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LT1620IGN Datasheet(PDF) 7 Page - Linear Technology |
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LT1620IGN Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LT1620/LT1621 APPLICATIONS INFORMATION between the IN+ and IN– inputs. Effective decoupling of supply rails is also imperative in these types of circuits, as large current transients are the norm. Power supply decoupling should be placed as close as possible to the ICs, and each IC should have a dedicated capacitor. Design Equations Sense resistor: RSENSE = VID/IMAX Current limit programming voltage: VPROG = VCC –[(10)(VID)] Voltage feedback resistors: RF1/RF2 = (VBATT(FLOAT) – 1.19)/1.19 End-of-Cycle Flag Application Figure 3 illustrates additional connections using the LT1620GN, including the end-of-cycle (EOC) flag feature. The EOC threshold is used to notify the user when the required load current has fallen to a programmed value, usually a given percentage of maximum load. The end-of-cycle output (MODE) is an open-collector pull- down; the circuit in Figure 3 uses a 10k pull-up resistor on the MODE pin, connected to VCC. The EOC flag threshold is determined through program- ming VPROG2. The magnitude of this threshold corre- sponds to 20 times the voltage across the sense amplifier inputs. As mentioned in the previous circuit discussion, the charging current level is set to correspond to a sense voltage of 80mV. The circuit in Figure 3 uses a resistor divider to create a programming voltage (VCC –VPROG2)of 0.5V. The MODE flag will therefore trip when the charging current sense voltage has fallen to 0.5V/20 or 0.025V. Thus, the end-of-cycle flag will trip when the charging current has been reduced to about 30% of the maximum value. Input Current Sensing Application Monitoring the load placed on the VIN supply of a charging system is achieved by placing a second current sense resistor in front of the charger VIN input. This function is useful for systems that will overstress the input supply (wall adapter, etc.) if both battery charging and other system functions simultaneously require high currents. This allows use of input supply systems that are capable of driving full-load battery charging and full-load system requirements, but not simultaneously. If the input supply current exceeds a predetermined value due to a combina- tion of high battery charge current and external system demand, the input current sense function automatically Figure 3. End-of-Cycle Flag Implementation with LT1620GN Figure 4. Input Current Sensing Application AVG PROG PROG2 AVG2 VCC IN+ SENSE IOUT VEE MODE IN– LT1620GN LT1620/21 • F03 CONNECTED AS IN FIGURE 2 R1 5.5k R2 50k C2 3.3 µF C1, 3.3 µF R3 10k END-OF-CYCLE (ACTIVE LOW) + AVG PROG VCC IN+ SENSE LT1620MS8 1 2 3 4 8 7 IOUT GND IN– 6 5 VSW 7 VIN 5 81 VFB 6 S/S 2 IFB 4 GND GND TAB 3 C1 1 µF 22 µF RP1 3k 1% RP2 12k 1% C2 1 µF R1 0.033 Ω L1B 10 µH 22 µF TO SYSTEM LOAD 4.7 µF L1A 10 µH 24 Ω VC 0.22 µF 0.1 µF X7R LT1513 RUN 5V 57k 6.4k 22 µF × 2 MBRS340 VBATT = 12.3V 1620/21 • F04 RSENSE 0.1 Ω + + + Li-ION |
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