delay (Point D in Figure 6),

which in turn enables the load. The 34652 is now in normal

operation mode and the retry counter resets.

Figure 8. Full Power MOSFET Turn-On and Current

Spike Associated with It. End of Charging Process

NORMAL MODE

If one of the start-up conditions (list on page 12) is violated

any time from the start of the Power MOSFET enhancement

process and thereafter during normal operation, the Power

MOSFET turns off and the power good output signals

deactivate, disabling the load, and a new timer cycle starts as

explained previously. The 34652 also monitors the load

current to prevent any overload or short circuit conditions

from happening in order to protect the load from damage.

LOAD CURRENT CONTROL

When in normal operation mode, the 34652 monitors the

load and provides two modes of current control as explained

in the paragraphs below.

Overcurrent Mode

The 34652 monitors the load for overcurrent conditions. If

the current going through the load becomes larger than the

overcurrent limit for longer than the overcurrent limit filter

timer of 100

µs, the overcurrent signal is asserted and the

gate of the Power MOSFET is discharged to try to regulate

the current at the ILIM value (Point A in Figure 9). The 34652

is in overcurrent mode for 3.0 ms

. If after a 3.0 ms filter timer

the device is still in overcurrent mode, the device turns off the

Power MOSFET and deactivates the power good output

signals (Point B in Figure 9). 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, then

the 34652 goes back to the normal operation mode and the

power good output signals stay activated as shown in

Figure 10, page 14. This way the device overcomes

temporary overcurrent situations and at the same time

protects the load from a more severe overcurrent situation.

Figure 9. Overcurrent Mode for More Than 3.0 ms

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