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LT1619 Datasheet(PDF) 10 Page - Linear Technology |
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LT1619 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LT1619 1619fa f RC f CORNER OS S S =≈ 1 2 5 π (The input impedance of the sense amplifier at the SENSE pin is 2500 Ω and ROS is typically less than 137Ω.) Typical values for ROS and CS are 100Ω and 1nF. The 100Ω value for ROS reduces Burst Mode threshold; use 10Ω and 10nF when this is not desireable. Figure 7. Implementing Undervoltage Lockout + – V V I 0 ZENER DIODE AVALANCHE DIODE BV < 5V I Figure 8. I-V Characteristics of Zener and Avalanche Breakdown Diodes S/S FB VC GND 8 7 6 5 1 2 3 4 VIN DRV GATE SENSE LT1619 R3 C1 R4 VIN 1619 F09 Figure 9. Filtering Input Voltage Ripple in UVLO Circuit Use of Shutdown Function to Modify Undervoltage Lockout The LT1619 is designed to operate from an input supply with voltage as low as 1.85V. Shutdown is activated when the S/S pin is pulled below 0.45V. The shutdown threshold is slightly greater than one junction diode forward voltage and has the temperature characteristics of a junction diode. The S/S pin is normally tied to the input when operating from a low voltage input source. Consider the 12V to – 65V isolated flyback converter (see Typical Applications). The converter draws 3A at low line while delivering 0.4A to the output. If the S/S pin is tied to the input, then the LT1619 will start switching as soon as VIN exceeds the internal UVLO threshold. With full load, the converter can draw much higher than the steady-state 3A from the input source during start-up. If the input source is current limited, the input voltage will collapse and latch low. The start-up problem can be prevented by adding a zener diode and a resistor to the S/S pin (Figure 7). This is equivalent to increasing undervoltage lockout voltage of the controller. Before VIN exceeds the zener voltage VZ, the S/S pin current is shunted to the ground through the Figure 6. Current Sense Filter for Improving Jitter Performance resistor R3. The voltage developed across R3 due to IS/S should be less than the shutdown threshold. The LT1619 remains off until VIN exceeds the sum of VZ and the shutdown threshold. True zener diodes (BV < 5V) and higher voltage avalanche diodes have different I-V charac- teristics (Figure 8). They need to be biased appropriately (value of R3) in order to obtain correct UVLO threshold. When implementing UVLO with converters with high input ripple voltages (such as flyback and forward), the circuit in Figure 7 is modified and shown in Figure 9. PWM COMPARATOR CURRENT SENSE AMPLIFIER SENSE CS RSENSE VSENSE + – ROS ID GND 1619 F06 LT1619 – + 5 4 S/S FB VC GND 8 7 6 5 1 2 3 4 VIN DRV GATE SENSE LT1619 R3 VZ VIN IS/S 1619 F07 IS/S VS/S = 0 R3 < SHUTDOWN THRESHOLD ≈ –2µA UVLO THRESHOLD = VZ + SHUTDOWN THRESHOLD ≈ VZ + VBE () IS/S VS/S = 0 APPLICATIO S I FOR ATIO |
Similar Part No. - LT1619_15 |
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Similar Description - LT1619_15 |
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