Datasheet LT3476 (Analog Devices) - 7

LT3476
ApplicAtions inForMAtion Layout Hints
protection feature to adequately protect the switch, it is The high speed operation of the LT3476 demands careful important that the CAP input sample a voltage at or near attention to board layout. Several items are worthy of note. the highest voltage reached by the SW node. As a result, The exposed pad of the package is the only GND terminal this OVP function will not provide adequate protection of the IC and is also important to thermal management from open load events in isolated power configurations for the IC, so it is crucial to achieve a good electrical and such as the 1:1 flyback, since input and output voltage thermal contact between the exposed pad and the ground magnitudes must be summed to obtain the voltage seen plane of the board. Also, the Schottky rectifier and the by the switch. capacitor between GND at the cathode of the Schottky 35V are in the high frequency switching path where current V(CAP) flow is discontinuous. These elements should be placed 20V LED so as to minimize the path between SW and the GND of DISCONNECT HERE the IC. To reduce EMI, it is important to minimize the area of the SW trace. Use a GND plane under SW to minimize I(SW) 1A/DIV interplane coupling to sensitive signals. To obtain good 0A current regulation accuracy and eliminate sources of 3476 F01 channel-to-channel coupling, the CAP and LED inputs of 20µs/DIV each channel of the LT3476 should be run as separate lines
Figure 1. LED Disconnect Transient
back to the terminals of the appropriate sense resistor. Since there is a small DC input bias current (~50µA) to
Setting the Switching Frequency
the LED and CAP inputs, resistance in series with these The switching frequency of the LT3476 is set by an exter- inputs should be minimized, otherwise there will be an nal resistor connected between the RT pin and GND. Do offset. Finally, the bypass capacitor on the VIN supply to not leave this pin open. Also, do not load this pin with a the LT3476 should be placed as close as possible to the capacitor. A resistor must always be connected for proper VIN terminal of the device. operation. See Table 1 below or see the Oscillator Frequency vs RT graph in the Typical Performance Characteristics for
Open-Circuit Protection/Overvoltage Lockout
resistor values and corresponding switching frequencies. The LT3476 has independent internal overvoltage/open- circuit protection (OVP) for all four converters, sensed
Table 1. Switching Frequency vs RT
through their respective CAP inputs. The purpose of the
SWITCHING FREQUENCY (kHz) RT (kΩ)
OVP feature is to protect the main switch of the device 200 140 from damage. In the boost configuration, if the LEDs are 400 61.9 disconnected from the circuit or fail open, the converter 1000 21 output voltage at CAP is clamped at the OVP voltage of 1200 16.2 35V (typ). Figure 1 shows the transient response of the 2000 8.25 step-up converter application with LED1 disconnected. With LED1 disconnected, the converter switches at cur- In general, a lower switching frequency should be used rent limit as the output ramps up to OVP. Upon reaching where either very high or very low switch duty cycle opera- the OVP clamp voltage, the converter will switch with a tion is required, or higher efficiency is desired. Selection reduced current limit to regulate the converter output of a higher switching frequency will allow use of smaller voltage at the OVP clamp. In the buck mode application value external components and yield a smaller solution shown in the Block Diagram, should the external supply size and profile. Also for high frequency PWM dimming, for CAP exceed the OVP clamp, then switching will be a higher switching frequency (shorter switching period) inhibited for the converter. In order for the overvoltage will give better dimming control since for turning on the 3476fb 7 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 Typical Application Related Parts