STPW12 datasheet(12/23 Pages) STMICROELECTRONICS

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Figure 18. Monitor signal vs. power

Ch1

1V/div

Ch2

1V/div

Ch3

1V/div

Ch4

1A/div

100µs/div

15W

Power

Monitor

Output voltage

Input voltage

Output Current

Figure 19. Monitor signal vs. power (zoom)

ts tf

Power

Monitor

Output voltage

Input voltage

Table 5. Monitor signal truth table

Device status

Mon / fault

Normal operation

IMON

Power limit triggered

High (for 10 μs)

Thermal shutdown

Low

Off-state

Low

Current limit triggered

High

The voltage at high logic level is typically 4.5 V, therefore the pin can be directly interfaced to a 5 V compliant

microcontroller or ADC device.

7.4

Auto-retry delay time setting

The auto-retry delay time, defined as the time between a power interruption event and the device restart, can be

set by choosing the appropriate value for the CS capacitor, according to the graph in Figure 16. Auto-retry delay

time vs. CS.

7.5

PWM mode description, masking time

In this mode the device internal power switch can be driven on/off by an external PWM signal, provided to the

PWM pin (square wave, maximum 2 kHz, duty cycle 20% - 100%). It is important to notice that in this mode the

power limit protection is kept inactive for a delay time (masking time) selected by an external RD resistor (see

Table 6. Masking time vs. RD), so to avoid false triggering during in-rush power events at each PWM signal rising

edge.

Eleven discrete steps are available for the PWM masking time. They can be selected by choosing an appropriate

value of the RD resistor, as shown in the following table, where the standard 1% resistors are used.

For a correct operation of the device, the RDelay pin must not be directly connected to GND, and the minimum

masking time (45 µs) is achieved by using RD = 1 kΩ.

The masking time is measured starting from when the PWM rising edge overcomes the VSPH threshold till the

monitor signal starts increasing (see Figure 7. Masking time, fault signal).

PWM masking time is active also during each start-up cycle (including auto-retry), in order to avoid that inrush

current flowing to the output capacitor triggers the power limit, which would cause a premature shutdown.

STPW12

Auto-retry delay time setting

DS12478 - Rev 1

page 12/23

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