Asking HELP from Electronic Gurus:
This charging IC is working fine, but I am not happy with the led output of the status. LTC4054 employ 3 status on the output -- high impedance, 20uA weak pulldown & 10mA strong pulldown -- and this 3 status is fine. But my problem is with strong pulldown even without a battery to charge.
[ QUOTE ]
The CHRG pin can provide an indication that the input voltage is greater than the undervoltage lockout threshold level. A weak pull-down current of approximately 20mA indicates that sufficient voltage is applied to VCC to begin charging. When a discharged battery is connected to the charger, the constant current portion of the charge cycle begins and the CHRG pin pulls to ground. The CHRG pin can sink up to 10mA to drive an LED that indicates that a charge cycle is in progress.
When the battery is nearing full charge, the charger enters the constant-voltage portion of the charge cycle and the charge current begins to drop. When the charge current drops below 1/10 of the programmed current, the charge cycle ends and the strong pull-down is replaced by the 20mA pull-down, indicating that the charge cycle has ended. If the input voltage is removed or drops below the undervoltage lockout threshold, the CHRG pin becomes high impedance.
To detect when the LTC4054 is in charge mode, force the digital output pin (OUT) high and measure the voltage at the CHRG pin. The N-channel MOSFET will pull the pin voltage low even with the 2k pull-up resistor. Once the charge cycle terminates, the N-channel MOSFET is turned off and a 20mA current source is connected to the CHRG pin. The IN pin will then be pulled high by the 2k pull-up resistor. To determine if there is a weak pull-down current, the OUT pin should be forced to a high impedance state. The weak current source will pull the IN pin low through the 800k resistor; if CHRG is high impedance, the IN pin will be pulled
[/ QUOTE ]
The statement "strong pulldown" during the charge and weak pulldown when completed seems correct. When an led is connected from B+ (5V) to chrg pin through 1K resistor, it lights up very bright during the charge as expected. When the charging is complete, the led is hardly lit up, but still visibly on, so of 20uA weak pull down is correct.
The misleading part is that when there is no battery connected to charge it lights up quite bright though not as bright as during the charging time. I am hoping to turn off the led when it is not really charging with a battery since a user may not notice if the battery contact is no good since the led is on anyway.
I don't know why LTC4054 is designed this way, but I am hoping that some gurus here can come out with a transistor circuit to cut off the led when not really charging in absence of the battery.
From my observation, I find this behavior of the chrg pin voltage when connected to B+ through 2K resistor (no led).
<ul type="square"> [*]1.44V during the constant current charging [*]1.78V when there is no battery [*]same as B+ when charging is complete [/list]
That means to turn on led if the charg pin voltage is below 1.6V and turn off if above 1.6V using a PNP transistor, with different resistor value, the voltage would be different. Any suggestion?
Another point which I can take charging status is from the PROG pin.
<ul type="square"> [*]When it is in constant current charging mode, the voltage is 1V. [*]As the current start reduce, the voltage start going down as low as 0.1V (10%) [*]Once the charging current eaches 10%, the voltage drops to 0.01V to indicate no charging. [/list]
So will it be easier if a NPN transistor is used to amplify this voltage to turn on led if above 0.05V?
Thanks a lot.
-- dj
