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LTC3707 数据表(PDF) 26 Page - Linear Technology |
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LTC3707 数据表(HTML) 26 Page - Linear Technology |
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26 / 32 page  LTC3707 26 3707fb APPLICATIONS INFORMATION Design Example As a design example for one channel, assume VIN = 12V(nominal), VIN = 22V(max), VOUT = 2V, IMAX = 5A, and f = 300kHz. The inductance value is chosen first based on a 30% ripple current assumption. The highest value of ripple current occurs at the maximum input voltage. Tie the FREQSET pin to the INTVCC pin for 300kHz operation. The minimum inductance for 30% ripple current is: ΔI L = V OUT (f)(L) 1– V OUT V IN ⎛ ⎝⎜ ⎞ ⎠⎟ A 4.7μH inductor will produce 25% ripple current and a 3.3μH will result in 36%. The peak inductor current will be the maximum DC value plus one half the ripple current, or 5.92A, for the 3.3μH value. Increasing the ripple current will also help ensure that the minimum on-time of 200ns is not violated. The minimum on-time occurs at maximum VIN: t ON(MIN) = V OUT V IN(MAX)f = 2V 22V(300kHz) = 303ns The RSENSE resistor value can be calculated by using the maximum current sense voltage specification with some accommodation for tolerances: R SENSE ≤ 60mV 5.92A ≈ 0.01Ω Since the output voltage is below 2.4V the output resistive divider will need to be sized to not only set the output voltage but also to absorb the SENSE pins specified input current. R1 (MAX) = 24k 0.8V 2.4V – V OUT ⎛ ⎝⎜ ⎞ ⎠⎟ = 24K 0.8V 2.4V – 1.8V ⎛ ⎝⎜ ⎞ ⎠⎟ = 32k Choosing 1% resistors; R1 = 25.5k and R2 = 32.4k yields an output voltage of 1.816V. The power dissipation on the top side MOSFET can be easily estimated. Choosing a Siliconix Si4412DY results in; RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input voltage with T(estimated) = 50°C: P MAIN = 2V 22V 5 ()2 1+(0.005)(50°C–25°C) [] 0.042 Ω ()+1.7 22V ()2 5A () 100pF () 300kHz () = 230mW A short-circuit to ground will result in a folded back current of: I SC = 25mV 0.01 Ω + 1 2 200ns(22V) 3.3µH ⎛ ⎝⎜ ⎞ ⎠⎟ = 3.2A with a typical value of RDS(ON) and δ = (0.005/°C)(20) = 0.1. The resulting power dissipated in the bottom MOSFET is: P SYNC = 22V – 2V 22V 3.2A ()2 1.1 () 0.042Ω () = 430mW which is less than under full-load conditions. CIN is chosen for an RMS current rating of at least 3A at temperature assuming only this channel is on. COUT is chosen with an ESR of 0.02 Ω for low output ripple. The output ripple in continuous mode will be highest at the maximum input voltage. The output voltage ripple due to ESR is approximately: VORIPPLE = RESR(ΔIL) = 0.02Ω(1.67A) = 33mVP–P |
类似零件编号 - LTC3707 |
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类似说明 - LTC3707 |
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