34652_07 Datasheet, PDF (16/26 Pages) Freescale Semiconductor, Inc – 2.0 A Negative Voltage Hot Swap Controller with Enhanced Programmability

FUNCTIONAL DEVICE OPERATION
PROTECTION FEATURES
Figure 15 shows how the 34652 uses the start-up timer to
make sure that the input voltage is above the undervoltage
falling threshold. The 34652 was in normal operation before
Point A. At Point A an undervoltage fault occurs. Then the
fault is cleared at Point B, and the 34652 initiates a start-up
sequence. Before the end of the start-up timer another
undervoltage fault occurs at Point C, so the 34652 does not
turn on the Power MOSFET. At Point D the fault is cleared
again for the length of the start-up timer. The 34652 turns on
the Power MOSFET and starts the charging process (refer to
Charging Process, pages 12–13).
Figure 15. Start-Up Timer Protection Against
Undervoltage Faults
OVERVOLTAGE
When the voltage on the OV pin exceeds the overvoltage
rising threshold for more than 1.0 ms, an overvoltage fault is
detected and one of the start-up conditions (list on page 12)
is violated. The 34652 turns off the Power MOSFET and
deactivates the power good output signals, thus disabling the
load. The 34652 stays in this state until the voltage on the OV
pin falls below the overvoltage falling threshold for more than
1.0 ms, signaling that the supply voltage is in the normal
operation range. Then a new start-up sequence initiates as
described on page 12.
The overvoltage detection circuit is also equipped with a
2.0 V hysteresis when in default mode. The hysteresis value
depends on the overvoltage detection threshold and can be
calculated as follows:
V_OVHY = V_{OV (RISING) *} [1 - (1.3 V - V_OVCHY) / 1.3 V ]
The waveforms for an overvoltage fault are shown in
Figure 16, page 16.
Figure 16. Overvoltage Fault
THERMAL SHUTDOWN
The thermal shutdown feature helps protect the internal
Power MOSFET and circuitry from excessive temperatures.
During start-up and thereafter during normal operation, the
34652 monitors the temperature of the internal circuitry for
excessive heat. If the temperature of the device exceeds the
thermal shutdown temperature of 160°C, one of the start-up
conditions (list on page 12) is violated, and the device turns
off the Power MOSFET and deactivates the power good
output signals. Until the temperature of the device goes
below 135°C, a new start-up sequence will not be initiated.
This feature is an advantage over solutions with an external
Power MOSFET, because it is not easy for a device with an
external MOSFET to sense the temperature quickly and
accurately. The thermal shutdown circuit is equipped with a
12 µs filter.
Thermal design is critical to proper operation of the 34652.
The typical R_DS(ON) of the internal Power MOSFET is
0.144 Ω at room ambient temperature and can reach up to
0.251 Ω at high temperatures. The thermal performance of
the 34652 can vary depending on many factors, among them:
• The ambient operating temperature (T_A).
• The type of PC board—whether it is single layer or multi-
layer, has heat sinks or not, etc.—all of which affects the
value of the junction-to-ambient thermal resistance (R_θJA).
• The value of the desired load current (I_LOAD).
When choosing an overcurrent limit, certain guidelines
need to be followed to make sure that if the load current is
running close to the overcurrent limit the 34652 does not go
into thermal shutdown. It is good practice to set the
parameters so that the resulting maximum junction
temperature is below the thermal shutdown temperature by a
safe margin.
Equation 1, on the following page can be used to calculate
the maximum allowable overcurrent limit based on the
maximum desired junction temperature or vice versa.
34652
16
Analog Integrated Circuit Device Data
Freescale Semiconductor