going from LiPo battery to wall power?

Hi Ehab,

I suspect that the circuit that maintains and monitors the battery is shutting the unit down when it detects that the battery is missing. Where are you attaching the 3.7 volts? To the charger Mini USB port or directly to the battery contacts? As long as your power supply has enough current capacity the voltage is all that matters. The unit will draw the current that it needs from the 3.7 volts. If you have a meter you can monitor the voltage when you turn the unit on. You may need a good capacitor on the output of your buck supply so and surge demand of the unit doesn't cause it to pull the power down momentarily. Some batteries have more connections than just the + and - power. If your battery has more than 2 connections the unit may need the battery in the circuit in order to power up.

John

Thank you for the response!
I have connected the buck converter's output directly to the battery pins (there are only 2 pins). if it makes any difference, I'm providing the converter with an input of 5v. You mentioned that I need to connect a capacitor to the output of the buck converter. How much exactly? I don't really know the formula to calculate that
oh, and the battery that came with the tablet is 3.7V 7800mAh

Hi Ehab,

5 volts input to the 3.3V converter might be a little low. Usually they need at least 2 volts higher input than output. I would start with a 470 uF capacitor at any voltage 10 volts or higher. If you have a way to measure the voltage of the converter when you turn on the unit you will be able to see if it can maintain the 3.3 volts required. If it drops below 3 volts you can try a 6 or a 7 volt supply to your converter. If you battery had only 2 terminals then we should be able to get this to work. Connecting to the battery terminal is what I would do too. Also on hooking up the capacitor make sure the plus lead of the capacitor goes to the plus lead of the converter and the negative lead of the capacitor goes to the negative lead of the converter.

John

I found this information:

Module Spec: non-isolated step-down (BUCK)

Rectification: non-synchronous rectification

Input voltage: 4V-35V

Output voltage: 1.23V-30V

Output Current: 3A (max)

Conversion efficiency: 92% (the highest)

Switching frequency: 150KHz

Output ripple: 30mV (max)

Load Regulation: ± 0.5%

Voltage Regulation: ± 2.5%

Operating Temperature: -40 ℃ to +85 ℃

The peripheral dimensions: 48 * 23 * 14 (L * W * H) (mm)

Notes: Default output is 20V from factory setting.

LM2596 Power Module

it seems like the 12v input is ok, so I'll try that in a bit. However, I'm curios; does the switching frequency have anything to do with this?

Ehab Charek wrote:

 

I do have, however, a few good 12v power supplies. Is it safe to use those instead?

 

Unfortunately I don't know, you'd need to check with the vendor of your DC-DC converter.

I'm all out of ideas I'm afraid!

Ehab Charek wrote:

 

I do have, however, a few good 12v power supplies. Is it safe to use those instead?

 

Unfortunately I don't know, you'd need to check with the vendor of your DC-DC converter.

I'm all out of ideas I'm afraid!

I found this information:

Module Spec: non-isolated step-down (BUCK)

Rectification: non-synchronous rectification

Input voltage: 4V-35V

Output voltage: 1.23V-30V

Output Current: 3A (max)

Conversion efficiency: 92% (the highest)

Switching frequency: 150KHz

Output ripple: 30mV (max)

Load Regulation: ± 0.5%

Voltage Regulation: ± 2.5%

Operating Temperature: -40 ℃ to +85 ℃

The peripheral dimensions: 48 * 23 * 14 (L * W * H) (mm)

Notes: Default output is 20V from factory setting.

LM2596 Power Module

it seems like the 12v input is ok, so I'll try that in a bit. However, I'm curios; does the switching frequency have anything to do with this?