Datasheet LT3742 (Analog Devices) - 9

LT3742
applicaTions inForMaTion Soft-Start and Shutdown
compares the FB pin to this ramp instead of to the 800mV The RUN/SS (Run/Soft-Start) pins are used to enable each reference; this slowly and smoothly increases the output controller independently, and to provide a user-program- voltage to its final value, while maintaining control of the mable soft-start function that reduces the peak input inductor current. Always check the inductor current and current and prevents output voltage overshoot during output voltage waveforms to ensure that the programmed start-up. To disable either controller, pull its RUN/SS pin soft-start time is long enough. A new soft-start cycle will to ground with an open-drain or open-collector device. be initiated whenever VIN drops low enough to trigger If both RUN/SS pins are pulled to ground, the LT3742 undervoltage lockout (programmed using the UVLO pin), is placed in shutdown mode, and quiescent current is or the LT3742 die temperature exceeds thermal shutdown. reduced to 20µA. Internal 1µA current sources pull up on A typical value for the soft-start capacitor is 1nF. each RUN/SS pin, and when either pin reaches 0.5V, that Soft-start is strongly recommended for all LT3742 appli- controller is enabled, along with the internal bias supply, cations, as it provides the least amount of stress on the gate drive boost regulator, voltage reference and master external power MOSFET and catch diode. Without soft-start, oscillator. If both outputs are always enabled together, both of these components will see the maximum current one soft-start capacitor can be used with both RUN/SS limit every start-up cycle. Figures 1a and 1b show start- pins tied together. up waveforms with and without soft-start for the circuit on the front page. Notice the large inductor current spike
The Benefits of Soft-Start
and the output voltage overshoot when soft-start is not When a capacitor is tied from the RUN/SS pin to ground, used. While this may be acceptable for some systems, the the internal 1µA pull-up current source generates a volt- addition of a single capacitor dramatically improves the age ramp on this pin. During start-up, the error amplifier start-up behavior of each DC/DC controller. VOUT VOUT 5V/DIV 5V/DIV IL 2A/DIV IL 2A/DIV 0.5ms/DIV 3742 F01a 0.5ms/DIV 3742 F01b
Figure 1a. Start-Up Waveforms Without Soft-Start Figure 1b. Start-Up Waveforms with 1nF Soft-Start Capacitor
3742fa 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts