MC34652 Datasheet, PDF (11/25 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 terminal to the negative voltage rail and the VPWR
terminal 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 terminals—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 terminal to decide on the Start-Up and Retry Delay
Timer value, and the device keeps checking the TIMER
terminal 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.
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—½CV².
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
duration of the energy dissipation in the Power MOSFET, the
circuit uses a two-level current approach to controlling the
inrush current and switching on the load as explained in the
following paragraphs.
When the Power MOSFET is turned on, the current limit is
set gradually from 0 A to I_CHG (between Points A and B in
Figure 7, page 12). The low charging current value and the
gradual rise time of I_CHG are either defaults or they can be
user programmable (2.0 ms rise time in the example in
Figure 7). The low charging current value of I_CHG is intended
to limit the temperature increase during the load capacitor
charging process, and the gradual rise to I_CHG is to prevent
transient dips in the input voltage due to sharp increases in
the current. This prevents the input voltage from drooping
due to current steps acting on the input line inductance, and
that in turn prevents a premature activation of the UV
detection circuit.
Figure 6. Start-Up Sequence
Analog Integrated Circuit Device Data
Freescale Semiconductor
34652
11