Datasheet LT8330 (Analog Devices) - 10

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APPLICATIONS INFORMATION SETTING THE OUTPUT VOLTAGE
The output voltage is programmed with a resistor divider from the output to the FBX pin. Choose the resistor values for a positive output voltage according to: IL 500mA/DIV R1 = R2 • (VOUT/1.60V – 1) Choose the resistor values for a negative output voltage VOUT 20V/DIV according to: R1 = R2 • (|V 8330 F03 OUT|/0.80V – 1) 500µs/DIV The locations of R1 and R2 are shown in the Block Dia-
Figure 3. Soft-Start Waveforms
gram. 1% resistors are recommended to maintain output voltage accuracy. INTVCC undervoltage (INTVCC < 2.5V) and/or thermal Higher-value FBX divider resistors result in the lowest input lockout (TJ > 170°C) will immediately prevent switching, quiescent current and highest light-load efficiency. FBX will reset the internal soft-start function and will pull down divider resistors R1 and R2 are usually in the range from VC. Once all faults are removed, the LT8330 will soft-start 25k to 1M. Most applications use a phase-lead capacitor VC and hence inductor peak current. from VOUT to FBX in combination with high-value FBX divider resistors (see Compensation in the Applications
FREQUENCY FOLDBACK
Information section). During start-up or fault conditions in which VOUT is very
SOFT-START
low, extremely small duty cycles may be required to maintain control of inductor peak current. The minimum The LT8330 contains several features to limit peak switch on-time limitation of the power switch might prevent these currents and output voltage (VOUT) overshoot during low duty cycles from being achievable. In this scenario start-up or recovery from a fault condition. The primary inductor current rise will exceed inductor current fall during purpose of these features is to prevent damage to external each cycle, causing inductor current to ‘walk up’ beyond components or the load. the switch current limit. The LT8330 provides protection High peak switch currents during start-up may occur in from this by folding back switching frequency whenever switching regulators. Since V FBX pin is close to GND (low V OUT is far from its final value, OUT levels). This frequency the feedback loop is saturated and the regulator tries to foldback provides a larger switch-off time, allowing inductor charge the output capacitor as quickly as possible, resulting current to fall enough each cycle (see Normalized Switch- in large peak currents. A large surge current may cause ing Frequency vs FBX Voltage in the Typical Performance inductor saturation or power switch failure. Characteristics section). The LT8330 addresses this mechanism with an internal soft-start function. As shown in the Block Diagram, the
THERMAL LOCKOUT
soft-start function controls the ramp of the power switch If the LT8330 die temperature reaches 170°C (typical), current by controlling the ramp of VC through M2. This the part will stop switching and go into thermal lockout. allows the output capacitor to be charged gradually toward When the die temperature has dropped by 5°C (nominal), its final value while limiting the start-up peak currents. the part will resume switching with a soft-started inductor Figure 3 shows the output voltage and supply current for peak current. the first page Typical Application. It can be seen that both the output voltage and supply current come up gradually. Rev. B 10 For more information www.analog.com Document Outline Features Applications Typical Application Description 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