Brown-In Function of FSBH0x70A
The VIN pin functions are disabled from FSBH0x70A,
but FSBH0x70A has brown-in protection in the VDD pin.
There is a discharge current internal from VDD to ground
during startup. The HV source current must be larger
than IDD-ST to charge the capacitor of VDD. Therefore, the
brown-in level can be determined by RHV according to
2VAC − 12
The FSBH-series uses feedback voltage (VFB) as an
indicator of the output load and modulates the PWM
frequency, as shown in Figure 30, 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 for reducing switching losses. As VFB
decreases below VFB-G (2.0V), the switching frequency is
fixed at 18kHz and FSBH-series enters “deep” green
mode to reduce the standby power consumption.
Figure 30. PWM Frequency
As VFB decreases below VFB-ZDC (1.6V), FSBH-series
enters into 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 31.
Figure 31. 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 32.
Figure 32. 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:
Besides the brownout protection and high/low line over-
power compensation, the FSBH0170/0270/0370 has
additional protections via the VIN pin, such as pull-
HIGH latch and pull-LOW auto-recovery that depends
on the application. As shown in Figure 33, VIN level is
higher than 4.7V, FSBH-series is latched until the VDD is
discharged. FSBH-series is auto-recovery when the VIN
level is lower than 0.3V.
Figure 33. VIN Pin Function
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 34, 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.
© 2009 Fairchild Semiconductor Corporation
FSBH0F70A, FSBH0170/A, FSBH0270/A, FSBH0370 • Rev. 1.0.3