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LTC4088EDE-1-PBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC4088EDE-1-PBF Datasheet(HTML) 10 Page - Linear Technology |
10 / 24 page LTC4088-1/LTC4088-2 10 40881fb OPERATIO Introduction The LTC4088-1/LTC4088-2 includes a PowerPath controller, battery charger, internal ideal diode, external ideal diode controller and a SUSPEND LDO. Designed specifically for USB applications, the PowerPath controller incorporates a precision average input current limited step-down switch- ing regulator to make maximum use of the allowable USB power. Because power is conserved, the LTC4088-1/ LTC4088-2 allows the load current on VOUT to exceed the current drawn by the USB port without exceeding the USB load specifications. The switching regulator and battery charger communicate to ensure that the average input current never exceeds the USB specifications. The ideal diodes from BAT to VOUT guarantee that ample power is always available to VOUT even if there is insuf- ficient or absent power at VBUS. Finally, to prevent battery drain when a device is con- nected to a suspended USB port, an LDO from VBUS to VOUT provides either low power or high power suspend current to the application. Input Current Limited Step Down Switching Regulator The power delivered from VBUS to VOUT is controlled by a 2.25MHz constant frequency step-down switching regulator. To meet the USB maximum load specification, the switching regulator contains a measurement and control system that ensures that the average input cur- rent remains below the level programmed at CLPROG. VOUT drives the combination of the external load and the battery charger. If the combined load does not cause the switching power supply to reach the programmed input current limit, VOUT will track approximately 0.3V above the battery voltage. By keeping the voltage across the battery charger at this low level, power lost to the battery charger is minimized. Figure 1 shows the power path components. If the combined external load plus battery charge current is large enough to cause the switching power supply to reach the programmed input current limit, the battery charger will reduce its charge current by precisely the amount necessary to enable the external load to be satisfied. Even if the battery charge current is programmed to exceed the allowable USB current, the USB specification for average input current will not be violated; the battery charger will reduce its current as needed. Furthermore, if the load cur- rent at VOUT exceeds the programmed power from VBUS, load current will be drawn from the battery via the ideal diodes even when the battery charger is enabled. The current at CLPROG is a precise fraction of the VBUS current. When a programming resistor and an averaging + – 0.3V 1.188V 3.6V CLPROG ISWITCH/N + – 15mV OV IDEAL DIODE PWM AND GATE DRIVE AVERAGE INPUT CURRENT LIMIT CONTROLLER AVERAGE OUTPUT VOLTAGE LIMIT CONTROLLER CONSTANT CURRENT CONSTANT VOLTAGE BATTERY CHARGER 2 GATE 8 VOUT 10 VOUTS 3 SW SYSTEM LOAD 3.5V TO (BAT + 0.3V) EXTERNAL IDEAL DIODE PMOS SINGLE CELL Li-Ion 408812 F01 12 BAT 9 VBUS TO USB OR WALL ADAPTER 11 + Figure 1 |
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