Datasheet LTC3602 (Analog Devices) - 8

LTC3602
OPERATION Dropout Operation Overtemperature Protection
When the input supply voltage decreases toward the output When using the LTC3602 in an application circuit, care voltage, the duty cycle increases toward the maximum must be taken not to exceed any of the ratings speci- on-time. Further reduction of the supply voltage forces the fi ed in the Absolute Maximum Ratings section. As an top switch to remain on for more than one cycle until it added safeguard, however, the LTC3602 does incorporate attempts to stay on continuously. In order to replenish the an overtemperature shutdown feature. If the junction voltage on the fl oating BOOST supply capacitor, however, temperature reaches approximately 150°C, both power the top switch is forced off and the bottom switch is forced switches will be turned off and the SW node will become on for approximately 85ns every sixteen clock cycles. This high impedance. After the part has cooled to below 115°C, achieves an effective duty cycle that can exceed 99%. The it will restart. output voltage will then be primarily determined by the input voltage minus the voltage drop across the upper
Voltage Tracking and Soft-Start
internal N-channel MOSFET and the inductor. Some microprocessors and DSP chips need two power supplies with different voltage levels. These systems often
Slope Compensation and Inductor Peak Current
require voltage sequencing between the core power supply Slope compensation provides stability in constant-fre- and the I/O power supply. Without proper sequencing, quency architectures by preventing subharmonic oscilla- latch-up failure or excessive current draw may occur that tions at duty cycles greater than 50%. It is accomplished could result in damage to the processor’s I/O ports or the internally by adding a compensating ramp to the inductor I/O ports of a supporting system device such as memory, current signal at duty cycles in excess of 30%. Normally, an FPGA or a data converter. To ensure that the I/O loads the maximum inductor peak current is reduced when are not driven until the core voltage is properly biased, slope compensation is added. In the LTC3602, however, tracking of the core supply and the I/O supply voltage is slope compensation recovery is implemented to reduce necessary. the variation of the maximum inductor peak current (and Voltage tracking is enabled by applying a ramp voltage to therefore the maximum available output current) over the the TRACK/SS pin. When the voltage on the TRACK pin range of duty cycles. is below 0.6V, the feedback voltage will regulate to this tracking voltage. When the tracking voltage exceeds 0.6V,
Short-Circuit Protection
tracking is disabled and the feedback voltage will regulate When the output is shorted to ground, the inductor cur- to the internal reference voltage. rent decays very slowly during a single switching cycle. The TRACK/SS pin is also used to implement an external To prevent current runaway from occurring, a secondary soft-start function. A 1.2μA current is sourced from this current limit is imposed on the inductor current. If the pin so that an external capacitor may be added to create inductor valley current increases to more than 4.5A, the a smooth ramp. If this ramp is slower than the internal top power MOSFET will be held off and switching cycles 1ms soft-start, then the output voltage will track this ramp will be skipped until the inductor current is reduced. during start up instead. Leave this pin fl oating to use the internal 1ms soft-start ramp.
Do not tie the TRACK/SS pin to INTVCC or to PVIN.
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