BQ76200: How are these components sized?

Hi User,

Each design orgainzation has its own rules on how to select a component, many use a 30 to  50% derating.  It gives some margin for operation and keeps components closer to a normal operating condition.

D1 and D5 are TVS to avoid transients on the battery and pack.  These can avoid damaging the electronics by limiting the peak voltage of the system.  If they are needed in the system they must be selected according to the needs of the system.  They should not leak during normal operation and clamp before damage occurs to any sensitive equipment.  Some designers do not like this approach for D1 since if it were to short it would be a direct short across the cells and open a fuse to the cells. D6 is a flyback diode, its purpose would be to clamp PACK+ near PACK-.  It would be a pulse current when the discharge FET opens under load, have a fast response, and stand off the normal operation voltage of the battery plus any transient.  D2, D3, D4 are transient limiting zeners which should not conduct with normal voltages and limit for the resistors selected within the voltage of the FET gate or pin limit described.

C3, C4, C6, C7 are ESD suppression diodes, they provide a high frequency path for ESD currents across the FETs or across the pack terminals.  2 are used in series so that a single component short does not short the FETs or PACK. Voltage should sustain the battery voltage.  The size may vary, 0.1 uF is typical. C2 is the charge pump capacitor, it should be selected depending on the load.  It will typically have the charge pump voltage, less than 15V.  See the data sheet for selection.  The BAT and PACK pin capacitors should be 10 nF as indicated, or smaller if needed, see http://www.ti.com/lit/pdf/slua794 

R7 is one of the pre-charge resistors.  If precharge is used select values to limit the current to a safe level for the cells in the under voltage condition.  Select a power rating or sufficient resistors to handle the dissipated power.  R6 and R16 are typically 10M resistors, these keep charge off the FETs when not driven by the BQ76200, they are large to minimize load on the charge pump and reduce battery current.  R14 is selected to control the switching speed of the FET and allow the part to operate.  See  see http://www.ti.com/lit/pdf/slua794 .  R17 is not normally used, but could be used as part of the PACK filter.  R18 is the primary PACK filter resistor, it will limit current into the PACK pin but can slow turn off if large.  See the apnote. R15 is a current limit resistor, it would conduct only in transient.  Use the recommended value, see the apnote.

Hi User,

I have usually seen the 50% applied to the component rating, so a 100V capacitor might not be used with more than a constant 50V, but again practices vary.  In cost sensitive applications where reliability may not be a high concern the component may be used at its rated value.  Many of the policies for derating are proprietary for the company where used.  If you read industry publications you will likely find some common guidelines.

For the flyback diode the TVS will clamp the voltage if both are used, so the standoff voltage may not be a concern.  Do look at the leakage, a higher voltage diode may be the better option at your DC level.  For current, when an inductive current path is opened the inductance wants the current to continue to flow and will increase the voltage, the diode allows a path for current flow to prevent excessive voltage rise.  The highest current is likely the battery short circuit, but the inductance in the load (test) path may be low.  The current through the diode will be a decaying peak, check the diode's ratings.  You might get further guidance from industry sources or manufacturers who specifically indicate diodes for flyback use.