ZHCSTR3B December 2010 – November 2023 TPS53315
PRODUCTION DATA
The TPS53315 does not have a dedicated oscillator to determine switching frequency. However, the device operates with pseudo-constant frequency by feed-forwarding the input and output voltages into the on-time one-shot timer. The adaptive on-time control adjusts the on-time to be inversely proportional to the input voltage and proportional to the output voltage .
This makes the switching frequency fairly constant in steady state conditions over a wide input voltage range. The switching frequency is selectable from eight preset values by a resistor connected between the RF pin and GND or between the RF pin and the VREG pin as shown in Table 6-1. Leaving the resistance open sets the switching frequency to 500 kHz.
RESISTOR (RRF) CONNECTIONS | SWITCHING FREQUENCY (kHz) |
---|---|
0 Ω to GND | 250 |
187 kΩ to GND | 300 |
619 kΩ to GND | 400 |
Open | 500 |
866 kΩ to VREG | 600 |
309 kΩ to VREG | 750 |
124 kΩ to VREG | 850 |
0 Ω to VREG | 970 |
The off-time is modulated by a PWM comparator. The VFB node voltage (the mid-point of resistor divider) is compared to the internal 0.6-V reference voltage added with a ramp signal. When the signal values match, the PWM comparator asserts a set signal to terminate the off-time (turn off the low-side MOSFET and turn on high-side MOSFET). The set signal is valid if the inductor current level is below the OCP threshold, otherwise the off-time is extended until the current level falls below the threshold.
Figure 6-1 and Figure 6-2 show two on-time control schemes.