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LT3433 Datasheet(PDF) 9 Page - Linear Technology |
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LT3433 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LT3433 3433ia The LT3433 contains circuitry to eliminate the current limit reduction associated with slope-compensation, or anti- slope compensation. As the slope compensation ramp is added to the sensed current, a similar ramp is added to the current limit threshold reference. The end result is that current limit is not compromised so the LT3433 can provide full power regardless of required duty cycle. Mode Switching The LT3433 senses operational duty cycle by directly monitoring VIN and VOUT. Voltage drops associated with pass and catch diodes are estimated internally such that mode switching occurs when the duty cycle required for continuous buck operation is greater than 75%. If such a condition exists, a second switch is enabled during the switch on time, changing operation to a dual switch bridged configuration. Because the voltage available across the switched inductor is greater in bridged mode, duty cycle will decrease. The output current in bridged mode is not continuous, so switch currents are considerably higher than while oper- ating in buck mode. In order to maximize available output power, continuous operation and low ripple currents are recommended. Switch currents will increase by a factor of 1/(1 – DC) during bridged mode, so this mode of operation is typically the gating item for converter drive capability. IOUT(MAX) = ISW(MAX) • (1 – DC) = [0.5A – ( ∆IL / 2)] • (1 – DC) where ∆IL is the ripple current in the inductor. It is also important to note that IOUT cannot be considered equivalent to ILOAD during bridged operation. Most of the converter’s switch drive power is derived from the gener- ated output supply, so IOUT must also accommodate this current requirement. During single-switch buck opera- tional conditions, switch drive current is negligible in terms of output current; however, during bridged opera- tion, these currents can become significant. These output derived switch drive currents will increase the current loading on VIN by the same 1/(1 – DC) factor as the switch currents. As maximum switch current is referenced to that coming from the VIN supply, the available maximum switch current will be reduced by this required drive current. IDRIVE = DC • 2 • ISW(MAX) • ISWDRIVE(MAX) Using 50mA/A for the required drive current for each switch yields the portion of switch current used to drive the switches is: ISW(DRIVE) = DC • 2 • ISW(MAX) • 0.05/(1 – DC) Removing drive currents from the available maximum switch current yields: ISW(MAX)'= ISW(MAX) • [1 – DC • 2 • ISW(MAX) • 0.05/(1 – DC)] where ISW(MAX)' is maximum switch current available to the load during bridged operation. The maximum load current can then be calculated as: ILOAD(MAX) = ISW(MAX)' • (1 – DC) which reduces to: ILOAD(MAX) = [0.5A – (∆IL/2)] • (1 – 1.1 • DC) Design Equations APPLICATIO S I FOR ATIO SW_H LT3433 VIN VOUT 3433 AI01 SW_L L Constants: VSWH = voltage drop across boosted switch VSWL = voltage drop across grounded switch VF = forward drop of external Schottky diodes f0 = operating frequency Duty Cycle (continuous operation): DCBUCK = (VOUT + 2VF)/(VIN – VSWH + VF) DCBRIDGED = (VOUT + 2VF)/(VOUT + VIN + 2VF – VSWH – VSWL) |
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