参数资料
型号: TPS54240QDGQRQ1
厂商: TEXAS INSTRUMENTS INC
元件分类: 稳压器
英文描述: 2.5 A SWITCHING REGULATOR, 2500 kHz SWITCHING FREQ-MAX, PDSO10
封装: GREEN, PLASTIC, MSOP-10
文件页数: 11/48页
文件大小: 1447K
代理商: TPS54240QDGQRQ1
SS/TR
TPS54240
EN
PWRGD
SS / TR
EN
PWRGD
VOUT 1
VOUT 2
R 1
R 2
R3
R 4
TPS54240
Vout2 + deltaV
Vssoffset
R1 =
VREF
Iss
VREF
R1
R2 =
Vout2 + deltaV
VREF
-
deltaV = Vout1
Vout2
-
R1 > 2800
Vout1
180
deltaV
-
SLVSAQ4A
– DECEMBER 2010 – REVISED APRIL 2011
DETAILED DESCRIPTION (continued)
Figure 35. Schematic for Ratiometric and Simultaneous Start-Up Sequence
Ratio-metric and simultaneous power supply sequencing can be implemented by connecting the resistor network
of R1 and R2 shown in Figure 35 to the output of the power supply that needs to be tracked or another voltage
reference source. Using Equation 7 and Equation 8, the tracking resistors can be calculated to initiate the Vout2
slightly before, after or at the same time as Vout1. Equation 9 is the voltage difference between Vout1 and Vout2
at the 95% of nominal output regulation.
The deltaV variable is zero volts for simultaneous sequencing. To minimize the effect of the inherent SS/TR to
VSENSE offset (Vssoffset) in the slow start circuit and the offset created by the pullup current source (Iss) and
tracking resistors, the Vssoffset and Iss are included as variables in the equations.
To design a ratio-metric start up in which the Vout2 voltage is slightly greater than the Vout1 voltage when Vout2
reaches regulation, use a negative number in Equation 7 through Equation 9 for deltaV. Equation 9 will result in a
positive number for applications which the Vout2 is slightly lower than Vout1 when Vout2 regulation is achieved.
Since the SS/TR pin must be pulled below 40mV before starting after an EN, UVLO or thermal shutdown fault,
careful selection of the tracking resistors is needed to ensure the device will restart after a fault. Make sure the
calculated R1 value from Equation 7 is greater than the value calculated in Equation 10 to ensure the device can
recover from a fault.
As the SS/TR voltage becomes more than 85% of the nominal reference voltage the Vssoffset becomes larger
as the slow start circuits gradually handoff the regulation reference to the internal voltage reference. The SS/TR
pin voltage needs to be greater than 1.3V for a complete handoff to the internal voltage reference as shown in
(7)
(8)
(9)
(10)
Copyright
2010–2011, Texas Instruments Incorporated
19
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