Datasheet LT3433 (Analog Devices) - 8

LT3433
U U W U APPLICATIO S I FOR ATIO Overview
condition as requiring a duty cycle greater than 75%. If such a condition exists, a second switch is enabled during The LT3433 is a high input voltage range, step-up/step- the switch on time, which acts to pull the output side of the down DC/DC converter IC using a 200kHz constant fre- inductor to ground. This “bridged” operation allows volt- quency, current mode architecture. Dual internal switches age conversion to continue when V allow the full input voltage to be imposed across the OUT approaches or exceeds V switched inductor, such that both step-up and step-down IN. modes of operation can be realized using the same single
Shutdown
inductor topology. The LT3433 incorporates a low current shutdown mode The LT3433 has provisions for high efficiency, low load where all IC functions are disabled and the V operation for battery-powered applications. Burst Mode IN current is reduced to 10µA. Pulling the SHDN pin down to 0.4V or operation reduces average quiescent current to 100µA in less activates shutdown mode. no load conditions. A low current shutdown mode can also be activated, reducing total quiescent current to 10µA.
Burst Mode Operation
Much of the LT3433’s internal circuitry is biased from an The LT3433 employs low current Burst Mode functionality internal low voltage linear regulator. The output of this to maximize efficiency during no load and low load condi- regulator is brought out to the VBIAS pin, allowing bypass- tions. Burst Mode function is disabled by shorting the ing of the internal regulator. The associated internal BURST_EN pin to either VBIAS or VOUT. Burst Mode circuitry can be powered directly from the output of the function is enabled by shorting BURST_EN to SGND. converter, increasing overall converter efficiency. Using externally derived power also eliminates the IC’s power In certain wide current range applications, the IC could dissipation associated with the internal V enter burst operation during normal load conditions. If the IN to VBIAS regulator. additional output ripple and noise generated by Burst Mode operation is not desired for normal operation,
Theory of Operation (See Block Diagram)
BURST_EN can be biased using an external supply that is disabled during a no-load condition. This enables Burst The LT3433 senses converter output voltage via the VFB Mode operation only when it is required. The BURST_EN pin. The difference between the voltage on this pin and an pin typically draws 35µA when Burst Mode operation is internal 1.231V reference is amplified to generate an error disabled (V voltage on the V BURST_EN ≥ 2V) and will draw no more than C pin which is, in turn, used as a threshold 75µA with V for the current sense comparator. BURST_EN = 2V. When the required switch current, sensed via the V During normal operation, the LT3433 internal oscillator C pin voltage, is below 30% of maximum, the Burst Mode runs at 200kHz. At the beginning of each oscillator cycle, function is employed. When the voltage on V the switch drive is enabled. The switch drive stays enabled C drops below the 30% load level, that level of sense current is latched until the sensed switch current exceeds the VC-derived into the IC. If the output load requires less than this latched threshold for the current sense comparator and, in turn, current level, the converter will overdrive the output slightly disables the switch driver. If the current comparator during each switch cycle. This overdrive condition forces threshold is not obtained for the entire oscillator cycle, the the voltage on the V switch driver is disabled at the end of the cycle for 250ns. C pin to continue to drop. When the voltage on V This minimum off-time mode of operation assures regen- C drops below the 15% load level, switching is disabled, and the LT3433 shuts down most of its internal eration of the VBST bootstrapped supply. circuitry, reducing quiescent current to 100µA. When the If the converter input and output voltages are close voltage on the VC pin climbs back to 20% load level, the IC together, proper operation in normal buck configuration returns to normal operation and switching resumes. would require high duty cycles. The LT3433 senses this 3433f 8