Datasheet

SS
t t1 t2= +
t2 +
C
SS
12 mA
ǒ
0.7 V * ǒ
V
OUT
R
BIAS
R1 ) R
BIAS
Ǔ
Ǔ
t1 +
C
SS
6 mA
ǒ
V
OUT
R
BIAS
R1 ) R
BIAS
Ǔ
3
SS SS
t C 58 10= ´ ´
TPS40140
www.ti.com SLUS660H –SEPTEMBER 2005–REVISED JUNE 2013
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, V
REF
= 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.
C
SS
is in Farads and t
SS
is given in seconds.
(1)
5.5 SOFT-START WITH PRE-BIASED OUTPUTS
For pre-biased outputs the TPS40140 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 TRKx pin and the output voltage, VOUT when
VOUT is pre-biased. TRKx rises due to the 6 μA current, until at 1 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 the 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 it 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 through Equation 4.
(2)
(3)
where
• C
SS
is in Farads
• t
SS
is in seconds
(4)
If there is no pre-bias (V
OUT
= 0 V), the equation reduces to case without pre-bias.
Copyright © 2005–2013, Texas Instruments Incorporated APPLICATION INFORMATION 17
Submit Documentation Feedback
Product Folder Links: TPS40140