3 TLV61290 Switching Frequency

The TLV61290 uses a hysteresis current control method. The following is the formula for calculating the switching frequency in continuous current mode (CCM).

where

• L is the inductor value
• V_IN is the input voltage
• V_OUT is the output voltage
• η is the converting efficiency
• f is the switching frequency
• I_LH is the inductor ripple current

The inductor current ripple of the TLV61290 is dynamically adjusted according to changes in input voltage, output voltage, and load (typical value is 1A). Based on the formula above, the operating frequency of the TLV61290 is not a fixed value, and this helps improve load transient performance. However, variations in switching frequency may introduce performance challenges in practical implementations, particularly when powering certain RF devices.

Figure 3-1 shows an example. The customer's application requires 4.5V output and maximum load capacity of 2A. When the input voltage is 3.7V, as shown in the test waveform below, a spurious signal exceeding RF specifications appears at ~1.3 MHz. Based on Figure 3-2 from actual testing, it can be seen that under this condition, the switching frequency of the device matches the frequency of the parasitic signal. This results in noise generated by the switching frequency affecting RF performance, even when the spread spectrum function is enabled.

Figure 3-1 RF Test Result (TLV61290 as Power Supply for RF Device)

Figure 3-2 Curve of Switching Frequency vs. Output Voltage

In summary, there are two ways to solve this problem:

1. Adjust the output voltage to avoid specific frequency band.

As shown in Figure 3-2, the switching frequency of the TLV61290 varies with changes in the output voltage. For example, when the switching frequency is 1.1 MHz, the radio frequency (RF) specifications have a large margin. So Vout can be adjusted to a lower value (such as 4.4V) to optimize RF performance.

2. Increase the output capacitance.

The frequency of the output voltage ripple corresponds to the switching frequency. If the output voltage ripple can be reduced, the energy amplitude at the fundamental frequency can also be reduced, thereby optimizing RF performance. Based on actual test results in Table 3-1, using four pieces 22μF output capacitors is sufficient to pass the RF test. The output capacitor model used is GRM188R61A226ME15D.