XC9248
Series
■OPERATIONAL EXPLANATION (Continued)
<Thermal Shutdown>
For protection against heat damage of the ICs, thermal shutdown function monitors chip temperature. The thermal shutdown
circuit starts operating and the Nch MOS driver transistor will be turned off when the chip’s temperature reaches 150℃. The LX
pin enters the CL discharged state and stops functioning at GND level (0V). When the temperature drops to 125℃ or less after
shutting of the current flow, the IC performs the soft-start function to initiate output startup operation.
<UVLO Circuit>
When the VIN voltage becomes 3.8V (TYP.) or lower, the Nch MOS driver transistor is forced OFF. The LX pin enters the CL
discharged state and stops functioning at GND level (0V). When the VIN voltage becomes 3.9V (TYP.) or higher, switching
operation takes place. By releasing the UVLO function, the IC performs the soft-start function to initiate output startup operation.
The soft-start function operates even when the VIN voltage falls momentarily below the UVLO detect voltage. The UVLO circuit
does not cause a complete shutdown of the IC, but causes pulse output to be suspended; therefore, the internal circuitry
remains in operation.
<Bootstrap method>
An Nch MOS driver Tr. is used for the High side driver, and a voltage higher than the VIN voltage is needed to turn the driver on.
For that purpose, the bootstrap method is used to generate a voltage higher than the VIN voltage. The CBST capacitance is
connected between BST and LX, and because the VLX voltage is lower than the 4.6V (TYP.) VL voltage that is the internal power
supply, CBST is charged from VL.
<VOUT short-circuit protection>
With the A type, when the output voltage VOUT is shorted to GND or is near a shorted state (the FB voltage is1/2 or lower), and
a current over the current limit flows to the High side or Low side driver Tr., a VOUT short circuit is detected and the High side and
Low side driver Trs. are immediately turned off and latched. The LX pin enters the CL discharged state and stops functioning at
GND level (0V). Once in the latched state, operation is resumed by either turning the IC off and restarting with the EN pin, or by
re-input into the VIN pin (the voltage is lowered below the under-voltage lockout detection voltage once).
<LX short-circuit protection>
If the event that the LX pin shorts to GND, LX short-circuit protection activates for protection from over-current due to rush
current and to protect the IC.
If the LX pin shorts to GND, High side current limiting will activate due to rush current when the High side driver Tr. turns on.
The High side driver Tr. turn offs, and the Low side driver Tr. turns on at the same time. At this time, if Low side current limiting
did not activate, an LX short-circuit is detected, and the Low side driver is turned off and latched at the same time as the High
side driver Tr. Once in the latched state, operation is resumed by either turning the IC off and restarting with the EN pin, or by
re-input into the VIN pin (the voltage is lowered below the under-voltage lockout detection voltage once).
<VOUT over-voltage protection>
To minimize output voltage overshoot, VOUT over-voltage protection activates when VOUT overshoot occurs due to the output
resistance changing from a heavy load to a light load or otherwise. When VOUT overshoot occurs and the FB voltage that senses
VOUT rises to 0.9V (TYP.) or more, the High side driver Tr. is immediately turned off and the Low side driver Tr. is turned on to
prevent VOUT overshoot. When the FB voltage falls to 0.8V (TYP.) or less due to hysteresis, the High side driver Tr. turns on at
the next clock cycle.
11/21
TOREX [ Torex Semiconductor ]
