Datasheet LT8335 (Analog Devices) - 10

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APPLICATIONS INFORMATION
the removal of the limitations to inductor value and duty cycle required to avoid sub-harmonic oscillations and the right half plane zero (RHPZ). While DCM provides these IL benefits, the trade-off is higher inductor peak current, lower 500mA/DIV available output power and reduced efficiency. VOUT 5V/DIV
SETTING THE OUTPUT VOLTAGE
The output voltage is programmed with a resistor divider 500µs/DIV 8335 F03 from the output to the FBX pin. Choose the resistor values
Figure 3. Soft-Start Waveforms
for a positive output voltage according to: ⎛ V The LT8335 addresses this mechanism with an internal R1 ⎞ =R2• OUT ⎜ –1⎟ ⎝1.60V ⎠ soft-start function. As shown in the Block Diagram, the soft-start function controls the ramp of the power switch Choose the resistor values for a negative output voltage current by controlling the ramp of VC through M2. This according to: allows the output capacitor to be charged gradually toward its final value while limiting the start-up peak currents. R1 ⎛ ⎞ =R2• ⎜VOUT⎜ ⎜ –1⎟ ⎝0.80V ⎠ Figure 3 shows the output voltage and supply current for the first page Typical Application. It can be seen that both The locations of R1 and R2 are shown in the Block Dia- the output voltage and supply current come up gradually. gram. 1% resistors are recommended to maintain output voltage accuracy. INTVCC undervoltage (INTVCC < 2.5V) and/or thermal lockout (T Higher-value FBX divider resistors result in the lowest input J > 170°C) will immediately prevent switching, will reset the internal soft-start function and will pull down quiescent current and highest light-load efficiency. FBX VC. Once all faults are removed, the LT8335 will soft-start divider resistors R1 and R2 are usually in the range from VC and hence inductor peak current. 25k to 1M. Most applications use a phase-lead capacitor 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 low, extremely small duty cycles may be required to
SOFT-START
maintain control of inductor peak current. The minimum on-time limitation of the power switch might prevent these The LT8335 contains several features to limit peak switch low duty cycles from being achievable. In this scenario currents and output voltage (VOUT) overshoot during inductor current rise will exceed inductor current fall during start-up or recovery from a fault condition. The primary each cycle, causing inductor current to ‘walk up’ beyond purpose of these features is to prevent damage to external the switch current limit. The LT8335 provides protection components or the load. from this by folding back switching frequency whenever High peak switch currents during start-up may occur in FBX pin is close to GND (low VOUT levels). This frequency switching regulators. Since VOUT is far from its final value, foldback provides a larger switch-off time, allowing inductor the feedback loop is saturated and the regulator tries to current to fall enough each cycle (see Normalized Switch- charge the output capacitor as quickly as possible, resulting ing Frequency vs FBX Voltage in the Typical Performance in large peak currents. A large surge current may cause Characteristics section). inductor saturation or power switch failure. 8335f 10 For more information www.linear.com/LT8335