SLYT846 February   2024 TPS62870 , TPS62870-Q1 , TPS62871 , TPS62871-Q1 , TPS62872 , TPS62872-Q1 , TPS62873 , TPS62873-Q1 , TPS62874-Q1 , TPS62875-Q1 , TPS62876-Q1 , TPS62877-Q1 , TPS6287B10 , TPS6287B15 , TPS6287B20 , TPS6287B25 , TPSM8287A06 , TPSM8287A10 , TPSM8287A12 , TPSM8287A15

 

  1.   1
  2. 1Introduction
  3. 2DCS-Control topology overview
  4. 3Fixed-frequency DCS-Control topology overview
  5. 4Switching frequency variation
  6. 5Lower-ripple power-save mode
  7. 6Stacking (paralleling) for higher (or lower) load currents
  8. 7Conclusion
  9. 8References

4 Switching frequency variation

In addition to maintaining the fast transient response, which can be further improved and tuned through the external compensation on the COMP pin, fixed-frequency DCS-Control provides a fixed switching frequency with a tight tolerance specification. Because the switching frequency is directly set with an oscillator instead of indirectly controlled with an on-timer, the frequency’s tolerance is specified in the device-specific data sheet. Table 1 and Table 2 compare the switching frequency specifications of the TPS62876-Q1, using the fixed-frequency DCS-Control topology, versus the typical frequency specification of the DCS-Control TPS62869 step-down converter.

Table 1 The TPS62876-Q1, using the fixed-frequency DCS-Control topology, specifies a ±10% tolerance of its four switching frequency options over the full temperature and input voltage ranges.
Parameter Test Conditions MIN TYP MAX Unit
fSW Switching Frequency fSW = 1.5MHz, PWM operation 1.35 1.5 1.65 MHz
fSW = 2.25MHz, PWM operation 2.025 2.25 2.475
fSW = 2.5MHz, PWM operation 2.25 2.5 2.75
fSW = 3MHz, PWM operation 2.7 3 3.3
Table 2 The TPS62869, using DCS-Control, only specifies a typical switching frequency.
Parameter Test Conditions MIN TYP MAX Unit
fSW PWM switching frequency IOUT = 1A, VOUT = 0.9V 2.4 MHz

Figure 4 and Figure 5 compare the actual variation of the switching frequency versus load current in an application. Both devices support power-save mode, which reduces the frequency at lower load currents (toward the left of both graphs). Operation in PWM mode (at higher currents) results in a precisely controlled switching frequency for fixed-frequency DCS-Control, while the switching frequency of DCS-Control increases slightly with an increasing load. In forced PWM mode (not shown), fixed-frequency DCS-Control maintains its constant frequency down to no load.

GUID-20240129-SS0I-BZHK-MHM2-WRPFX29HZLLJ-low.svg Figure 4 Switching frequency variation of the TPS62869 with DCS-Control.
GUID-20240129-SS0I-ZB7G-Z1GX-DDFRFQSH7K5M-low.svg Figure 5 Switching frequency variation of the TPSM8287A12 power module with fixed-frequency DCS-Control.

Besides power-save mode, there are two conditions where the switching frequency can deviate from the frequency set by the oscillator: during a strong load transient and if the minimum on-time is reached. When applying a heavy load, the high-side MOSFET may be on for longer than a full switching period, and when removing a heavy load, it may be off for longer than a full switching period. Both scenarios result in one or more pulses that are not present because of the extended on- or off-times.

If the minimum on-time of the high-side MOSFET is reached, both fixed-frequency DCS-Control and DCS-Control reduce the switching frequency in order to meet the minimum on-time and maintain output-voltage regulation. This is improved performance compared to some current-mode devices that maintain the frequency but let the output voltage rise in order to meet the required minimum on-time. While both fixed-frequency DCS-Control and DCS-Control reduce the switching frequency in the same way [2], fixed-frequency DCS-Control has fewer operating conditions during which the minimum on-time is reached, and the frequency reduced, because of its lower minimum on-time. For example, the TPS62876-Q1 specifies the 44ns maximum value of its minimum on-time at a 5V input voltage and across the operating temperature. Such a low value of minimum on-time enables lower output-voltage applications in automotive and aerospace and defense, for example, to operate in the higher-frequency region sometimes required by the overall system.