XD9260/XD9261Series
■OPERATIONAL EXPLANATION (Continued)
<Reference voltage source, soft start function>
The reference voltage forms a reference that is used to stabilize the output voltage of the IC.
After “H” level is fed to CE pin, the reference voltage connected to the error amp increases linearly during the soft start interval.
This allows the voltage divided by the internal R1 and R2 resistors and the reference voltage to be controlled in a balanced
manner, and the output voltage rises in proportion to the rise in the reference voltage. This operation prevents rush input current
and enables the output voltage to rise smoothly.
If the output voltage does not reach the set output voltage within the soft start time, such as when the load is heavy or a large
capacity output capacitor is connected, the balancing of the voltage divided by the internal resistors R1 and R2 and the
reference voltage is lost, however, the current restriction function activates to prevent an excessive increase of input current,
enabling a smooth rise of the output voltage.
<PWM/PFM selection circuit>
Regarding XD9260 which has PWM control method, it works with a continuous conduction mode, and operates at a stable
switching frequency by means of an on time (ton) that is determined by the input voltage and output voltage regardless of the load.
Regarding XD9261 which has PWM/PFM auto switching control method, it works with a discontinuous conduction mode at
light loads, and lowers the switching frequency to reduce switching loss and improve efficiency.
<CE function>
Operation starts when “H” voltage is input into the CE pin. The IC can be put in the shutdown state by inputting “L” voltage into
the CE pin. In the shutdown state, the supply current of the IC is 0μA (TYP.), and the Pch MOS driver Tr. and Nch MOS switch
Tr. for synchronous rectification turn off. The CE pin is a CMOS input and the sink current is 0μA.
<UVLO Circuit>
When the VIN voltage becomes 2.00V (TYP.) or lower, the P-ch 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 2.10V
(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 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.
<Thermal Shutdown>
For protection against heat damage of the ICs, thermal shutdown function monitors chip temperature. The thermal shutdown
circuit starts operating and the P-ch MOS driver and N-ch MOS driver transistor will be turned off when the chip’s temperature
reaches 150℃. When the temperature drops to 120℃ or less after shutting of the current flow, the IC performs the soft-start
function to initiate output startup operation.
<Short-circuit protection function>
The B type short-circuit protection circuit protects the device that is connected to this product and to the input/output in
situations such as when the output is accidentally shorted to GND. The short-circuit protection circuit monitors the output
voltage, and when the output voltage falls below the short-circuit protection threshold voltage, it turns off the Pch MOS driver Tr
and latches it. Once in the latched state, operation is resumed by turning off the IC from the CE pin and then restarting, or by
re-input into the VIN pin.
<CL High Speed Discharge>
The B type 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 MOS switch transistor located between the VOUT pin and
the GND pin. When the IC is disabled, electric charge at the output capacitor (CL) is quickly discharged so that it may avoid
application malfunction.
Output Voltage Dischage characteristics
RDCHG = 210 (TYP) C =10
F
μ
Ω
L
τ
V=VOUT(T)×e – t /
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
t=τln (VOUT(T) / V)
-
VOUT = 1.2V
V: Output voltage after discharge
VOUT(T): Output voltage
t: Discharge time
V
OUT = 1.8V
--
OUT = 3.3V
---
V
τ: CL×RDCHG
CL: Capacitance of Output capacitor
R
DCHG: CL auto-discharge resistance,
but it depends on supply voltage.
0
2
4
6
8
10 12 14 16 18 20
Discharge Time: t(ms)
12/26
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