Datasheet LT3433 (Analog Devices) - 10

LT3433
U U W U APPLICATIO S I FOR ATIO
The requirement for avoiding current mode instability is
Converter Capabilities
that the rising slope of sensed inductor ripple current (S1) The output current capability of an LT3433 converter is is greater than the falling slope (S2). At duty cycles greater affected by a myriad of variables. The current in the than 50% this is not true. To avoid the instability condition, switches is limited by the LT3433. Switch current is a false signal is added to the sensed current with a slope measured coming from the V (S IN supply, and does not X) that is sufficient to prevent current mode instability, directly translate to a limitation in load current. This is or S1 + SX ≥ S2. This leads to the following relations: especially true during bridged mode operation when the SX ≥ S2(2DC – 1)/DC converter output current is discontinuous. If the forward voltages of a converter’s catch and pass During bridged mode operation, the converter output diodes are defined as VF1 and VF2, then: current is discontinuous, or only flowing to the output while the switches are off (not to be confused with discon- S2 = (VOUT + VF1 + VF2)/L tinuous switcher operation). As a result, the maximum Solving for L yields a relation for the minimum inductance output current capability of the converter is reduced from that will satisfy slope compensation requirements: that during buck mode operation by a factor of roughly L 1 – DC, not including additional losses. Most converter MIN = (VOUT + VF1 + VF2)(2DC – 1)/(DC • SX) losses are also a function of DC, so operational duty cycle The LT3433 maximizes available dynamic range using a must be accurately determined to predict converter load slope compensation generator that generates a continu- capabilities. ously increasing slope as duty cycle increases. The slope compensation waveform is calibrated at 80% duty cycle to VIN generate an equivalent slope of at least 0.05A/µs. The equation for minimum inductance then reduces to: SW_H L D1 MIN = (VOUT + VF1 + VF2)(15e-6) LT3433 L D2 For example, with VOUT = 5V and using VF1 + VF2 = 1.1V V SW_L OUT (cold): LMIN = (5 + 1.1)(15e-6) = 91.5µH 3433 AI02
Slope Compensation Requirements
Application variables:
Typical Minimum Inductor Values vs VOUT
VIN = Converter input supply voltage 350 VOUT = Converter programmed output voltage 300 VBST = Boosted supply voltage (VBST – VSWH) 250 DC = Operational duty cycle µH) ( f 200 O = Switching frequency L MIN IMAX = Peak switch current limit 150 ∆IL = Inductor ripple current 100 ISW = Average switch current or peak switch current less half the ripple current (I 50 MAX – ∆IL/2) 4 6 8 10 12 14 16 18 20 R V SWH = Boosted switch “on” resistance OUT (V) 3433 AI01 RSWL = Grounded switch “on” resistance L = Inductor value 3433f 10