34652_07 Datasheet, PDF (13/26 Pages) Freescale Semiconductor, Inc – 2.0 A Negative Voltage Hot Swap Controller with Enhanced Programmability

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
Figure 7. Power MOSFET Turn-On and the Gradual
Increase in the Charging Current from 0 A to I_CHG
(2.0 ms in Example)
The I_CHG 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 I_CHG to I_LIM 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 - V_IN ⎜< 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.
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 I_LIM 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 8. Full Power MOSFET Turn-On and Current
Spike Associated with It. End of Charging Process
Analog Integrated Circuit Device Data
Freescale Semiconductor
Figure 9. Overcurrent Mode for More Than 3.0 ms
34652
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