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TPS40200: Inverting Buck Boost Converter configuration

Josko M
Intellectual 840 points
Part Number: TPS40200
Other Parts Discussed in Thread: PMP7207, TLV27L1, TLV271

Hi,

since this is a controller and not converter, we have some questions about inverting Buck-Boost configuration:  

1) Is it mandatory to reference following pins to the negative output voltage (-Vout), if the inverting configuration will be used: 
- SS Pin 2
- RC Pin 1
- GND Pin 5
?

2) According to available reference designs related to TPS40200 in inverting configuration (PMP7207, PMP5026, and PMP1903) , all above mentioned pins are always referenced to -Vout. 
In this way the max. negative output voltage will be limited to -28V if the input voltage is +24V (VDDmax=52V). However, there is a Reference design where they are connected to reference ground 0V: PMP3034. Are both ways correct?

3) Do you see any issues, if above mentioned pins would be connected to 0V reference (GND) instead to -Vout? 
In this way higher voltages could be generated. 

We need actually solution, how generate negative voltage in range from -10V to -95V, max. lout = 5mA with TPS40200. Input voltage will be 24V +/-1V. 
In order to dynamically adjust the output voltage in wide range the PWM output from microcontroller will be used. 
Actually, PMP3034 would fit our application. We only have to connect filtered and buffered PWM signal to U2, pin 4 of TLV27L1. 

Can we go ahead with PMP3034 design?  

Br
Josko

     

  • 0
    TI__Expert 8245 points

    Hi,

    Both connections are right, the difference is when you connect GND pin to system GND you need to design an extra circuit for feedback but if you connect negative output to GND pin, the feedback circuit is simple. Actually the reference design is converting a positive voltage to a negative, if you want to get a -90V from -10V maybe you need to search for a boost solution.

  • 0
    TI__Mastermind 22299 points

    Hi Josko,

    For inverting buck boost we usually recommend referencing everything to the -VOUT node, this keeps the IC, Output and compensation all referenced to the same node which is simpler and would have a better response. However this does limit the |-VOUT|+VIN to the ABSMAX of the IC yes.

    Following the PMP3034 design should work fine, it uses TLV271 as an inverter to make VOUT positive to the feedback. This should be OK.

    Let me know if you have further questions,

    -Orlando

  • 0
    Intellectual 840 points

    Hi Iven, Hi Orlando,

    thanks on your feedback. 

    Further questions would be regarding the component sizing. 

    Topic 1:
    Due to space constraints on PCB we would like to use as smallest packages as possible for MOSFET, Diode and Inductor. 
    Parts used in PMP3034 design are quite bulky: 
    - MOSFET Id=3.1A
    - Diode If=1A
    - Inductor Irms=1.15A

    I haven't dived deep in calculations, but rough estimation for minimum Isw at boost converter design should be at laest: Isw=(Vout x Iout)/Vin
    For PMP3034 it is as follows: 
    - Vin = 10.8V ... 13.2V
    - Vout = -150V
    - Iout = 5 mA
    This yields to min Isw=~70mA, therefore it is confusing why in PMP3034 design such large MOSFET have been used? 
    The total power dissipation for a MOSFET consist of conduction, switching and gate loses.

    Conduction loses for IRFI9634: Pc=Rds*Isw^2*Rds_temp.comp@+85°C= 1*(0.07+0,01)^2*1.5 = 9.6mW 
    Switching loses for IRFI9634 :Ps=(VIN-VOU)*Iout*fsw*(Qgs2+Qgd)/Ig = (13,2+150)*0,08*200000*(8e-9+18e-9)/0.125 = 0,543 W
    Gate losses for IRFI9634:  Pg=Qdtot*Vg*fsw= 38e-9*10*200000=76mW

    Ptot=Pc+Ps+Pg = 0,0096+0,076+0,543 = 0,63W. For RthJA=65°C/W the temperature rise will be 41°C.  
    However if a MOSFET with lower gate charge (e.g. FQT3P20, SOT-223) would be used here, the power dissipation due to switching losses can be rather reduced. 

    Topic 2:
    Second question is the resistor choice for overcurrent threshold. Rlim= 0,05R. This yield the current threshold 2A typ. or (1,1A ... 2.8A for temp. range)
    It is clear that Ilim threshold must be higher than IL+ILripple/2 but the setting in PMP3034 design is confusing.  

    Please could you comment these to topics?  

    Br
    Josko

  • 0 in reply to Josko M
    TI__Expert 8245 points

    Hi,

    Yes, you can use a smaller size MOSFET if the thermal dissipation is good enough. For the topic 2, it maybe OK to choose a DR74 inductor when not considering current limit condition but you are right as a larger inductor here will be better.