SLVSE72A September 2017 – September 2017 TPS565208
The TPS564208 is a 5-A synchronous step-down converter. The proprietary D-CAP2™ mode control supports low ESR output capacitors such as specialty polymer capacitors and multi-layer ceramic capacitors without complex external compensation circuits. The fast transient response of D-CAP2™ mode control can reduce the output capacitance required to meet a specific level of performance.
The main control loop of the TPS565208 is adaptive on-time pulse width modulation (PWM) controller that supports a proprietary D-CAP2™ mode control. The D-CAP2™ mode control combines adaptive on-time control with an internal compensation circuit for pseudo-fixed frequency and low external component count configuration with low-ESR ceramic output capacitors. It is stable even with virtually no ripple at the output.
At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after internal one-shot timer expires. This one shot duration is set inversely proportional to the converter input voltage, VIN, and proportional to the output voltage VO, to maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below the reference voltage. An ripple is added to reference voltage to simulate output ripple, eliminating the need for ESR induced output ripple from D-CAP2TM mode control.
The TPS565208 has an internal 1.0-ms soft-start. When the EN pin becomes high, the internal soft-start function begins ramping up the reference voltage to the PWM comparator.
If the output capacitor is pre-biased at startup, the device initiates switching and starts ramping up only after the internal reference voltage becomes greater than the feedback voltage VFB. This scheme ensures that the converter ramps up smoothly into regulation point.
The output over-current limit (OCL) is implemented using a cycle-by-cycle valley detect control circuit. The inductor current is monitored during the OFF state by measuring the low-side FET drain to source voltage. This voltage is proportional to the switch current. To improve accuracy, the voltage sensing is temperature compensated.
During the on time of the high-side FET switch, the switch current increases at a linear rate determined by VIN, VOUT, and the output inductor value. During the on time of the low-side FET switch, this current decreases linearly. The average value of the switch current is the load current IOUT. If the monitored current is above the OCL level, the converter maintains low-side FET on and delays the creation of a new set pulse, even the voltage feedback loop requires one, until the current level becomes OCL level or lower. In subsequent switching cycles, the on-time is set to a fixed value and the current is monitored in the same manner.
There are some important considerations for this type of over-current protection. The load current is higher than the over-current threshold by one half of the peak-to-peak inductor ripple current. Also, when the current is being limited, the output voltage tends to fall as the demanded load current may be higher than the current available from the converter. This may cause the output voltage to fall. When the VFB voltage falls below the UVP threshold voltage, the UVP comparator detects it. And then, the device shuts down after the UVP delay time (typically 24 µs) and re-starts after the hiccup time (typically 14.9 ms).
When the over current condition is removed, the output voltage returns to the regulated value.
UVLO protection monitors the internal regulator voltage. When the voltage is lower than UVLO threshold voltage, the device is shut off. This protection is non-latching.
The device monitors the temperature of itself. If the temperature exceeds the threshold value (typically 172°C), the device is shut off. This is a non-latch protection.
When the input voltage is above the UVLO threshold and the EN voltage is above the enable threshold, the TPS565208 operates in the normal switching mode. Normal continuous conduction mode (CCM) occurs when the minimum switch current is above 0 A. In CCM, the TPS565208 operates at a quasi-fixed frequency of 550 kHz.
When the TPS565208 is operating in normal CCM, it may be placed in standby by asserting the EN pin low.