TPS54383, TPS54386
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SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
Output Voltage Sequencing
The TPS5438x allows single-pin programming of output voltage startup sequencing. During power-on, the state
of the SEQ pin is detected. Based on whether the pin is tied to BP, to GND, or left floating, the outputs behave
as described in Table 1.
Table 1. Sequence States
SEQ PIN STATE
MODE
EN1
EN2
Ignored by the device.when VEN2
enable threshold voltage
<
Tie EN1 to < enable threshold voltage
for BP to be active when VEN2
enable threshold voltage
>
BP
Sequential, Output 2 then Output 1
Active
Tie EN1 to > enable threshold voltage
for low quiescent current (BP inactive)
when VEN2 > enable threshold voltage
Ignored by the device.when VEN1
enable threshold voltage
<
Tie EN2 to < enable threshold voltage
for BP to be active when VEN1
enable threshold voltage
>
GND
Sequential, Output 1 then Output 2
Active
Tie EN2 to > enable threshold voltage
for low quiescent current (BP inactive)
when VEN1 > enable threshold voltage
Independent or Ratiometric, Output 1
and Output 2
Active. EN1 and EN2 must be tied
together for Ratio-metric startup.
Active. EN1 and EN2 must be tied
together for Ratio-metric startup.
(floating)
If the SEQ pin is connected to BP, then when Output 2 is enabled, Output 1 is allowed to start approximately 400
µs after Output 2 has reached regulation; that is, sequential startup where Output 1 is slave to Output 2. If EN2 is
allowed to go high after the outputs have been operating, then both outputs are disabled immediately, and the
output voltages decay according to the load that is present.
If the SEQ pin is connected to GND, then when Output 1 is enabled, Output 2 is allowed to start approximately
400 µs after Output 1 has reached regulation; that is, sequential startup where Output 2 is slave to Output 1. If
EN1 is allowed to go high after the outputs have been operating, then both outputs are disabled immediately,
and the output voltages decay according to the load that is present.
SEQ = BP
Sequential
SEQ = GND
Sequential
CH2 then CH1
CH1 then CH2
5-V VOUT1
(2 V/div)
5-V VOUT1
(2 V/div)
3.3-V VOUT2
(2 V/div)
3.3-V VOUT2
(2 V/div)
T - Time - 1 ms/div
Figure 18. SEQ Pin TIed to BP
T - Time - 1 ms/div
Figure 19. SEQ Pin Tied to GND
14
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