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LT3433IFE Datasheet(PDF) 10 Page - Linear Technology |
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LT3433IFE Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT3433 3433f APPLICATIO S I FOR ATIO The requirement for avoiding current mode instability is that the rising slope of sensed inductor ripple current (S1) is greater than the falling slope (S2). At duty cycles greater than 50% this is not true. To avoid the instability condition, a false signal is added to the sensed current with a slope (SX) that is sufficient to prevent current mode instability, or S1 + SX ≥ S2. This leads to the following relations: SX ≥ S2(2DC – 1)/DC If the forward voltages of a converter’s catch and pass diodes are defined as VF1 and VF2, then: S2 = (VOUT + VF1 + VF2)/L Solving for L yields a relation for the minimum inductance that will satisfy slope compensation requirements: LMIN = (VOUT + VF1 + VF2)(2DC – 1)/(DC • SX) The LT3433 maximizes available dynamic range using a slope compensation generator that generates a continu- ously increasing slope as duty cycle increases. The slope compensation waveform is calibrated at 80% duty cycle to generate an equivalent slope of at least 0.05A/ µs. The equation for minimum inductance then reduces to: LMIN = (VOUT + VF1 + VF2)(15e-6) For example, with VOUT = 5V and using VF1 + VF2 = 1.1V (cold): LMIN = (5 + 1.1)(15e-6) = 91.5µH Converter Capabilities The output current capability of an LT3433 converter is affected by a myriad of variables. The current in the switches is limited by the LT3433. Switch current is measured coming from the VIN supply, and does not directly translate to a limitation in load current. This is especially true during bridged mode operation when the converter output current is discontinuous. During bridged mode operation, the converter output current is discontinuous, or only flowing to the output while the switches are off (not to be confused with discon- tinuous switcher operation). As a result, the maximum output current capability of the converter is reduced from that during buck mode operation by a factor of roughly 1 – DC, not including additional losses. Most converter losses are also a function of DC, so operational duty cycle must be accurately determined to predict converter load capabilities. VOUT (V) 4 50 100 150 200 250 812 16 20 3433 AI01 300 350 610 14 18 Slope Compensation Requirements Typical Minimum Inductor Values vs VOUT SW_H LT3433 VIN VOUT 3433 AI02 SW_L L D1 D2 Application variables: VIN = Converter input supply voltage VOUT = Converter programmed output voltage VBST = Boosted supply voltage (VBST – VSWH) DC = Operational duty cycle fO = Switching frequency IMAX = Peak switch current limit ∆IL = Inductor ripple current ISW = Average switch current or peak switch current less half the ripple current (IMAX – ∆IL/2) RSWH = Boosted switch “on” resistance RSWL = Grounded switch “on” resistance L = Inductor value |
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