SLVAFL5 September   2023 TPS61299

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction of Water Heater
  5. 2Low Voltage Start-up Boost Converter Introduction
    1. 2.1 General Introduction About TPS61299
    2. 2.2 Introduction about Traditional Start-up Process
    3. 2.3 Down Mode and Boost Mode Operation
  6. 3TPS61299 Ultra-low Voltage Start-up Design
    1. 3.1 Ultra Low Voltage Start-up Set up
    2. 3.2 The Way AVIN influence Start-up Process
    3. 3.3 The Way Vout Influence Start-up Process
    4. 3.4 The Way L Influence Start-up process
      1. 3.4.1 Why With the Larger Inductance, The Smaller PVIN Start-up Can Be Achieved?
      2. 3.4.2 Why Does The Inductance Continue to Increase, The Minimum PVIN to Start Up Is Not Lower
      3. 3.4.3 Why 80 mV Can Start Normally While 70 mV Fail to?
  7. 4Ultra-low Voltage Start-up Design
    1. 4.1 Test Waveform
  8. 5Summary
  9. 6References

3.2 The Way AVIN influence Start-up Process

Table 3-2 shows the start-up time △T varies with different AVIN, with VOUT =3.3 V, PVIN = 150 mV and L =1 uH. △T is the time when VOUT ramps from 10% to 90% of the target voltage. With the increase of AVIN, the startup time is reduced gradually.

Table 3-2 Start-up Time for Different AVIN
AVIN (V) △T(ms)
0.7 55.4
1 81.6
1.2 88.6
1.5 92.8
2 96.1
3 244

Figure 3-2 and Figure 3-3 shows the start-up process with AVIN=0.7 V and 2 V ,with PVIN= 150 mV, VOUT=3.3 V, L=1uH. As is seen, the start-up process is faster with AVIN = 0.7 V. The start-up time varies under the two set-ups is mainly because the Down Mode period. The difference resulting from various AVIN is because the internal block should detect the AVIN and compare it with Vout to decide whether to exit down mode or enter Boost mode.

GUID-20230828-SS0I-ZBBG-ZPVL-6TDCS7VBG8D7-low.pngFigure 3-2 Start-up Waveform with AVIN = 0.7 V
GUID-20230828-SS0I-2CXQ-TM8J-L0MPTN7VWSGQ-low.pngFigure 3-3 Start-up Waveform with AVIN = 2 V

In the initial stage as shown in Figure 2-3, when Vin is higher than Vout, the TPS61299 enters Down Mode, which schematic can been seen in Figure 2-4. Afterwords, the device exits down mode and enters BOOST Mode until VOUT ramps to AVIN. The lower AVIN is , the earlier the device exits down mode operation and enters boost operation. While under boost mode the efficiency is much higher because the high side MOSFET fully turns on other than works in saturation area, which means more energy can be converted to output thus the converter could regulate to target VOUT sooner.

Under down mode, during the Ton period, the Low side MOSFET turns on and the inductor current ramps up; during the Toff period, the Low side MOSFET turns off and High side MOSFET turns on in turn. According to the volt-second balance principle of inductor, the differentiation of inductor in Equation 1 and Equation 2:

During Ton period:

Equation 1.   L d i d t = P V I N

During the Toff period:

Equation 2. L d i d t = S W - P V I N

The Ton period is much the same under the two cases because the PVIN is both 150 mV. While during Toff period, the voltage drop across inductor varies since the SW node voltage differs under different AVIN. The SW node voltage varies for that the source node of high side PMOS is charged to AVIN+Vgs(th), as a result of PMOS internal Cgs. Figure 3-4 shows the inductor current with AVIN=2 V and 0.7 V , the off time is much longer with AVIN=0.7V.

GUID-20230906-SS0I-MZF6-4G5W-STKT9KF9LTMT-low.svg Figure 3-4 Inductor Current with Different AVIN

In summary, the recommendation is to choose lower AVIN to achieve ultra low VIN start-up. However, AVIN must be higher than UVLO of TPS61299, which means the minimum AVIN is 0.7 V.