XCL208/XCL209
Series
■OPERATIONAL DESCRIPTION(Continued)
<Short-Circuit Protection>
The short-circuit protection circuit monitors the internal R1 and R2 divider voltage (Type F: FB pin voltage). 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 driver transistor, the short-circuit protection quickly operates to turn off
and to latch the driver transistor. In the latch state, the operation can be resumed by either turning the IC off and on via the
CE pin, or by restoring power supply to the VIN pin.
Also, when sharp load transient happens, a voltage drop at the VOUT is propagated through CFB, as a result, short circuit
protection may operate in the voltage higher than short-circuit protection voltage.
<UVLO Circuit>
When the VIN pin voltage becomes 1.4V (TYP.) or lower, the P-channel 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, by
releasing the UVLO state then the soft-start function initiates output startup operation. The soft-start function operates even
when the VIN pin voltage falls momentarily below the UVLO operating voltage same as releasing the UVLO function. 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 IPFM, the IC keeps the P-ch MOSFET on.
In this case, on-time (tON) that the P-ch MOSFET is kept on can be given by the following formula.
t
ON = L×IPFM / (VIN－VOUT) →IPFM①
<PFM Duty Limit>
In the PFM control operation, the maximum PFM Duty Limit is set to 200% (TYP.). Therefore, under the condition that the
step-down ratio is small, it’s possible for P-ch MOSFET to be turned off even when coil current doesn’t reach to IPFM. →IPFM②
IPFM①
IPFM②
<CL High Speed Discharge>
The XCL208B/XCL209B and the XCL208F/XCL209F can quickly discharge the electric charge at the output capacitor (CL)
when a low signal to the CE pin which enables a whole IC circuit put into OFF state, is inputted via the N-ch transistor located
between the LX pin and the VSS pin. When the IC is disabled, electric charge left at the output capacitor (CL) is quickly
discharged so that it may avoid application malfunction. Discharge time is set by the CL auto-discharge resistance (RDCHG) and
the output capacitance (CL). By setting time constant as τ(τ=CL x RDCHG), discharge time of the output voltage is calculated
by the following formula.
V = VOUT(T) x e –t/ or t=τln (VOUT(T) / V)
τ
V : Output voltage after discharge
100
CL=10uF
CL=20uF
CL=50uF
90
80
70
60
50
40
30
20
10
0
V
OUT(T) : Output voltage
t: Discharge time,
τ: CL x RDCHG
CL : Output capacitance (CL)
R
DCHG : CL auto-discharge resistance
0
10
20
30
40
50
60
70
80
90 100
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TOREX [ Torex Semiconductor ]
