Datasheet LT3758, LT3758A (Analog Devices) - 14

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APPLICATIONS INFORMATION FBX Frequency Foldback
load transient response, when compared to the LT3758. When V New internal circuits ensure that the transient from not OUT is very low during start-up or a GND fault on the output, the switching regulator must operate at switching to switching at high current can be made in low duty cycles to maintain the power switch current a few cycles. The optimum values depend on the con- within the current limit range, since the inductor cur- verter topology, the component values and the operat- rent decay rate is very low during switch off time. The ing conditions (including the input voltage, load current, minimum on-time limitation may prevent the switcher etc.). To compensate the feedback loop of the LT3758/ from attaining a sufficiently low duty cycle at the pro- LT3758A, a series resistor-capacitor network is usually grammed switching frequency. So, the switch current connected from the VC pin to GND. Figure 1 shows the will keep increasing through each switch cycle, exceed- typical VC compensation network. For most applications, ing the programmed current limit. To prevent the switch the capacitor should be in the range of 470pF to 22nF, peak currents from exceeding the programmed value, the and the resistor should be in the range of 5k to 50k. A LT3758 contains a frequency foldback function to reduce small capacitor is often connected in parallel with the RC the switching frequency when the FBX voltage is low (see compensation network to attenuate the VC voltage ripple the Normalized Switching Frequency vs FBX graph in the induced from the output voltage ripple through the inter- Typical Performance Characteristics section). nal error amplifier. The parallel capacitor usually ranges in value from 10pF to 100pF. A practical approach to design During frequency foldback, external clock synchroniza- the compensation network is to start with one of the cir- tion is disabled to prevent interference with frequency cuits in this data sheet that is similar to your applica- reducing operation. tion, and tune the compensation network to optimize the performance. Stability should then be checked across all
Thermal Lockout
operating conditions, including load current, input voltage If LT3758 die temperature reaches 165°C (typical), the and temperature. part will go into thermal lockout. The power switch will be turned off. A soft-start operation will be triggered. The
SENSE Pin Programming
part will be enabled again when the die temperature has For control and protection, the LT3758 measures the dropped by 5°C (nominal). power MOSFET current by using a sense resistor (RSENSE) between GND and the MOSFET source. Figure 4 shows a
Loop Compensation
typical waveform of the sense voltage (VSENSE) across the Loop compensation determines the stability and transient sense resistor. It is important to use Kelvin traces between performance. The LT3758/LT3758A use current mode the SENSE pin and RSENSE, and to place the IC GND as control to regulate the output which simplifies loop com- close as possible to the GND terminal of the RSENSE for pensation. The LT3758A improves the no-load to heavy proper operation. VSENSE ∆VSENSE = χ • VSENSE(MAX) VSENSE(MAX) VSENSE(PEAK) t DTS TS 3758 F04
Figure 4. The Sense Voltage During a Switching Cycle
Rev F 14 For more information www.analog.com Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Applications Related Parts Typical Applications Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts