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LT3650EDD-4.1PBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3650EDD-4.1PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3650-4.1/LT3650-4.2 11 36504142fb VIN Input Supply The LT3650 is biased directly from the charger input supply through the VIN pin. This supply provides large switched currents, so a high quality, low ESR decoupling capacitor is recommended to minimize voltage glitches on VIN. The VIN decoupling capacitor (CVIN) absorbs all input switching ripple current in the charger, so it must have an adequate ripple current rating. RMS ripple current (ICVIN(RMS)) is: II V V V V CVIN RMS CHG MAX BAT IN IN BAT () () ~ •• ⎛ ⎝⎜ ⎞ ⎠⎟ ⎡ ⎣⎣ ⎢ ⎤ ⎦ ⎥ − ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 1 1 2 which has a maximum at VIN = 2 • VBAT, where: ICVIN(RMS) = ICHG(MAX)/2 The simple worst-case of 1/2 • ICHG(MAX) is commonly used for design. Bulk capacitance is a function of desired input ripple volt- age ( ΔVIN), and follows the relation: CI VV V F IN BULK MAX BAT IN IN () • / () = ⎛ ⎝⎜ ⎞ ⎠⎟ Δ μ 10μF is typically adequate for most charger applications. BOOST Supply The BOOST bootstrapped supply rail drives the internal switch and facilitates saturation of switch transistor. Oper- ating range of the BOOST pin is 0V to 4.5V, as referenced to the SW pin. Connect a 1μF or greater capacitor from the BOOST pin to the SW pin. The voltage on the decoupling capacitor is refreshed through a diode, with the anode connected to/from either the battery output voltage or an external source, and the cathode connected to the BOOST pin. Rate the diode av- erage current greater than 0.1A, and its reverse voltages greater than VIN(MAX). VIN/ BOOST Start-Up Requirement The LT3650 operates with a VIN range of 4.75V to 32V, however, a start-up voltage requirement exists due to the nature of the nonsynchronous step-down switcher topology used for the charger. If there is no BOOST supply available, the internal switch requires (VIN – VSW) > 3V to operate. This requirement does not exist if the BOOST supply is available and (VBOOST – VSW) > 2V. When an LT3650 charger is not switching, the SW pin is at the same potential as the battery, which can be as high as VBAT(FLT). For reliable start-up, the VIN supply must be at least 3V above the SW pin. The minimum start-up speci- fication of VIN at or above 7.5V provides ample margin to satisfy this requirement. Once switching begins, the BOOST supply capacitor gets charged such that (VBOOST – VSW) > 2V, and the VIN requirement no longer applies. In low VIN applications, the BOOST supply can be powered by an external source for start-up, eliminating the VIN start-up requirement. VBAT Output Decoupling An LT3650 charger output requires bypass capacitance connected from the BAT pin to ground (CBAT). A 10μF ce- ramic capacitor is required for all applications. In systems where the battery can be disconnected from the charger output, additional bypass capacitance may be desired for visual indication of a no-battery condition (see the Status Pins section). If it is desired to operate a system load from the LT3650 charger output when the battery is disconnected, additional bypass capacitance is required. In this type of application with the charger being used as a DC/DC converter, exces- sive ripple and/or low amplitude oscillations can occur without additional output bulk capacitance. For these ap- plications, place a 100μF low ESR nonceramic capacitor (chip tantalum or organic semiconductor capacitors such as Sanyo OS-CONs or POSCAPs) from BAT to ground, in parallel with the 10μF ceramic bypass capacitor. This additional bypass capacitance may also be required in systems where the battery is connected to the charger through long wires. The voltage rating on CBAT must meet or exceed the battery float voltage. APPLICATIONS INFORMATION |
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