PM8800 STMicroelectronics, PM8800 Datasheet - Page 25

• Manufacturer Part Number: PM8800
• Description: Integrated IEEE 802.3af compliant PoE-PD interface and PWM controller with support of external source
• Manufacturer: STMicroelectronics

PM8800A
6.3
The PM8800A can work also with a clock reference provided by an external source whose
frequency is higher than the one programmed by the R
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.
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
C
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
t
ss
ss
(
ms
capacitor connected to the SS pin, which progressively increases the duty cycle of the
)
=
0.23 C
⋅
SS
(
nF
)
T
resistor. The presence of the
PWM section
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