YB1517
Step-up DC-DC Converter & White LED Driver
Functional Block
Figure 3: YB1517 Block Diagram
Operation
The YB1517 uses a constant frequency,
current mode control scheme to regulate
the output voltage. Its operation can be
understood by referring to the block
diagram in Figure 3. At the start of each
oscillator cycle, a voltage proportional to
the switch current is added to a ramp
output and the resulting sum is fed into the
positive terminal of the PWM comparator
(comparator-1). When this voltage exceeds
the level of the comparator negative input,
the peak current has been reached, and
the SR latch (in Control Logic) is reset and
turns off the power switch. The voltage at
the negative input of the comparator comes
from the output of the error amplifier. The
error amplifier sets the correct peak current
level to keep the output in regulation. If the
error amplifier’s output increases, more
current is delivered to the output; if it
decreases, less current is delivered.
YB1517 MRev.1.0
Application Information
Soft Start and Current Limit
The internal soft start circuit minimizes the
inrush current during turning on YB1517.
The maximum switch current is limited to
about 900 mA by the chip.
Over Voltage Protection
The YB1517 has an internal over voltage
protection circuit which also acts as an
open-circuit protection. In the cases of
open circuit or the LEDs failure, the LEDs
are disconnected from the circuit, and the
feedback voltage will be zero. The
YB1517 will then switch to a high duty
cycle resulting in a high output voltage,
which may cause SW pin voltage to
exceed its maximum 34V rating. The
YB1517 will shutdown automatically until
7
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