bq2000T Programmable Multi-Chemistry Fast-Charge Management IC Features General Description ➤ S a fe m a nag e m e nt o f f as t charge for NiCd, NiMH, or LiIon battery packs ➤ High-frequency switching controller for efficient and simple charger design ➤ Pre-charge qualification for detecting shorted, damaged, or overheated cells ➤ Fast-charge termination by ∆T/ ∆ t m i ni m um c ur r e nt (Li-Ion), maximum temperature, and maximum charge time The bq2000T is a programmable, monolithic IC for fast-charge management of nickel cadmium (NiCd), nickel metal-hydride (NiMH), or lithium-ion (Li-Ion) batteries in single- or multi-chemistry applications. The bq2000T detects the battery chemistry and proceeds with the optimal charging and termination algorithms. This process eliminates undesirable undercharged or overcharged conditions and allows accurate and safe termination of fast charge. ➤ Selectable top-off mode for achieving maximum capacity in NiMH batteries ➤ Programmable trickle-charge mode for reviving deeply discharged batteries and for postcharge maintenance ➤ Built-in battery removal and insertion detection ➤ Sleep mode for low power consumption Depending on the chemistry, the bq2000T provides a number of charge termination criteria: n For safety, the bq2000T inhibits fast charge until the battery voltage and temperature are within user-defined limits. If the battery voltage is below the low-voltage threshold, the bq2000T uses trickle-charge to condition the battery. For NiMH batteries, the bq2000T provides an optional top-off charge to maximize the battery capacity. The integrated high-frequency comparator allows the bq2000T to be the basis for a complete, high-efficiency power-conversion circuit for both nickel-based and lithium-based chemistries. Rate of temperature rise, ∆T/∆t (for NiCd and NiMH) n Minimum charging current (for Li-Ion) n Maximum temperature n Maximum charge time Pin Names Pin Connections SNS 1 8 MOD VSS 2 7 VCC LED 3 6 RC BAT 4 5 TS SNS Current-sense input VSS System ground LED Charge-status output BAT Battery-voltage input 8-Pin DIP or Narrow SOIC or TSSOP PN-2000.eps SLUS149A–FEBRUARY 2000 1 TS Temperature-sense input RC Timer-program input VCC Supply-voltage input MOD Modulation-control output bq2000T RC Pin Descriptions SNS RC input used to program the maximum charge-time, hold-off period, and trickle rate during the charge cycle, and to disable or enable top-off charge Current-sense input Enables the bq2000T to sense the battery current via the voltage developed on this pin by an external sense-resistor connected in series with the battery pack VSS System Ground LED Charge-status output Supply-voltage input MOD Modulation-control output Functional Description Battery-voltage input The bq2000T is a versatile, multi-chemistry batterycharge control device. See Figure 1 for a functional block diagram and Figure 2 for the state diagram. Battery-voltage sense input. A simple resistive divider, across the battery terminals, generates this input. TS VCC Push-pull output that controls the charging current to the battery. MOD switches high to enable charging current to flow and low to inhibit charging- current flow. Open-drain output that indicates the charging status by turning on, turning off, or flashing an external LED BAT Timer-program input Temperature-sense input Input for an external battery-temperature monitoring circuit. An external resistive divider network with a negative temperature-coefficient thermistor sets the lower and upper temperature thresholds. TS Voltage Reference BAT OSC Voltage Comparator ADC ∆T/∆t ALU Clock Phase Generator Timer Charge Control LED Voltage Comparator MOD RC Internal OSC SNS VCC VSS BD2000T.eps Figure 1. Functional Block Diagram 2 bq2000T 4.0V < VCC < 6.0V Charge Initialization VBAT < VSLP Sleep Mode VMCV < VBAT < VSLP Battery Voltage (checked at all times) VSLP < VBAT < VCC VBAT < VMCV VTS > VHTF Charge Suspended VTS < VHTF Battery Temperature (checked at all times) VLBAT < VBAT < VMCV and VHTF < VTS < VLTF VBAT < VLBAT or VTS > VLTF Battery Conditioning Current Regulation ∆T/∆t (after hold-off period), or VTS < VTCO or Time = MTO VLBAT < VBAT < VMCV and VHTF < VTS < VLTF VCC Reset Maintenance Charge No Time < MTO and VBAT > VMCV Voltage Regulation Top-Off Selected? Yes Current Taper or Time = MTO Time = MTO or VTS < VTCO Top-Off VBAT > VMCV Done VBAT > VMCV VCC Reset or Battery Replacement or Capacity Depletion (Li-Ion) SD2000T.eps Figure 2. State Diagram 3 bq2000T Battery Chemistry Initiation and Charge Qualification The bq2000T detects the battery chemistry by monitoring the battery-voltage profile during fast charge. If the voltage on BAT input rises to the internal VMCV reference, the IC assumes a Li-Ion battery. Otherwise the bq2000T assumes NiCd/NiMH chemistry. The bq2000T initiates a charge cycle when it detects n Application of power to VCC n Battery replacement n Exit from sleep mode n Capacity depletion (Li-Ion only) As shown in Figure 6, a resistor voltage-divider between the battery pack’s positive terminal and VSS scales the b a t t er y v olt a g e m ea s u r ed a t p in BAT. I n a mixed-chemistry design, a common voltage-divider is used as long as the maximum charge voltage of the nickel-based pack is below that of the Li-Ion pack. Otherwise, different scaling is required. Immediately following initiation, the IC enters a charge-qualification mode. The bq2000T charge qualification is based on battery voltage and temperature. If voltage on pin BAT is less than the internal threshold, VLBAT , the bq2000T enters the charge-pending state. This condition indicates the possiblility of a defective or shorted battery pack. In an attempt to revive a fully depleted pack, the bq2000T enables the MOD pin to trickle-charge at a rate of once every 1.0s. As explained in the section “Top-Off and Pulse-Trickle Charge,” the trickle pulse-width is user-selectable and is set by the value of the resistance connected to pin RC. Once the chemistry is determined, the bq2000T completes the fast charge with the appropriate charge algorithm (Table 1). The user can customize the algorithm by programming the device using an external resistor and a capacitor connected to the RC pin, as discussed in later sections. NiCd and NiMH Batteries During this period, the LED pin blinks at a 1Hz rate, indicating the pending status of the charger. Following qualification, the bq2000T fast-charges NiCd or NiMH batteries using a current-limited algorithm. During the fast-charge period, it monitors charge time, temperature, and voltage for adherence to the termination criteria. This monitoring is further explained in later sections. Following fast charge, the battery is topped off, if top-off is selected. The charging cycle ends Similarly, the bq2000T suspends fast charge if the battery temperature is outside the VLTF to VHTF range. (See Table 4.) For safety reasons, however, it disables the pulse trickle, in the case of a battery over-temperature condition (i.e., VTS < VHTF). Fast charge begins when the battery temperature and voltage are valid. Current IMAX Qualification VMCV Voltage Fast Charge Phase 1 VLBAT Phase 2 Voltage Trickle Current IMIN Time GR2000CA.eps Figure 3. Lithium-Ion Charge Algorithm 4 bq2000T Table 1. Charge Algorithm Battery Chemistry NiCd or NiMH Li-Ion Charge Algorithm 1. Charge qualification 2. Trickle charge, if required 3. Fast charge (constant current) 4. Charge termination (∆T/∆t, time) 5. Top-off (optional) 6. Trickle charge 1. Charge qualification 2. Trickle charge, if required 3. Two-step fast charge (constant current followed by constant voltage) 4. Charge termination (minimum current, time) with a trickle maintenance-charge that continues as long as the voltage on pin BAT remains below VMCV. charge. This feature provides the additional charge time required for Li-Ion cells. Lithium-Ion Batteries Maximum Temperature (NiCd, NiMH, Li-Ion) The bq2000T uses a two-phase fast-charge algorithm for Li-Ion batteries (Figure 3). In phase one, the bq2000T regulates constant current until VBAT rises to VMCV. The bq2000T then moves to phase two, regulates the battery with constant voltage of VMCV, and terminates when the charging current falls below the IMIN threshold. A new charge cycle is started if the cell voltage falls below the VRCH threshold. A negative-coefficient thermistor, referenced to VSS and placed in thermal contact with the battery, may be used as a temperature-sensing device. Figure 5 shows a typical temperature-sensing circuit. During fast charge, the bq2000T compares the battery temperature to an internal high-temperature cutoff threshold, VTCO. As shown in Table 4, high-temperature termination occurs when voltage at pin TS is less than this threshold. During the current-regulation phase, the bq2000T monitors charge time, battery temperature, and battery voltage for adherence to the termination criteria. During the final constant-voltage stage, in addition to the charge time and temperature, it monitors the charge current as a termination criterion. There is no post-charge maintenance mode for Li-Ion batteries. ∆T/∆t (NiCd, NiMH) When fast charging, the bq2000T monitors the voltage at pin TS for rate of temperature change detection, ∆T/∆t. The bq2000T samples the voltage at the TS pin every 16s and compares it to the value measured 2 samples earlier. This feature terminates fast charge if this voltage declines at a rate of Charge Termination Maximum Charge Time (NiCD, NiMH, and Li-Ion) VCC V 161 Min The bq2000T sets the maximum charge-time through pin RC. With the proper selection of external resistor and capacitor, various time-out values may be achieved. Figure 4 shows a typical connection. Figure 5 shows a typical connection diagram. Minimum Current (Li-Ion Only) The bq2000T monitors the charging current during the voltage-regulation phase of Li-Ion batteries. Fast charge is terminated when the current is tapered off to 7% of the maximum charging current. Please note that this threshold is different for the bq2000. The following equation shows the relationship between the RMTO and CMTO values and the maximum charge time (MTO) for the bq2000T: MTO = RMTO ∗ CMTO ∗ 35,988 MTO is measured in minutes, RMTO in ohms, and CMTO in farads. (Note: RMTO and CMTO values also determine other features of the device. See Tables 2 and 3 for details.) Initial Hold-Off Period The values of the external resistor and capacitor connected to pin RC set the initial hold-off period. During this period, the bq2000T avoids early termination by disabling the ∆T/∆t feature. This period is fixed at the For Li-Ion cells, the bq2000T resets the MTO when the battery reaches the constant-voltage phase of the 5 bq2000T 2 VCC VSS 7 bq2000T CMTO RC 6 RMTO F2000T RCI.eps Figure 4. Typical Connection for the RC Input VCC 2 VSS 7 VCC RT1 bq2000T 5 TS RT2 N Battery T Pack C F2000TTMC.eps Figure 5. Temperature Monitoring Configuration BAT+ 2 VSS RB1 bq2000T 4 BAT RB2 F2000TBVD.eps Figure 6. Battery Voltage Divider 6 bq2000T programmed value of the maximum charge time divided by 32. hold-off period = Charge Current Control The bq2000T controls the charge current through the MOD output pin. The current-control circuit supports a switching-current regulator with frequencies up to 500kHz. The bq2000T monitors charge current at the SNS input by the voltage drop across a sense-resistor, RSNS, in series with the battery pack. See Figure 9 for a typical current-sensing circuit. RSNS is sized to provide the desired fast-charge current (IMAX): maximum time - out 32 Top-Off and Pulse-Trickle Charge An optional top-off charge is available for NiCd or NiMH batteries. Top-off may be desirable on batteries that have a tendency to terminate charge before reaching full capacity. To enable this option, the capacitance value of CMTO connected to pin RC (Figure 4) should be greater than 0.13µF, and the value of the resistor connected to this pin should be less than 15kΩ. To disable top-off, the capacitance value should be less than 0.07µF. The tolerance of the capacitor needs to be taken into account in component selection. IMAX = 0.05 RSNS If the voltage at the SNS pin is greater than VSNSLO or less than VSNSHI, the bq2000T switches the MOD output high to pass charge current to the battery. When the SNS voltage is less than VSNSLO or greater than VSNSHI, the bq2000T switches the MOD output low to shut off charging current to the battery. Figure 8 shows a typical multi-chemistry charge circuit. Once enabled, the top-off is performed over a period equal to the maximum charge time at a rate of 116 that of fast charge. Voltage Input Following top-off, the bq2000T trickle-charges the battery by enabling the MOD to charge at a rate of once every 1.0 second. The trickle pulse-width is user-selectable and is set by the value of the resistor RMTO, which is on pin RC. Figure 7 shows the relationship between the trickle pulse-width and the value of RMTO. The typical tolerance of the pulsewidth below 150kΩ is ±10%. As shown in Figure 6, a resistor voltage-divider between the battery pack’s positive terminal and VSS scales the battery voltage measured at pin BAT. For Li-Ion battery packs, the resistor values RB1 and RB2 are calculated by the following equation: RB1 VCELL = N∗ −1 RB2 VMCV During top-off and trickle-charge, the bq2000T monitors battery voltage and temperature. These functions are suspended if the battery voltage rises above the maximum cell voltage (VMCV) or if the temperature exceeds the high-temperature fault threshold (VHTF). where N is the number of cells in series and VCELL is the manufacturer-specified charging voltage. The end-to-end input impedance of this resistive divider network should be at least 200kΩ and no more than 1MΩ. 160 140 120 Shows Tolerance Pulsewidth—ms 100 80 60 40 20 4 3 2 1 2 4 6 8 10 50 100 150 200 250 RMTO—kΩ 2000PNvB3.eps Figure 7. Relationship Between Trickle Pulse-Width and Value of RMTO 7 bq2000T Q1 FMMT718 D4 DC+ S1A D3 MMSD914LT C6 47UF R9 120 OHMS 1000PF Q3 MMBT3904LT1 VCC D6 BZT52-C5V1 BAT+ D2 ZHCS1000 D5 MMSD914LT C8 R7 1K L1 47UH Q2 MMBT3904LT1 R8 220 OHMS C3 R2 2K C7 10UF C4 4.7PF 0.0022UF R1 D1 RED 1 2 3 4 U1 SNS VSS LED BAT C2 0.1 R4 210K C5 10UF R13 10.5K C9 0.33UF 100K 8 7 6 5 MOD VCC RC TS R12 THERM 100K bq2000T C1 0.1 R14 23.2K R11 6.81K R6 C10 0.01UF CHEMISTRY 221K BAT R5 200K R10 1.1K NOTES: R3 0.05 OHM 1. For Li-Ion, the CHEMISTRY is left floating. For NiCd/NiMH, the CHEMISTRY is tied to BAT2. DC input voltage: 9–16V 3. Charge current: 1A Pn1031a02.eps 4. L1: 3L Global P/N PKSMD-1005-470K-1A Figure 8. Single-Cell Li-Ion, Three-Cell NiCd/NiMH 1A Charger 8 bq2000T Table 2. Summary of NiCd or NiMH Charging Characteristics Parameter Value Maximum cell voltage (VMCV) 2V Minimum pre-charge qualification voltage (VLBAT) 950mV High-temperature cutoff voltage (VTCO) 0.225 ∗ VCC High-temperature fault voltage (VHTF) 0.25 ∗ VCC Low-temperature fault voltage (VLTF) 0.5 ∗ VCC bq2000T fast-charge maximum time out (MTO) RMTO ∗ CMTO ∗ 35,988 Fast-charge charging current (IMAX) 0.05/RSNS Hold-off period MTO/32 Top-off charging current (optional) IMAX/16 Top-off period (optional) MTO Trickle-charge frequency 1Hz Trickle-charge pulse-width See Figure 7 A NiCd or NiMH battery pack consisting of N series-cells may benefit by the selection of the RB1 value to be N-1 times larger than the RB2 value. bq2000T compares this voltage against its internal threshold voltages to determine if charging is safe. These thresholds are the following: In a mixed-chemistry design, a common voltage-divider is used as long as the maximum charge voltage of the nickel-based pack is below that of the Li-Ion pack. Otherwise, different scaling is required. n Temperature Monitoring n The bq2000T measures the temperature by the voltage at the TS pin. This voltage is typically generated by a negative-temperature-coefficient thermistor. The n High-temperature cutoff voltage: VTCO = 0.225 ∗ VCC This voltage corresponds to the maximum temperature (TCO) at which fast charging is allowed. The bq2000T terminates fast charge if the voltage on pin TS falls below VTCO. High-temperature fault voltage: VHTF = 0.25 ∗ VCC This voltage corresponds to the temperature (HTF) at which fast charging is allowed to begin. Low-temperature fault voltage: VLTF = 0.5 ∗ VCC This voltage corresponds to the minimum temperature Table 3. Summary of Li-Ion Charging Characteristics Parameter Value Maximum cell voltage (VMCV) 2V Minimum pre-charge qualification voltage (VLBAT) 950mV High-temperature cutoff voltage (VTCO) 0.225 ∗ VCC High-temperature fault voltage (VHTF) 0.25 ∗ VCC Low-temperature fault voltage (VLTF) 0.5 ∗ VCC bq2000T fast-charge maximum time-out (MTO) 2 ∗ RMTO ∗ CMTO ∗ 35,988 Fast-charge charging current (IMAX) 0.05/RSNS Hold-off period MTO/32 Minimum current (for fast-charge termination) IMAX/14 Trickle-charge frequency (before fast charge only) 1Hz Trickle-charge pulse-width (before fast charge only) See Figure 7 9 bq2000T Table 4. Temperature-Monitoring Conditions Temperature Condition Action VTS > VLTF Cold battery—checked at all times Suspends fast charge or top-off and timer Allows trickle charge—LED flashes at 1Hz rate during pre-charge qualification and fast charge VHTF < VTS < VLTF Optimal operating range Allows charging VTS < VHTF Suspends fast-charge initiation, does not allow Hot battery—checked during charge qualitrickle charge—LED flashes at 1Hz rate during fication and top-off and trickle-charge pre-charge qualification VTS < VTCO Battery exceeding maximum allowable temperature—checked at all times (LTF) at which fast charging or top-off is allowed. If the voltage on pin TS rises above VLTF, the bq2000T suspends fast charge or top-off but does not terminate charge. When the voltage falls back below VLTF, fast charge or top-off resumes from the point where suspended. Trickle-charge is allowed during this condition. Terminates fast charge or top-off Sleep Mode The bq2000T features a sleep mode for low power consumption. This mode is enabled when the voltage at pin BAT is above the low-power-mode threshold, VSLP. During sleep mode, the bq2000T shuts down all internal circuits, drives the LED output to high-impedance state, and drives pin MOD to low. Restoring BAT below the VMCV threshold initiates the IC and starts a fast-charge cycle. Table 4 summarizes these various conditions. Charge Status Display The charge status is indicated by open-drain output LED. Table 5 summarizes the display output of the bq2000T. Table 5. Charge Status Display Charge Action State Rf LED Status Battery absent High impedance Pre-charge qualification 1Hz flash Trickle charge (before fast charge) 1Hz flash Fast charging Low Power Supply ground Top-off or trickle (after fast charge, NiCd, NiMH only) High impedance bq2000 ground Charge complete High impedance Sleep mode High impedance Charge suspended (VTS > VLTF) 1Hz flash RSNS 1 SNS Cf 2 BAT- VSS bq2000T 2000TCS.eps Figure 9. Current-Sensing Circuit 10 bq2000T Absolute Maximum Ratings Symbol Parameter Minimum Maximum Unit VCC VCC relative to VSS -0.3 +7.0 V VT DC voltage applied on any pin, excluding VCC relative to VSS -0.3 +7.0 V TOPR Operating ambient temperature -20 +70 °C TSTG Storage temperature -40 +125 °C TSOLDER Soldering temperature - +260 °C Note: (TA = TOPR; VCC = 5V ± 20% unless otherwise specified) Symbol VTCO VHTF VLTF Parameter Temperature cutoff High-temperature fault Low-temperature fault VMCV Maximum cell voltage VLBAT Minimum cell voltage TS input change for ∆T/∆t detection VSLP High threshold at SNS, resulting in MOD-low Low threshold at SNS, resulting in MOD-high Sleep-mode input threshold VRCH Recharge threshold VSNSHI VSNSLO 10s max. Permanent device damage may occur if Absolute Maximum Ratings are exceeded. Functional operation should be limited to the Recommended DC Operating Conditions detailed in this data sheet. Exposure to conditions beyond the operational limits for extended periods of time may affect device reliability. DC Thresholds VTHERM Notes Rating 0.225 * VCC 0.25 * VCC 0.5 * VCC Tolerance ± 5% ± 5% ± 5% Unit V V V 2.00 ± 0.75% V 950 VCC 161 ± 5% mV ± 25% V/Min 50 ±10 mV Voltage at pin SNS -50 ±10 mV Voltage at pin SNS VCC - 1 ±0.5 V Applied to pin BAT VMCV - 0.1 ±0.02 V At pin BAT − 11 Notes Voltage at pin TS Voltage at pin TS Voltage at pin TS VBAT > VMCV inhibits fast charge Voltage at pin BAT bq2000T Recommended DC Operating Conditions (TA = TOPR) Symbol Condition Minimum Typical Maximum Unit Notes VCC Supply voltage 4.0 5.0 6.0 V ICC Supply current - 0.5 1 mA Exclusive of external loads ICCS Sleep current - - 5 µA VBAT = VSLP VTS Thermistor input 0.5 - VCC V VTS < 0.5V prohibited VOH Output high VCC - 0.2 - - V MOD, IOH = 20mA VOL Output low - - 0.2 V MOD, LED, IOL = 20mA IOZ High-impedance leakage current - - 5 µA LED Isnk Sink current - - 20 mA MOD, LED RMTO Charge timer resistor 2 - 250 kΩ CMTO Charge timer capacitor 0.001 - 1.0 µF Note: All voltages relative to VSS except as noted. Impedance Symbol Parameter Minimum Typical Maximum Unit RBAT Battery input impedance 10 - - MΩ RTS TS input impedance 10 - - MΩ RSNS SNS input impedance 10 - - MΩ Minimum Typical Maximum Unit Timing Symbol (TA = TOPR; VCC = 5V ± 20% unless otherwise specified) Parameter dMTO MTO time-base variation -5 - +5 % fTRKL Pulse-trickle frequency 0.9 1.0 1.1 Hz 12 bq2000T Data Sheet Revision History Change No. Note: Page No. Description Nature of Change Was: 14% Is: 7% 1 5 Minimum current termination 1 3 Added state diagram 1 7 Changed capacitor value for enabling top-off Was: 0.13µF Is: 0.26µF 1 8 Figure 8 Schematic updated 1 10 VTCO, VHTF, VLTF Tolerance updated 2 9 Minimum current (for fast charge Was: IMAX/7 Is: IMAX/14 termination) Change 1 = May 1999 B changes to Final from Jan. 1999 Preliminary data sheet. Change 2 = February 2000 changes from May 1999 B. Ordering Information bq2000T Package Option: PN = 8-pin narrow plastic DIP SN = 8-pin narrow SOIC TS = 8-pin TSSOP Device: bq2000T Multi-Chemistry Fast-Charge IC with ∆T/∆t Detection 13 bq2000T 8-Pin DIP (PN) 8-Pin PN (0.300" DIP) Inches D E1 E A B1 A1 L C B S e G Millimeters Dimension A Min. Max. Min. Max. 0.160 0.180 4.06 4.57 A1 0.015 0.040 0.38 1.02 B 0.015 0.022 0.38 0.56 B1 0.055 0.065 1.40 1.65 C 0.008 0.013 0.20 0.33 D 0.350 0.380 8.89 9.65 E 0.300 0.325 7.62 8.26 E1 0.230 0.280 5.84 7.11 e 0.300 0.370 7.62 9.40 G 0.090 0.110 2.29 2.79 L 0.115 0.150 2.92 3.81 S 0.020 0.040 0.51 1.02 8-Pin SOIC Narrow (SN) 8-Pin SN (0.150" SOIC) Inches 14 Millimeters Dimension A Min. Max. Min. Max. 0.060 0.070 1.52 1.78 A1 0.004 0.010 0.10 0.25 B 0.013 0.020 0.33 0.51 C 0.007 0.010 0.18 0.25 D 0.185 0.200 4.70 5.08 E 0.150 0.160 3.81 4.06 e 0.045 0.055 1.14 1.40 H 0.225 0.245 5.72 6.22 L 0.015 0.035 0.38 0.89 bq2000T 8-Pin TSSOP ~ TS Package Suffix Millimeters Dimension A Min. Max. - A1 0.05 B C Inches Min. Max. 1.10 - 0.043 0.15 0.002 0.006 0.18 0.30 0.007 0.012 0.09 0.18 0.004 0.007 D 2.90 3.10 0.115 0.122 E 4.30 4.48 0.169 0.176 e 0.65BSC 0.0256BSC H 6.25 6.50 0.246 0.256 L 0.50 0.70 0.020 0.028 Notes: 1. Controlling dimension: millimeters. Inches shown for reference only. 2 'D' and 'E' do not include mold flash or protrusions. Mold flash or protrusions shall not exceed 0.15mm per side 3 Each lead centerline shall be located within ±0.10mm of its exact true position. 4. Leads shall be coplanar within 0.08mm at the seating plane. 5 Dimension 'B' does not include dambar protrusion. 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