TS3405 Datasheet, PDF (6/10 Pages) Taiwan Semiconductor Company, Ltd – Single Synchronous Buck PWM Controller

Functional Description
Start Up
The TS3405 automatically initializes upon receipt of power. The Power-On Reset (POR) function continually monitors
the bias voltage at the Vcc pin. The POR function initiates the Soft Start (SS) operation after the supply voltage
exceeds its POR threshold.
Over Current Protection (OCP)
The over current function protects the converter from a shorted output by using the lower MOSFET’s on-resistance,
Rds(on), to monitor the current. Therefore, even the power input voltage is greater than Vcc, TS3405 still can support
this. This method enhances the converter’s efficiency and reduces cost by eliminating a current sensing resistor.
The TS3405’s OCP threshold is a fixed value, -300mV, when Phase voltage is less –300mV, the next on-cycle will not
be initialized.
Over Voltage Protection (OVP)
An over voltage protection comparator is monitoring the COMP. When COMP voltage is less than 0.3V, the Soft Start
(SS) process is initiated.
Soft Start (SS)
Both POR and OVP initiate the soft start sequence after the over current set point has been sampled. Soft Start
clamps virtually the error amplifier output (COMP pin) and reference input (non-inverting terminal of the error amp) to
the internally generated Soft Start voltage. The oscillator’s triangular waveform is compared to the ramping error
amplifier voltage. This generates Phase pulses of increasing width that charge the output capacitor(s). when the
internally generated Soft Start voltage exceeds the COMP pin voltage, the output voltage is in regulation. This method
provides a rapid and controlled output voltage rise. The entire startup sequence typically takes about 5mS.
Current Sinking
The TS3405 incorporates a MOSFET shoot-through protection method which allows a converter to sink current as well
as source current. Care should be exercised when designing a converter with the TS3405 when it is known that the
converter may sink current.
When the converter is sinking current, it is behaving as a boost converter that is regulating its input voltage. This
means that the converter is boosting current into the buck converter input power, if the buck converter input power has
the same supply source which supplies the bias voltage, Vcc to the TS3405. if there is nowhere for this current to go,
such as to other distributed loads on the Vcc rail, through a voltage limiting protection device, or other methods, the
capacitance on the Vcc bus will absorb the current. This situation will allow voltage level of the Vcc rail to increase. If
the voltage level of the rail is boosted to a level that exceeds the maximum voltage rating of the TS3405, then the IC
will experience an irreversible failure and the converter will no longer be operational. Ensuring that there is a path for
the current to follow other than the capacitance on the rail will prevent this failure mode.
TS3405
6-10
2003/12 rev. A