# Main Project - Current Sharing Notes
[TOC]
## 1. Introduction
* 12V Current Sharing
* Use LTC4370 looks good too (5V ~ 12V)

## 2. Principles of Current Sharing
To connect the power supplies in parallel to provide more current or power to the load to improve the reliability of our robot since the power of a single power supply is not sufficient to the whole robot (cheetah and wheelbase).

There are number of method and we choose Forced Current Sharing which is compare the supply output current by circuit through the feedback. It can evenly sharing load and best voltage regulation. In a nutshell, it is more stable.
:3

## 3. Schematics Design
### 3.1. MOSFET
* Choose a suitable MOSFET
* Compare the SUM85N03-06P (the one in the LTC4370 datasheet) and IRFR48Z (the one u chose)

The selection of the MOSFET is mainly base on the Gate-Source Threshold Voltage - Vgs(th)(min) and Vgs(th)(max) and the Drain-Source ON Resistance - Rds(on). And we can compare the 2 MOSFET by those criteria. And the cost is a consideration too.

| IRFR48Z | SUM85N03-06P |
| -------- | -------- |
|  | |

The Minimum and Maximum V_gs of IRFR48Z is 2.0V and 4.0V while for SUM85N03-06P is 1.0V to 3.0V. The higher V_gs mean it take a higher voltage to turn the MOSFET on. But our voltage for regulating the MOSFET should be 5.0V. So that is a is not a big concern.

For the R_ds(on), the lower the resistance is the lower, the power dissipation of the MOSFET is. Both of the MOSFET have a max. r. of 11 m ohm.

In taobao, the price of the IRFR48Z is around .50 dollar which is cheaper than SUM85N03-06P of aorund 2 dollar.

For me, choosing IRFR48Z is better and we use it before and we have spare to use.

:3

| Criteria | SUM85N03-06P | IRFR48Z | IRLR3705 | IRFR3710Z |
|:--------------- | ------------ |:------- |:-------- |:--------- |
| Max BV_DSS | 30 V | 55 V | 55 V | 100 V |
| V_GS(MAX) | 20 V | 20 V | 16 V | 20 V |
| R_ds(ON) | 8 mR | 11 mR | 8 mR | 18 mR |
| Max Power (25C) | 100 W | 91 W | 130 W | 140 W |
| Taobao Price | $2 | $0.5 | $0.5 | $0.7 |
☆

### 3.2. VCC Pin

* IC can work down to 0V => if Vin < 2.9V, supply 2.9V-6V to VCC externally
* But we are operating in 5v and 12v => no need external power supply
* Instead, use 0.1µF bypass capacitor between VCC and GND

### 3.3. RANGE Pin

The RANGE pin resistor is decided by the design trade-off between the sharing capture range and the power dissipated in the MOSFET. A larger Range pin resistor increases the capture range at the expense of enhanced power dissipation and reduced load voltage. On the other hand, supplies with tight tolerances can afford a smaller capture range and therefore cooler operation of the MOSFETs. Hence we should calculate a proper value for the range pin.

The calculation can be found in the application information of the LTC4370 data sheet. Here is the formula:


VFR is the forward regulation voltage and we are regulating 3-cell battery and the max. and min. voltage is 12.6v and 11.1v. By calculation we can get the resistance required is around 150K Ohm.

For 5v regulation, we often use TPS54360 and the error is 4% which means the max. and min. voltage is 5.1v and 4.9v. By calculation we can get the resistance required is around 20K Ohm.

:3

### 3.4. Shunt Resistor

Shunt resistor can be used for the current sensing by measuring the voltage drop across the resistor. For example, This means that the current is equal to the voltage divided over the resistance, or: I = V / R = 0.030 / 0.001 = 30 A. But after crossing the shunt resistor the voltage will drop. Sharing error (a percentage error) equals:


Since we are checking on the 0m50 (0.5 mOhm) shunt resistor and the current will not excceed 15A. Hence the voltage drop is 0.2mV by calculate between the current difference and the shunt resistor value. As long as the voltage drop is acceptable, we can use the shunt resistor.

:3

## Other things
* We hv 0m50 (0.5 mOhm) shunt resistors (originally for power mgmt)

## Previous Notes by Judy
### Current Progress
Though the UCC39002 normal operating voltage is 12V, it seems can actually apply higher voltage like 24V.
RBIAS1 & RBIAS2 - Voltage Divider --> midpoint is about 12V
- **CANNOT be too big**
- Supply a small base drive current to the BJT
- current sensing ic makes me so confused
- https://www.ti.com/lit/an/slua311/slua311.pdf
- Example without sense pin!!


Sensing current -- Power Management Board


### To-do
- [x] MOSFET used in the datasheet
- [x] Find the correct value rc for the LTC4370
- [x] Check whether use shunt resistor or not

### Example
- UCC29002 but 24V (Actually input 24V but output 12V)
https://e2echina.ti.com/question_answer/analog/power_management/f/24/t/75419
- 均流 (Formula is included)
https://www.powerctc.com/zh-hant/node/4577
- So many discussion (in Chinese)
http://bbs.21dianyuan.com/thread-32860-1-1.html
- less than 5V example
https://www.analog.com/en/technical-articles/paralleling-linear-regulators-made-easy.html
![](https://i.imgur.com/bog7asB.png

### LTCXXXX (Ideal Diode Controller)
#### :X:LTC4357 (Not sharing current)
- load sharing
- Datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/4357fd.pdf
#### LTC4372/LTC4373
- Datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/ltc4372-4373.pdf
#### LTC4370
- MAX: 20V (Solution?)
- Datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/4370f.pdf

### UCXXXX
#### UC3902
- Simple Circuit
- Seems cannot afford 24V (MAX: 20V)
- Sensing Current is needed
#### UC3907
- Max can afford 35V
- https://www.itread01.com/content/1545882700.html
- Too complicated
- Sensing Current is needed

#### UCC39002
- Can afford 24V and so cheap
- But Sensing Current is needed

:D