|Description||Green Mode Fairchild Power Switch (FPS™)|
|FSBH0270W Datasheet PDF : 14 Pages |
The FSBH-series uses feedback voltage (VFB) as an
indicator of the output load and modulates the PWM
frequency, as shown in Figure 26, such that the
switching frequency decreases as load decreases. In
heavy-load conditions, the switching frequency is
100kHz. Once VFB decreases below VFB-N (2.5V), the
PWM frequency starts to linearly decrease from 100kHz
to 18kHz to reduce switching losses. As VFB decreases
below VFB-G (2.0V), the switching frequency is fixed at
18kHz and the FSBH-series enters “deep” green mode to
reduce the standby power consumption.
Figure 26. PWM Frequency
As VFB decreases below VFB-ZDC (1.6V), the FSBH-
series enters burst-mode operation. When VFB drops
below VFB-ZDC, FSBH-series stops switching and the
output voltage starts to drop, which causes the feedback
voltage to rise. Once VFB rises above VFB-ZDC, switching
resumes. Burst mode alternately enables and disables
switching, thereby reducing switching loss to improve
power saving, as shown in Figure 1.
Figure 28. Constant Power Control
The FSBH-series provides full protection functions to
prevent the power supply and the load from being
damaged. The protection features include:
Open-Loop / Overload Protection (OLP)
When the upper branch of the voltage divider for the
shunt regulator (KA431 shown) is broken, as shown in
Figure 29, or over current or output short occurs, there
is no current flowing through the opto-coupler transistor,
which pulls the feedback voltage up to 6V.
When feedback voltage is above 4.6V for longer than
56ms, OLP is triggered. This protection is also triggered
when the SMPS output drops below the nominal value
longer than 56ms due to the overload condition.
VDD Over-Voltage Protection (OVP)
VDD over-voltage protection prevents IC damage caused
by over voltage on the VDD pin. The OVP is triggered
when VDD voltage reaches 28V. Debounce time (typically
130µs) prevents false trigger by switching noise.
Over-Temperature Protection (OTP)
The SenseFET and the control IC are integrated,
making it easier to detect the temperature of the
SenseFET. As the temperature exceeds approximately
142°C, thermal shutdown is activated.
Figure 27. Burst-Mode Operation
H/L Line Over-Power Compensation
To limit the output power of the converter constantly,
high/low line over-power compensation is included.
Sensing the converter input voltage through the VIN pin,
the high/low line compensation function generates a
relative peak-current-limit threshold voltage for constant
power control, as shown in Figure 28.
© 2011 Fairchild Semiconductor Corporation
FSBH0F70WA/0170W/0270W • Rev. 1.0.2
Figure 29. OLP Operation
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