Datasheet
SLUS564C − JULY 2003 − REVISED OCTOBER 2008
13
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APPLICATION INFORMATION
ENABLE AND SOFT-START
Each SMPS is switched into standby mode separately by grounding the corresponding ENBLx pin. The 5-V
supply is enabled if either the DDR
, ENBL1 or ENBL2 pin(s) goes high ( >2.2 V).
Softstart of each SMPS is achieved by slowly ramping the error amplifier reference voltage by following a
buffered version of the SSTRTx pin voltage. Designers can achieve their own start-up sequencing by simply
provide external timing signals since the startup times do not depend on the load current. The softstart time is
programmable by external capacitor connected from SSTRTx pin to the ground. Each SSTRTx pin sources
constant current, typically 2.3 µA. The output voltage of the SMPS ramps up from 0 V to its target regulation
voltage as the SSTRTx pin voltage increases from 0 V to 1.2 V. This gives the softstart time formula to be,
C
SSTRT
(Farads) +
T
SSTRT
(sec) 2.3 10
*6
1.2
The soft-start capacitor is discharged upon UVLO, OVP or UVP is detected as well as ENBLx is set low.
OUTPUT DISCHARGE (SOFT-STOP)
When an SMPS is turned off by ENBLx asserted low or the part enters a fault mode, both top and bottom drivers
are turned off. This may leave the output in a high impedance state that allows the voltage to persist for some
time. Output voltage should be discharged prior to the next power up. To achieve this, connect the output to the
VO1_VDDQ or VO2 pins.
These pins turn on a 6-Ω resistor to ground during an off or fault condition. Both the VO1_VDDQ and VO2 pin
must be discharged to 0.3 V before the TPS51020 restarts. The TPS51020 has the flexibility of adding a resistor
in series with the VOx pin and the output voltage in order to reduce the discharge current and reduce the total
power dissipation within the device. It should be noted that when this resistor is added the discharged voltage
threshold changes according to the following equation:
V
DISCHARGE
+
ǒ
R
EXTERNAL
) R
DS(on)
Ǔ
R
DS(on)
0.3
where
D R
EXTERNAL
is the series resistor between VOx and the output
D R
DS(on)
= 6 Ω