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TPSM843A22: Dynamic performace when change external sync

Hailey Chen
Intellectual 2860 points
Part Number: TPSM843A22
Other Parts Discussed in Thread: LMZ31710, TPS54824

Tool/software:

Hi, expert:

  I see TPSM843A22 could support switch internal clock to external sync, I hope to know how about the dynamic performance when switch to external sync or switch to internal clock, do you have some test result could provide? 

  I know the TPSM843A22 is the better generation than LMZ31710, for LMZ31710 "It is not recommended to switch from CLK mode back to RT mode because the internal switching frequency drops to 100 kHz first before returning to the switching frequency set by the RT resistor (RRT)"  I want to ask what have changed in TPSM843A22, why TPSM843A22 could support switch, I need get this information and promote TPSM843A22 to my customer. 

 In addition, TPSM843A22 only could support  five selectable lever switching frequencies: 500 kHz, 750 kHz, 1 MHz, 1.5 MHz, and 2.2 MHz, right? not support any external frequency that supports 500-2.2M， like 600Khz.

  Thanks!

  • 0
    TI__Mastermind 22595 points

    Hailey,

    Figure 7-3 shows the free running to sync to free running https://www.ti.com/lit/ds/symlink/tpsm843a22.pdf#page=14

    The LMZ31710 has a PLL and the TPSM843A22 does not have a PLL.   

    On the TPSM843A22, the external sync signal must be within +/- 20% of the internal switching frequency for sync operation.  

    If the ext. sync signal goes missing, the internal clock takes over immediately.   To switch at 600kHz with external clock, use an FSEL setting of 750kHz. 

    Table 7.2 shows on page 13 https://www.ti.com/lit/ds/symlink/tpsm843a22.pdf#page=14 the external sync frequency range from 400kHz to 2640kHz. 

    To reduce device size, fewer pins are available for configuration, pins must be multi-function like MSEL (CL, compensation, soft start) on the TPSM843A22. 

    To have multi-function pins, pin strapping is needed so that a specific resistor value on pin adjusts multiple settings.   

    The TPSM843A22 uses the pin strap method to select the switching frequency, since the pin strap decode circuit is there for the MSEL. 

     LMZ31710 must detect the external signal on the RT/CLK pin to switch over to the sync signal and to switch back to internal clock, the PLL must transition to 100kHz to switch over.   https://www.ti.com/lit/ds/symlink/lmz31710.pdf#page=22

    The TPS54824 has a newer generation RT/CLK PLL circuit that switches over after 10us loss of signal, because of customer feedback on the LMZ31710 method.