Electronic Components Datasheet Search |
|
UCC3957-1 Datasheet(PDF) 4 Page - Texas Instruments |
|
UCC3957-1 Datasheet(HTML) 4 Page - Texas Instruments |
4 / 8 page 4 UCC3957 -1/-2/-3/-4 APPLICATION INFORMATION Overview The UCC3957 provides complete protection against over-discharge, over-charge and overcurrent for a three or four cell Lithium-Ion battery pack. It uses a “flying ca- pacitor” technique to sample the voltage across each battery cell and compare it to a precision reference. If any cell is in over or under-voltage, the internal state ma- chine takes the appropriate action to prevent further charge or discharge. High-side P-MOSFETs are used to independently control charge and discharge current. Typ- ical application circuits are shown in Figures 1 and 2. Connecting the Cell Stack When connecting the cell stack to the circuit, it is impor- tant to do it in the proper order. First, the bottom of the stack should be connected to AN4 . Next, the top of the stack should be connected to VDD. The cell taps can then be connected to AN1-AN3 in any order. Choosing Three or Four Cells For three cell packs, the cell count pin (CLCNT) should be connected to the DSPLY pin, and the AN3 pin should be tied to the AN4 pin. For four cell applications, the CLCNT pin should be grounded (to AN4) and the AN3 pin will be connected to the positive terminal of the bot- tom cell in the stack. Under-voltage Protection When any cell is found to be over-discharged (below the Normal to Undercharge threshold), the state machine turns off both high-side FETs and enters the sleep mode, where current consumption drops to about 3.5uA. It re- mains in sleep mode until the application of a charger is sensed by the Wake Up (WU) pin being raised above VDD. Charging Once a charger has been applied, the Charge FET will be turned on as long as the Charge Enable input (CHGEN) is pulled up to the DSPLY pin. If the CHGEN input is left open (or connected to AN4), the Charge FET will remain off. During charge, the Discharge FET will be off (current will be conducted through its body diode) until the cell volt- ages are all above the Undercharge to Normal threshold. Once the cell voltages are above this threshold, the Dis- charge FET will be turned on, minimizing power dissipa- tion. Open Wire Protection The UCC3957 provides protection against broken cell sense connections within the pack. If the sense connec- tion to one of the cells (pins AN1, 2 or 3) should become disconnected, weak internal current sources will make the cells connected to that wire appear to be in over- charge and charging of the pack will be prevented. Over-voltage Protection and the “Smart Discharge” Feature If any cell is charged to a voltage exceeding the Normal to Overcharge threshold, the Charge FET will be turned off, preventing further charge current. Hysteresis keeps the Charge FET off until the cell voltages have dropped below the Overcharge to Normal threshold. In most pro- tector designs, the Charge FET is held off completely within this voltage band. During this time, discharge cur- rent must be conducted through the body diode of the Charge FET. This forward voltage drop can be as high as 1V, causing significant power dissipation in the Charge FET and wasting precious battery power. The UCC3957 has a unique “Smart Discharge” feature that allows the Charge FET to come back on (for dis- charge only) while still in the overcharge hysteresis band. This greatly reduces power dissipation in the Charge FET. This is accomplished by sensing the voltage drop across the current sense resistor. If this drop exceeds 15mV (corresponding to 0.6 amps of discharge current using a .025 sense resistor), the Charge FET is turned back on. This threshold assures that only discharge cur- rent will be conducted. In an example using a 20m FET with a 1V body diode drop and a 1 amp load, the power dissipation in Q1 would be reduced from 1 watt to 0.02 watts. Note that a similar technique is not used during charge (when the Discharge FET is off due to cells being in undervoltage) because the charge current should be low while the cells are in undervoltage. Protection Against a Runaway Charger The use of a small N-channel level shifter (Q3 in the ap- plication diagrams) allows the IC to interface with the high-side Charge FET (Q1), even in the presence of a runaway charger. Only the drain-source voltage rating of the charge FET limits the charge voltage that the protec- tion circuit can withstand. The Wakeup (WU) pin is de- signed to handle input voltages greater than VDD, as long as the current is limited. In the examples shown, the Charge FET’s gate-source resistor (R1) provides this current limiting. Note that in Figure 2, a resistor and zener (R2 and VR1) have been added to protect Q1 against any possibility of a voltage transient exceeding its maximum gate-source rating. Overcurrent Protection The UCC3957 protects the battery pack from an over- load or a hard short circuit using a two-tier overcurrent protection scheme. The overcurrent protection is de- |
Similar Part No. - UCC3957-1 |
|
Similar Description - UCC3957-1 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |