34652_07 Datasheet, PDF (12/26 Pages) Freescale Semiconductor, Inc – 2.0 A Negative Voltage Hot Swap Controller with Enhanced Programmability

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
START-UP SEQUENCE
When power is first applied to the 34652 by connecting the
VIN pin to the negative voltage rail and the VPWR pin to the
positive voltage rail, the 34652 keeps the Power MOSFET
turned off, deactivates the power good output signals, and
resets the retry counter. If the device is disabled, no further
activities will occur and power-up would not start. If the device
is enabled, it starts to establish an internally regulated supply
voltage required for the internal circuitry. The Power
MOSFET will stay off until the start of the charging process.
After Power-ON Reset (POR) and once the Undervoltage
Lockout (UVLO) threshold is cleared, the 34652 checks for
external components on four pins — the UV, the ILIM , the
ICHG , and 128 µs later the OV — to set the levels of the
Undervoltage Threshold, the Overcurrent Limit, the Charging
Current Limit, and the Overvoltage Threshold, respectively.
The device also checks for external components on the
TIMER pin to decide on the Start-Up and Retry Delay Timer
value, and the device keeps checking the TIMER pin
continuously throughout the operation.
The device then initiates the start-up timer (Point A in
Figure 6) and checks for the start-up conditions (see next
paragraph). The duration of the timer is either a default or a
user-programmable value. For undervoltage and overvoltage
faults during power up the 34652 retries infinitely until normal
input voltage is attained. If the die temperature ever
increased beyond the thermal shutdown threshold or the
device is disabled, then the start-up timer resets and the retry
counter increments. If after 10 retries the die temperature is
still high and the device is still disabled, the 34652 will not
retry again and the power in the device must be recycled or
the device must be disabled to reset the retry counter.
Figure 6. Start-Up Sequence
Start-Up Conditions
The start-up conditions are as follows:
• Input voltage is below the overvoltage turn-off threshold.
This threshold is either a default or user-programmable
value.
• Input voltage is above the undervoltage turn-off threshold.
This threshold is either a default or user-programmable
value.
• Die temperature is less than thermal shutdown
temperature.
• Device is enabled.
If the start-up conditions are satisfied for a time equal to
the length of the start-up timer and the retry counter is less
than or equal to 10, the device starts to turn on the Power
MOSFET gradually to control the inrush current that charges
up the load capacitor to eventually switch on the load (Point B
in Figure 6).
Charging Process
When charging a capacitor from a fixed voltage source, a
definite amount of energy will be dissipated in the control
circuit, no matter what the control algorithm is. This energy is
equal to the energy transferred to the capacitor — ½ CV ².
With this in mind, the Power MOSFET in the 34652 cannot
absorb this pulse of energy instantaneously, so the pulse
must be dissipated over time. To limit the peak power
dissipation in the Power MOSFET and to spread out the
34652
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Analog Integrated Circuit Device Data
Freescale Semiconductor