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XC9235B08DMR-G Datasheet(PDF) 14 Page - Torex Semiconductor

Part No. XC9235B08DMR-G
Description  600mA Driver Tr. Built-In, Synchronous Step-Down DC/DC Converters
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Maker  TOREX [Torex Semiconductor]
Homepage  http://www.torex.co.jp
Logo TOREX - Torex Semiconductor

XC9235B08DMR-G Datasheet(HTML) 14 Page - Torex Semiconductor

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XC9235/XC9236/XC9237 Series
■OPERATIONAL DESCRIPTION (Continued)
<Short-Circuit Protection>
The short-circuit protection circuit monitors the internal R1 and R2 divider voltage from the VOUT pin (refer to FB point in
the block diagram shown in the previous page). In case where output is accidentally shorted to the Ground and when
the FB point voltage decreases less than half of the reference voltage (Vref) and a current more than the ILIM flows to
the Pch MOS driver transistor, the short-circuit protection quickly operates to turn off and to latch the driver transistor.
For the D/E/F/G series, it does not matter how much the current limit, once the FB voltage become less than the
quarter of reference voltage (VREF), the short-circuit protection operates to latch the Pch MOS driver transistor. In
latch mode, the operation can be resumed by either turning the IC off and on via the CE/MODE pin, or by restoring
power supply to the VIN pin.
When sharp load transient happens, a voltage drop at the VOUT is propagated to the FB point through CFB, as a result,
short circuit protection may operate in the voltage higher than 1/2 VOUT voltage.
<UVLO Circuit>
When the VIN pin voltage becomes 1.4V or lower, the Pch MOS driver transistor output driver transistor is forced OFF to
prevent false pulse output caused by unstable operation of the internal circuitry. When the VIN pin voltage becomes 1.8V
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 pin voltage falls momentarily below
the UVLO operating 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.
<PFM Switch Current>
In PFM control operation, until coil current reaches to a specified level (IPFM), the IC keeps the Pch MOS driver transistor
on. In this case, time that the Pch MOS driver transistor is kept on (TON) can be given by the following formula.
TON= L
×IPFM / (VINVOUT)
→IPFM
< PFM Duty Limit >
In PFM control operation, the PFM duty limit (DTYLIMIT_PFM) is set to 200% (TYP.). Therefore, under the condition that the
duty increases (e.g. the condition that the step-down ratio is small), it’s possible for Pch MOS driver transistor to be turned
off even when coil current doesn’t reach to IPFM.
→IPFM
図 IPFM ①
Ton
Lx
I Lx
IPFM
0mA
図 IPFM ②
IPFM
0mA
Lx
I Lx
FOSC
PFMデューティ制限
PFM Duty Limit


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