Datasheet LT8335 (Analog Devices) - 9

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APPLICATIONS INFORMATION
While in Burst Mode operation, the current limit of the LT8335. Therefore, R3 and R4 should be large enough to switch is approximately 500mA, resulting in the output minimize their effect on efficiency at light loads. voltage ripple shown in Figure 2. Increasing the output capacitance will decrease the output ripple proportionally.
INTV
As the output load ramps upward from zero the switch-
CC REGULATOR
ing frequency will increase but only up to the fixed 2MHz A low dropout (LDO) linear regulator, supplied from VIN, defined by the internal oscillator as shown in Figure 1. The produces a 3V supply at the INTVCC pin. A minimum 1µF output load at which the LT8335 reaches the fixed 2MHz low ESR ceramic capacitor must be used to bypass the frequency varies based on input voltage, output voltage, INTVCC pin to ground to supply the high transient currents and inductor choice. required by the internal power MOSFET gate driver. No additional components or loading is allowed on this pin. The INTVCC rising threshold (to allow soft start and IL switching) is typically 2.6V. The INTVCC falling threshold 500mA/DIV (to stop switching and reset soft start) is typically 2.5V. VOUT 10mV/DIV
DUTY CYCLE CONSIDERATION
The LT8335 minimum on-time, minimum off-time and 8335 F02 switching frequency (fOSC) define the allowable minimum 5µs/DIV and maximum duty cycles of the converter (see Minimum
Figure 2. Burst Mode Operation
On-Time, Minimum Off-Time, and Switching Frequency in the Electrical Characteristics table).
PROGRAMMING INPUT TURN-ON AND TURN-OFF
Minimum Allowable Duty Cycle =
THRESHOLDS WITH EN/UVLO PIN
Minimum On-Time(MAX) • fOSC(MAX) The EN/UVLO pin voltage controls whether the LT8335 is Maximum Allowable Duty Cycle = enabled or is in a shutdown state. A 1.6V reference and a 1 – Minimum Off-Time(MAX) • fOSC(MAX) comparator A6 with built-in hysteresis (typical 80mV) allow the user to accurately program the system input voltage The required switch duty cycle range for a Boost converter at which the IC turns on and off (see the Block Diagram). operating in continuous conduction mode (CCM) can be The typical input falling and rising threshold voltages can calculated as: be calculated by the following equations: V D IN(MAX) MIN =1– V OUT + VD V R3+R4 IN(FALLING,UVLO(–)) =1.60• R4 V D IN(MIN) MAX =1– V V R3+R4 OUT + VD IN(FALLING,UVLO(–)) =1.68• R4 where VD is the diode forward voltage drop. If the above V duty cycle calculations for a given application violate the IN current is reduced below 1µA when the EN/UVLO pin voltage is less than 0.2V. The EN/UVLO pin can be con- minimum and/or maximum allowed duty cycles for the nected directly to the input supply V LT8335, operation in discontinuous conduction mode IN for always-enabled operation. A logic input can also control the EN/UVLO pin. (DCM) might provide a solution. For the same VIN and V When operating in Burst Mode operation for light load OUT levels, operation in DCM does not demand as low a duty cycle as in CCM. DCM also allows higher duty cycle currents, the current through the R3 and R4 network can operation than CCM. The additional advantage of DCM is easily be greater than the supply current consumed by the 8335f For more information www.linear.com/LT8335 9