LTC3103 OPERATION Power Good Status OutputShort-Circuit Protection The PGOOD pin is an open-drain output which indicates When the output is shorted to ground, the error amplifier the output voltage status of the step-down converter. If the will saturate high and the high side switch will turn on at output voltage falls 10% below the regulation voltage, the the start of each cycle and remain on until the current limit PGOOD open-drain output will pull low. A built-in deglitch- trips. During this minimum on-time, the inductor current ing delay prevents false trips due to voltage transients on will increase rapidly and will decrease very slowly during load steps. The output voltage must rise 2% above the the remainder of the period due to the very small reverse falling threshold before the pull-down will turn off. The voltage produced by a hard output short. To eliminate the PGOOD output will also pull low during overtemperature possibility of inductor current runaway in this situation, the shutdown and undervoltage lockout to indicate these fault switching frequency is reduced to approximately 300kHz conditions. The PGOOD output is valid 1ms after the buck when the voltage on FB falls below 0.3V. converter is enabled. When the converter is disabled the open-drain device is forced on into a low impedance state. BST Pin Function The PGOOD pull-up voltage must be below the 6V absolute The input switch driver operates from the voltage gener- maximum voltage rating of the pin. ated on the BST pin. An external capacitor between the SW and BST pins and an internal synchronous PMOS boost Current Limit switch are used to generate a voltage that is higher than The peak inductor current limit comparator shuts off the the input voltage. When the synchronous rectifier is on buck switch once the internal limit threshold is reached. (SW is low) the internal boost switch connects one side Peak switch current is no less than 400mA. of the capacitor to VCC replenishing its charge. When the synchronous rectifier is turned off the input switch is turned Slope Compensation on forcing SW high and the BST pin is at a potential equal Current mode control requires the use of slope com- to VCC + SW relative to ground. pensation to prevent sub-harmonic oscillations in the A comparator ensures there is sufficient voltage across inductor current waveform at high duty cycle operation. In the boost capacitor to guarantee start-up after long sleep some current mode ICs, current limiting is performed by periods or if starting up into a pre-biased output. clamping the error amplifier voltage to a fixed maximum which leads to a reduced output current capability at low Undervoltage Lockout step-down ratios. Slope compensation is accomplished The LTC3103 has an internal UVLO which disables the on the LTC3103 internally through the addition of a com- converter if the supply voltage decreases below 2.1V pensating ramp to the current sense signal. The current (typical), the converter will be disabled. The soft-start for limiting function is completed prior to the addition of the the converter will be reset during undervoltage lockout to compensation ramp and therefore achieves a peak inductor provide a smooth restart once the input voltage increases current limit that is independent of duty cycle. above the undervoltage lockout threshold. The RUN pin can alternatively be configured as a precise undervoltage lockout (UVLO) on the VIN supply with a resistive divider connected to the RUN pin. 3103fa 10 For more information www.linear.com/LTC3103 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts
