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LTC3609 Datasheet(PDF) 13 Page - Linear Technology |
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LTC3609 Datasheet(HTML) 13 Page - Linear Technology |
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13 / 26 page  LTC3609 3609fb applicaTions inForMaTion MinimumOff-timeandDropoutOperation The minimum off-time, tOFF(MIN), is the smallest amount of time that the LTC3609 is capable of turning on the bot- tom MOSFET, tripping the current comparator and turning the MOSFET back off. This time is generally about 250ns. The minimum off-time limit imposes a maximum duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle is reached, due to a dropping input voltage for example, then the output will drop out of regulation. The minimum input voltage to avoid dropout is: V V t t t IN MIN OUT ON OFF MIN ON ( ) ( ) = + A plot of maximum duty cycle vs frequency is shown in Figure 3. SettingtheOutputVoltage The LTC3609 develops a 0.6V reference voltage between the feedback pin, VFB, and the signal ground as shown in Figure 6. The output voltage is set by a resistive divider according to the following formula: VOUT =0.6V 1+ R2 R1 Toimprovethefrequencyresponse,afeed-forwardcapaci- tor, C1, may also be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line. InductorSelection Given the desired input and output voltages, the induc- tor value and operating frequency determine the ripple current: Δ IL = VOUT f L 1 − VOUT VIN Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors and output voltage ripple. Highest efficiency operation is obtained at low frequency with small ripple current. However, achieving this requires a large inductor. There is a tradeoff between component size, efficiency and operating frequency. A reasonable starting point is to choose a ripple current that is about 40% of IOUT(MAX). The largest ripple current occurs at the highest VIN. To guarantee that ripple current does not exceed a specified maximum, the inductance should be chosen according to: L = VOUT f ΔIL(MAX) 1− VOUT VIN(MAX) Figure3.MaximumSwitchingFrequencyvsDutyCycle 2.0 1.5 1.0 0.5 0 0 0.25 0.50 0.75 3609 F03 1.0 DROPOUT REGION DUTY CYCLE (VOUT/VIN) Figure2.CorrectingFrequencyShiftwithLoadCurrentChanges CVON 0.01µF RVON2 100k RVON1 30k CC VOUT RC (2a) (2b) VON ITH LTC3609 CVON 0.01µF RVON2 10k Q1 2N5087 RVON1 3k 10k CC 3609 F02 VOUT INTVCC RC VON ITH LTC3609 |
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