## Add negative charge pumping for boost converter

* Last week:
* We added LDO for boost IC, then we got better ripple rejection and spike eliminate
* Positive
* This week:
* We'd like to generate **Negative** rail form the boost converter

### Circuit


### Analyze

||Ton|Toff|
|---|---|---|
|Pos. side|Vsw=0<br>Inductor charge current<br>Co+ discharged voltage |Vo=Vi*(eiff)<br>Inductor discharge current <br> Co+ charged voltage|
|Neg. side|Co- discharged|Co- charged to SW pin voltage <br>Current through R_chg and D2|

#### Off stage

#### On stage

### ~~Test result (very strange)~~

~~**Voltage drop while neg. side is connected**~~

~~Maybe R / C / L value should be fixed~~

Today morning fixed:

Maybe is connection of bread board

So I'd like to adopt hold board make and test again

#### To neg. wire open: Normally

#### To neg. wire connected: Boost out and LDO out are dropped

### Summery

#### Pros

* Don't need to feed negative voltage for application with +/- rail
* 5Vusb, 18650(2.7v ~ 4.2V) can generate +/- 12V power rail with very low cost
* Very high efficiency(>90%)

#### Cons

* Negative rail is decide by boost ic
* Default voltage when Rf and Rg is empty
* Using LDO set target negative voltage
* [-V_target < -(V_default + V_drop)](https://i.imgur.com/IXvlQqg.png)
* Drop and heat
* -12V to -5V @ 500mA, some package would > 80 deg

#### etc.

* Make hole board version
* Optimize with R/L/C

### Reference

* [TI: Generating Negative and Positive Voltage Rail With Boost
Converter TLV61048](https://www.ti.com/lit/pdf/slvaej3)
* [Last week report](https://hackmd.io/@BelleII-CDC-Trig-NTU-DeWei/rJ-lptHa3)
* [Generating Negative Output Voltage from Positive Input Voltage Using MAX17291 Boost Converter IC with Active Discharge Feature](https://www.analog.com/en/design-notes/generating-negative-output-voltage-from-positive-input-voltage.html)