PM8800ATR datasheet(25/35 Pages) STMICROELECTRONICS

tên linh kiện	PM8800ATR

Giải thích chi tiết về linh kiện	Integrated IEEE 802.3af compliant PoE-PD interface and PWM controller with support of external source

PDF	35 Pages

Scroll/Zoom	100%

nhà sản xuất	STMICROELECTRONICS [STMicroelectronics]

Trang chủ	http://www.st.com

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25 / 35 page

PM8800A

PWM section

25/35

The PM8800A can work also with a clock reference provided by an external source whose

frequency is higher than the one programmed by the RT resistor. The presence of the

external resistor is mandatory also when the reference is provided from outside the IC.

The external source must coupled to the RT with a 100 pF capacitor and have a minimum

peak amplitude of 2.8 V. Minimum pulse width of 50 ns has to be assured for proper

operation.

When synchronized the PWM frequency is equal to the external clock reference.

6.3

Soft start

The soft-start feature of the PM8800A allows the load voltage to ramp-up in a safe and

controlled manner. This is achieved by a 10

µA internal current source charging an external

Css capacitor connected to the SS pin, which progressively increases the duty cycle of the

PWM pulse, since the reference of the error amplifier is clamped with this value. Duration of

the start-up time and external capacitor are linked by the following relation:

SS voltage is actively kept at 2.3 V by the internal control circuitry, which manages also

over-current and fault conditions.

Figure 16.

Typical soft start waveform

Ch1 = 5Vout, Ch2 = soft start, Ch4 = I input

ss ms

()

0.23 C

SS nF

()

⋅

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