Bq2057C - The voltage drop across Rsns resistor is not 105mV

What is the battery capacity? Can you measure the voltage on each pin? 

I tested on our EVM, but cannot duplicate the issue. 

Is there any particular reason that you chose this part? Have you looked at bq24040 for single cell linear charger? 

The battery capacity that i tested the circuit with was 5200mah (2 cells of 2600mah in parallel). I also tried with a single cell 3000mah but the result is the same. 

When Battery NOT connected

Vcc- 5v

Bat- 4.08v   (I was expecting this to be at 4.2v)

SNS- 5v

TS - 2.46v

CC- 4.98v

comp- 5v

Vss- 0v

when Battery is connected (5200mah)

SNS-   4.69v

Bat-  3.65v  (I was not expecting this either)

Vcc- 4.8v

Ts- 2.37v

Vss- 0v

CC- 3.82v

Comp-4.80v

I choose this part because of the current requirement. My project requires a charging current of 1.5A to 1.8 A.  This part was listed under 2A chargers in element14.

I hope you have got all the information you need.  Oh and also I am not using the EVM.

Regards

bq2057C is for single cell with battery regulation at 4.2V. If you are charging 2-cell Li-ion battery, please use bq2057W.

What is the battery voltage you are expecting? 

I experimented with BQ2057C a couple more days with a single cell of 3000mah,

The pin voltages were as follows,

SNS-   4.8v

Bat-  3.74v  

Vcc- 5v

Ts- 2.52v

Vss- 0v

CC- 4.1v

Comp-5v

The charging current was ~200mA for Rsns of 0.2ohm. And the voltage drop across Rsns is ~42mV.  

Is this tested on the same IC? 

The max rating on the BAT pin is VCC+0.3V. From your previous post, VCC is 5V. You are using 2 cell battery which voltage can be as high as 8.4V. 

Where are you measuring the current?

From the schematic, voltage drop across the sense resistor should be VCOMP - VSNS.

From the data you provided, this voltage should be  5V - 4.8V = 0.2V.

The and the sense resistor is 0.2ohm. This indicates the current goes through the sense resistor is 1A. 

I cannot duplicate the issue on the EVM which is based on the schematic you provided.

This is the exact schematic that I am using.

I am using 2 cells in parallel.  The total capacity of the pack is 5600mah. The terminal voltage the 100% charged pack is 4.2v, So I think BQ2057C is a correct choice.

 The data that I posted the last time was incorrect due to some lose connection on the board. I have again checked the circuit today.I have very carefully rigged up the circuit . And these are the latest pin voltages of BQ2057C.

SNS - 4.86v

BAT - 3.73v

VCC- 4.97v

TS- 2.47v

COMP- 4.97v

CC-  3.65v

VSS- GND

STAT- 2v

Rsns=0.2 ohms

The expected current is ~500mA.

The current that I am measuring at the Battery terminal is ~250mA

I get that the voltage across Rsns is 110mV and the charging current should be ~500mA,  but its not.

Additional Information.

1. The voltage at the base of the Pass- transistor is 4.08v

2. The width tracks on the PCB on which BQ2057 is mounted are of 10mils

3. No extra heat sink is provided for the pass-transistor. 

I think i have asked the question before, how are you measuring the battery charging current?

The circuit in your diagram is the same as the circuit on our EVM. I strongly recommend to order an EVM and compare the result. With the information you provided, i do not see why the charging current is not 500mA.

0.11V/0.2ohm = ~500mA

VSNS = 4.86V, VCC=3.65V, VBAT = 3.73V, the pnp should be on. 

Also, if you are connecting the battery in parallel, you should be able to use other single cell li-ion chargers. 

- if you need a stand-alone basic charger, please check bq24040.

- if you need a charger with power-path, please have a look at bq24075

- if you need a switch mode charger with higher charge current, please check out bq2419x

I am measuring the current using a Digital Multi-Meter. Connected in series with the +ve terminal of the battery.  I will consider your recommendations.

Thanks 

Hi Rakesh,

Could you try to modify Rb to 500 ohm (original is 1K ohm at Base of FZT788B (BJT).

hope could solve your problem.

Driscoll