SC4905A/B
POWER MANAGEMENT
Application Information (Cont.)
START UP SEQUENCE
PRELIMINARY
Initially during the power up, the SC4905 is in the under
voltage lock out condition. As the VDD supply exceeds
the UVLO limit of the SC4905 and the VFF pin exceeds
the line under voltage lock out of about 1.2V, the inter-
nal reference, oscillator, and logic circuitry are powered
up.
The OUT driver is not enabled until the line under voltage
lock out limit is reached. At that point, once the FB pin
has reached above 600mV, the output driver is enabled.
As the output voltage starts to increase, the error signal
from the error amplifier starts to decrease. If isolation is
required, the error amplifier output can drive the LED of
the opto isolator. The output of the opto is connected in
a common emitter configuration with a pull up resistor to
a reference voltage connected to the FB pin of the
SC4905. The voltage level at the FB pin provides the duty
cycle necessary to achieve regulation.
2002 Semtech Corp.
11
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