STPW12 datasheet(13/23 Pages) STMICROELECTRONICS

13 / 23 page

7.6

Details about the start-up sequence

The device is not equipped with a soft-start control, in order to comply with the maximum PWM frequency,

therefore in certain applications, featuring high output capacitance after the device and depending on the load

characteristics, the current limit protection could occur during the start-up. The device is immediately turned off

and restarted as described in Section 7.2 Current limit and thermal protections. This kind of on-off cycle can

happen several times until the COUT is fully charged and as a result, the start-up is prolonged.

When VOUT reaches the target value, if power limit threshold is overcome and PWM is still high, due to a short

masking time the device could turn off. The device then starts again after the auto-retry delay time has elapsed.

For all of the above reasons, RD and CS have to be selected carefully in order to achieve the start-up sequence

and time that satisfies the needs of the application.

The UVLO internal signal is deglitched by a delay of 60 μs (typ.) after EN assertion, in order to ensure that all the

circuit functions are ready for the start-up. UVLO, EN, and PWM signals need being correctly asserted so that the

masking time is activated.

During the start-up, attention must be paid in case a specific sequence/connection of VIN, EN and PWM is

needed. The following diagrams show two examples of start-up with masking time activation. Figure 20. Start-up

with PWM tied to VIN shows a start-up with PWM pin connected directly to VIN (PWM function is not used in the

application), whereas Figure 21. Start-up with EN tied to VIN shows a start-up with EN pin tied to VIN (EN function

is not used in the application).

Figure 20. Start-up with PWM tied to VIN

Monitor

Enable

Input voltage=PWM voltage

Output current

Ch1

3V/div

Ch2

2V/div

Ch3

2A/div

Ch4

2V/div

20µs/div

Figure 21. Start-up with EN tied to VIN

Input voltage=Enable voltage

Ch1

3V/div

Ch2

2V/div

Ch3

2A/div

Ch4

2V/div

20µs/div

Monitor

PWM

Output current

With reference to Figure 20. Start-up with PWM tied to VIN, at time t1 the EN signal activates the UVLO

comparator and the UVLO deglitch time starts. At deglitch time expiration (t2) all signals are high, the device is

activated and the masking time starts.

Instead, in the case described in Figure 21. Start-up with EN tied to VIN, the enable is applied to VIN connection,

so the UVLO deglitch time has already expired when PWM signal is asserted. The masking time starts together

with PWM.

STPW12

Details about the start-up sequence

DS12478 - Rev 1

page 13/23

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