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本帖最后由 超級狗 于 2013-5-8 12:50 编辑 : f& x6 y) i$ A/ P' E
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看一下芯片資料中的這一段，是不是輸入和輸出的電平差太小？（應用是 12V 轉 11V）/ Y5 l& B. Y( Z* g2 M0 ^
! A: o+ @# W* \4 p7 f5 {5 v; R5 xLow-Dropout Operation and Bootstrap Voltage (BOOT)& A# d# b9 y! ^: `
The TPS5401 has an integrated boot regulator, and requires a small ceramic capacitor between the BOOT and PH pins to provide the gate-drive voltage for the high-side MOSFET. The BOOT capacitor is refreshed when the high-side MOSFET is off and the low-side diode conducts. The value of this ceramic capacitor should be 0.1 mF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher is recommended because of the stable characteristics over temperature and voltage. To improve dropout, the TPS5401 is designed to operate at 100% duty cycle as long as the BOOT-to-PH pin voltage is greater than 2.1 V. When the voltage from BOOT to PH drops below 2.1 V, the high-side MOSFET is turned off using an UVLO circuit, which allows the low-side diode to conduct and refresh the charge on the BOOT capacitor. Because the supply current sourced from the BOOT capacitor is low, the high-side MOSFET can remain on for more switching cycles than are required to refresh the capacitor; thus, the effective duty cycle of the switching regulator is high. The effective duty cycle during dropout of the regulator is mainly influenced by the voltage drops across the power MOSFET, inductor resistance, low-side diode, and printed circuit board resistance. During operating
, L8 e' J; l6 ~2 X4 wconditions in which the input voltage drops and the regulator is operating in continuous conduction mode, the high-side MOSFET can remain on for 100% of the duty cycle to maintain output regulation, until the BOOT-to-PH voltage falls below 2.1 V.
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! }; r7 `( }' F. f, L$ u% z# rAttention must be taken in maximum-duty-cycle applications which experience extended time periods with light loads or no load. When the voltage across the BOOT capacitor falls below the 2.1-V UVLO threshold, the high-side MOSFET is turned off, but there may not be enough inductor current to pull the PH pin down to recharge the BOOT capacitor. The high-side MOSFET of the regulator stops switching because the voltage across the BOOT capacitor is less than 2.1 V. The output capacitor then decays until the difference in the input voltage and output voltage is greater than 2.1 V, at which point the BOOT UVLO threshold is exceeded, and the device starts switching again until the desired output voltage is reached. This operating condition persists until the input voltage and/or the load current increases. It is recommended to adjust the VIN stop voltage greater than the BOOT UVLO trigger condition at the minimum load of the application using the adjustable VIN UVLO feature with resistors on the EN pin.0 J2 M# G6 ]) Z, d. y# e
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发表于 2013-5-3 21:25
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楼主
发表于 2013-5-8 08:54