參數(shù)資料
型號(hào)： TPS40054PWPR
廠商： TEXAS INSTRUMENTS INC
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO16
封裝： GREEN, PLASTIC, HTSSOP-16
文件頁(yè)數(shù)： 5/35頁(yè)
文件大?。?/td> 866K
代理商： TPS40054PWPR
PROGRAMMING SOFT START
t
START w 2p
L
C
O
(seconds)
(12)
( )
SS / SD
START
FB
I
C
t
F
V
=
÷
è
(13)
www.ti.com ...................................................................................................................................... SLUS593F – DECEMBER 2003 – REVISED SEPTEMBER 2008
The TPS4005x uses a closed-loop soft-start system to ensure a controlled ramp of the output during startup. The
reference voltage used for the startup is derived in the following manner. A capacitor (CSS/SD) is connected to the
SS/SD pin. There is a ramped voltage generated at this pin by charging CSS/SD with a current source. A value of
0.85 V is subtracted from the voltage at the SS/SSD pin and is applied to a non-inverting input of the error
amplifier. This is the effective soft-start ramp voltage, VSSRMP. The error amplifier also has the 0.7-V reference
(VFB) voltage applied to a non-inverting input. The structure of the error amplifier input stage is such that the
lower of VFB or VSSRMP becomes the dominant voltage that the error amplifier uses to regulate the FB pin. This
provides a clean, closed-loop startup while VSSRMP is lower than VFB and a precision reference regulated supply
as VSSRMP climbs above VFB. To ensure a controlled ramp-up of the output voltage, the soft-start time should be
greater than the L-CO time constant as described in Equation 12.
where
t
START is the startup ramp time in s
L is the filter inductance in H
C
O is the filter capacitance in F
There is a direct correlation between tSTART and the input current required during start-up. The faster tSTART, the
higher the input current required during start-up. This relationship is describe in more detail in the section titled,
Programming the Current Limit which follows. The soft-start capacitance, CSS/SD, is described in Equation 13.
For applications in which the VIN supply ramps up slowly (typically between 50 ms and 100 ms), it may be
necessary to increase the soft-start time to between approximately 2 ms and 5 ms to prevent nuisance UVLO
tripping. The soft-start time should be longer than the time that the VIN supply transitions between 6 V and 7 V.
where
I
SS/SD is the soft-start charge current (typical value is 2.3 A)
V
FB is the feedback reference voltage (typical value is 0.7 V)
Copyright 2003–2008, Texas Instruments Incorporated
13
Product Folder Link(s): TPS40054 TPS40055 TPS40057
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