Buck Regulator Support

Hi shmueld,

You may take a look at LTC3307A: LTC3307A Datasheet and Product Info | Analog Devices or LTM8074: LTM8074 Datasheet and Product Info | Analog Devices

Dear Ron,

Thanks for getting back to me - the LTM8074 looks very promising !

A couple of questions 

It appears I could use Vin as +12V or +5V - however on the data sheets on the TYPICAL PERFORMANCE CHARACTERISTICS then min Vin shown is +12V. Do you have any information on the performance for Vin=+5V ?

Also is my following understanding correct that from graph of the Efficiency and Power Loss, VOUT = 3.3V, fSW = 2MHz

that @1A there would be a power loss of 0.5W ? And given θJA = 48.9°C/W,  thus I would get 24.45 °C in heat dissipation above the ambient temperature ? I assume that this would be less for Vin = 5+5V ?

From the graph of the Input vs Load Current VOUT = 3.3V with load current 1A with Vin = +12V ==> Thus input current = approx. 0.3 A - does this mean that I will need to add an additional power of 300mA to my +12V supply above my Load current ?

When was the chip first released and do you have any details of its current life cycle ?

Thank you so much once again

Shmuel 

From the typical performance characteristics graphs, you could see there is a pattern with the graphs of 12Vin to 36Vin. I believe the 5Vin would follow this pattern so you can estimate the values.

I think you're correct that you would get 24.45 °C with your equation above. I would also assume this is less for 5Vin.

The graph of the Input vs Load Current VOUT = 3.3V shows the relationship of input and output power. This means that if your input source can supply 1A, you can power up the chip since it only need 0.3A. However, if your source is supplying other circuits, you need to check that it is supplying enough current to the LTM8074.

LTM8074 was released in 2018 and I believe it will be active for years to come due to its popularity.

Dear Ron,

Thank you for your quick and informative reply.

We have gone ahead and bought the DC2753A and DC2494A as we might need more power - however we have space critical needs. Please could you clarify the following :-

1. For the LTM8074 in Table 1. for CIN2** ==> "**An input bulk capacitor is required." What value and size would you recommend for this Bulk Cap? Also if we use multiple chips would each chip need this additional Bulk Cap or would one be enough at the supply voltage ? Also in Figure 5 why was this Bulk Cap not shown ?

The above "input bulk capacitor" is this what is referred to in the data sheets as follows :-

"If the input supply is poorly controlled or the LTM8074 is hot-plugged into an energized supply, ........ controlling input voltage overshoot is add an electrolytic bulk cap to the VIN net. ......The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit."

If yes then if our supply is "controlled" properly and or NOT "hot-plugged into an energized supply" then  can we safely use this design WITHOUT this Bulk Cap ?

2. For the LTM8074 in Table 1. It does not give the values for Vout=+1.1. Could you possibly give me the associated values for such an output ?

3. If don't connect a CAP to TR/SS pin is there a default soft Start ? I couldn't find in the manual how to calculate the SS value associated with this CAP ?

Best regards

Shmuel

Dear Ron,

Thank you for your quick and informative reply.

We have gone ahead and bought the DC2753A and DC2494A as we might need more power - however we have space critical needs. Please could you clarify the following :-

1. For the LTM8074 in Table 1. for CIN2** ==> "**An input bulk capacitor is required." What value and size would you recommend for this Bulk Cap? Also if we use multiple chips would each chip need this additional Bulk Cap or would one be enough at the supply voltage ? Also in Figure 5 why was this Bulk Cap not shown ?

The above "input bulk capacitor" is this what is referred to in the data sheets as follows :-

"If the input supply is poorly controlled or the LTM8074 is hot-plugged into an energized supply, ........ controlling input voltage overshoot is add an electrolytic bulk cap to the VIN net. ......The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit."

If yes then if our supply is "controlled" properly and or NOT "hot-plugged into an energized supply" then  can we safely use this design WITHOUT this Bulk Cap ?

2. For the LTM8074 in Table 1. It does not give the values for Vout=+1.1. Could you possibly give me the associated values for such an output ?

3. If don't connect a CAP to TR/SS pin is there a default soft Start ? I couldn't find in the manual how to calculate the SS value associated with this CAP ?

Best regards

Shmuel

Looking at the DC2753 demo board, the bulk cap used is a SUNCON 63CE22BS. You can check its size on its datasheet. It is not listed on the required parts in the demo manual so I think as long as your input supply is clean, you can choose to remove the bulk cap.

There is an equation to solve Rfb based on your desired output voltage on page 10 of the datasheet. 

Back to the demo manual, the TR/SS capacitor which is C4 is listed as a required component. I think it is best to leave it connected.

Since you are getting the demo board, you should be able to easily test and verify your design before making your own circuit.

Dear Ron,

Thank you so much for getting back to me. Please could you clarify the following :-

1, For the LTM8074 in Table 1. for CIN2** ==> "**An input bulk capacitor is required." If we used multiple chips would each chip need this additional Bulk Cap or would one be enough at the supply voltage ? Also in Figure 5 why was this Bulk Cap not shown ?

2. The above "input bulk capacitor" is this what is referred to in the data sheets as follows :-

"If the input supply is poorly controlled or the LTM8074 is hot-plugged into an energized supply, ........ controlling input voltage overshoot is add an electrolytic bulk cap to the VIN net. ......The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit."

If yes then if our supply is "controlled" properly and or NOT "hot-plugged into an energized supply" then  can we safely use this design WITHOUT this Bulk Cap ?

3. For the LTM8074 in Table 1. It does not give the values for Vout=+1.1. Could you possibly give me the associated values for such an output specifically :-

CIN2, COUT, fSW, RT(kΩ), MAXIMUM fSW, MIN RT(kΩ)

4. If don't connect a CAP to TR/SS pin is there a default soft Start ? I couldn't find in the manual how to calculate the Soft Start(SS) value associated with this CAP ? If I wanted a different SS how could I calculate it ?

Best Regards

Shmuel 

Dear Ron,

Thank you for all your help - did you manage to look at my last response ?

Best regards

Shmuel 

Dear Ron,

Any news ?

Also I understand that in Burst Mode at light loads the LTM8074 is much more efficient c.f. to Pulse-Skipping mode - but how does the output voltage ripple compare ?

Bets regards

Shmuel 

Hi Shmuel,

Apologies for the late response. I highly suggest using the LTpowerCAD design tool found on the product page of LTM8074. It would help in choosing component values for your desired Vout. The soft-start computation is found on page 10 of the datasheet on the TR/SS pin description.

Once you get the demo board, you can start to work from there for your design.

Dear Ron,

Thanks for getting back to me ! 

Thank you I have already started to look at the LTpowerCAD design tool. Nonetheless please could you still answer the following questions below :-

1, For the LTM8074 in Table 1. for CIN2** ==> "**An input bulk capacitor is required." If we used multiple chips would each chip need this additional Bulk Cap or would one be enough at the supply voltage ? Also in Figure 5 why was this Bulk Cap not shown ?

2. The above "input bulk capacitor" is this what is referred to in the data sheets as follows :-

"If the input supply is poorly controlled or the LTM8074 is hot-plugged into an energized supply, ........ controlling input voltage overshoot is add an electrolytic bulk cap to the VIN net. ......The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit."

If yes then if our supply is "controlled" properly and or NOT "hot-plugged into an energized supply" then  can we safely use this design WITHOUT this Bulk Cap ?

3. For the LTM8074 in Table 1. It does not give the values for Vout=+1.1. Could you possibly give me the associated values for such an output specifically :-

CIN2, COUT, fSW, RT(kΩ), MAXIMUM fSW, MIN RT(kΩ)

4. Also I understand that in Burst Mode at light loads the LTM8074 is much more efficient c.f. to Pulse-Skipping mode - but how does the output voltage ripple compare ?

5. In the data sheet its states "The soft-start ramp time is approximated by the equation t = 0.39 • C where C is in μF" is this t in sec ? If so If I put 0.01uF like in the demo board thus this will give me 3.9mS ? What is the rise time of the output if the SS pin is left floating ?

Thanks for all your help once again

Bets reagrds

Shmuel