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參數(shù)資料
| 型號(hào)： | TPS53851RHHT |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, PQCC36 |
| 封裝： | PLASTIC, VQFN-36 |
| 文件頁數(shù)： | 9/66頁 |
| 文件大?。?/td> | 1264K |
| 代理商： | TPS53851RHHT |
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T
SS + CSS
58
103
t1 +
C
SS
6 mA
V
OUT
R
BIAS
R1 ) RBIAS
t2 +
C
SS
12 mA
0.7 V *
V
OUT
R
BIAS
R1 ) RBIAS
www.ti.com
SLUS985 – DECEMBER 2009
5.4
TRACK (SOFT-START WITHOUT PRE-BIASED OUTPUT)
A capacitor connected to the TRKx pins sets the power-up time. When UVLO_CEx is high and the internal
power-on reset (POR) is cleared, the calibrated current source, starts charging the external soft start
capacitor with 12-
μA. The PGOOD pin is held low during the start up. The rising voltage across the
capacitor serves as a reference for the error amplifier, U10 and U14. When the soft start voltage reaches
the level of the reference voltage, VREF = 0.7 V, the converter’s output reaches the regulation point and
further voltage rise of the soft-start voltage has no effect on the output. When the soft start voltage
reaches 1.4 V, the powergood (PGOOD) function is cleared to be reported on the PGOOD pin. Normally
the PGOOD pin goes high at this time. Equation 1 is used to calculate the value of the soft-start capacitor.
Css is in Farads and Tss is given in seconds.
(1)
5.5
SOFT-START WITH PRE-BIASED OUTPUTS
For pre-biased outputs the TPS53851 uses two levels of soft-start current that charge the soft-start
capacitor connected to the TRKx pin(s). PWM switching begins when the TRKx voltage rises to the
voltage present on the FBx pin. When the first PWM pulse occurs, the charging current is increased to 12
μA. Figure 5-2 shows the typical waveforms present on the TRACKx pin and the output voltage, VOUT
when VOUT is pre-biased. TRKx rises due to the 6
μA current, until at T1 the voltage on TRKx equals the
pre-biased voltage on the FBx pin, at time t1. At this time the soft-start current is increased to 12
μA and
TRKx rises with an increase in it's slope. When TRKx reaches 0.7 V, at time t2, the output should be in
regulation. The voltage on the TRKx pin continues to rise. When the TRKx voltage is 1.4 V, at time t3, the
PGOODx signal is enabled. The TRKx voltage continues to rise to 2.4 V where is is clamped internally.
This approach provides for an accurate detection of the threshold where FBx = TRKx. Figure 5-3 is a
block diagram of the implementation. The calculation for the soft start time, due to pre-bias, includes the
time from t0 to t1, plus the time from t1 to t2, as shown in Equation 2 and Equation 3.
(2)
(3)
where
CSS is in Farads
tSS is in seconds
tSS = t1+t1
If there is no pre-bias (VOUT = 0 V), the equation reduces to case without pre-bias.
Copyright 2009, Texas Instruments Incorporated
APPLICATION INFORMATION
17
Product Folder Link(s): TPS53851
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