Datasheet LT3483, LT3483A (Analog Devices) - 6

LT3483/LT3483A
OPERATION
The LT3483/LT3483A use a constant off-time control (LT3483) or 400mA (LT3483A), comparator A1 resets scheme to provide high efficiency over a wide range of the latch, which turns off Q1 after about 80ns. Inductor output currents. Operation can be best understood by current flows through the internal Schottky D1 to GND, referring to the Block Diagram. When the voltage at the charging the flying capacitor. Once the 300ns off-time has FB pin is approximately 0V, comparator A3 disables most elapsed, and internal diode current drops below 250mA of the internal circuitry. Output current is then provided (as detected by comparator A2), Q1 turns on again and by external capacitor COUT, which slowly discharges until ramps up to the switch current limit. This switching action the voltage at the FB pin goes above the hysteresis point continues until the output capacitor charge is replenished of A3. Typical hysteresis at the FB pin is 10mV. A3 then (until the FB pin decreases to 0V), then A3 turns off the enables the internal circuitry, turns on power switch Q1, internal circuitry and the cycle repeats. The inverting charge and the currents in external inductors L1A and L1B be- pump topology replaces L1B with the series combination gin to ramp up. Once the switch current reaches 200mA D2 and R2.
APPLICATIONS INFORMATION CHOOSING A REGULATOR TOPOLOGY
to the supply voltage. The inverting flyback also performs well in a step-up/invert application, but it occupies more
Inverting Charge Pump
board space compared with the inverting charge pump. The inverting charge pump regulator combines an induc- Also, the maximum |VOUT| using the flyback is less than tor-based step-up with an inverting charge pump. This can be obtained with the charge pump—it is reduced from configuration usually provides the best size, efficiency 38V by the magnitudes of VIN and ringing at the switch and output ripple and is applicable where the magnitude of node. Under a short-circuit condition at the output, a pro- V prietary technique limits the switch current and prevents OUT is greater than VIN. Negative outputs to –38V can be produced with the LT3483/LT3483A in this configuration. damage to the LT3483/LT3483A even with supply voltage For cases where the magnitude of V as high as 16V. As an option, a 0.47µF capacitor may be OUT is less than or equal to V added between terminals D and SW of LT3483/LT3483A IN, use a 2-inductor or transformer configuration such as the inverting flyback. to suppress ringing at SW. In the inverting charge pump configuration, a resistor
Inductor Selection
is added in series with the Schottky diode between the negative output and the D pin of the LT3483/LT3483A. The Several recommended inductors that work well with the purpose of this resistor is to smooth/reduce the current LT3483/LT3483A are listed in Table 1, although there are spike in the flying capacitor when the switch turns on. A many other manufacturers and devices that can be used. 10Ω resistor works well for a Li+ to –8V application, and Consult each manufacturer for more detailed information the impact to converter efficiency is less than 3%. The and for their entire selection of related parts. Many differ- resistor values recommended in the applications circuits ent sizes and shapes are available. For inverting charge also limit the switch current during a short-circuit condi- pump regulators with input and output voltages below tion at the output. 7V, a 4.7µH or 6.8µH inductor is usually the best choice. For flyback regulators or for inverting charge pump
Inverting Flyback
regulators where the input or output voltage is greater than 7V, a 10µH inductor is usually the best choice. A The inverting flyback regulator, shown in the –5V application larger value inductor can be used to slightly increase the circuit, uses a coupled inductor and is an excellent choice available output current, but limit it to around twice the where the magnitude of the output is less than or equal 3483fc 6 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics pin functions block diagram operation applications information typical applications Package Description Revision History Typical Application Related Parts