LM3485LED EVAL National Semiconductor, LM3485LED EVAL Datasheet - Page 8

• Manufacturer Part Number: LM3485LED EVAL
• Description: BOARD EVALUATION LM3485LED
• Manufacturer: National Semiconductor

Specifications of LM3485LED EVAL

• Current - Output / Channel: 1.4A
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 30V
• Features: Dimmable
• Voltage - Input: 5 ~ 30V
• Utilized Ic / Part: LM3485
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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Block Diagram
Functional Description
OVERVIEW
The LM3485 is buck (step-down) DC-DC controller that uses
a hysteretic control scheme. The comparator is designed with
approximately 10mV of hysteresis. In response to the voltage
at the FB pin, the gate drive (PGATE pin) turns the external
PFET on or off. When the inductor current is too high, the
current limit protection circuit engages and turns the PFET off
for approximately 9µs.
Hysteretic control does not require an internal oscillator.
Switching frequency depends on the external components
and operating conditions. Operating frequency reduces at
light loads resulting in excellent efficiency compared to other
architectures.
2 external resistors can easily program the output voltage.
The output can be set in a wide range from 1.242V (typical)
to V
HYSTERETIC CONTROL CIRCUIT
The LM3485 uses a comparator based voltage control loop.
The feedback is compared to a 1.242V reference and a 10mV
hysteresis is designed into the comparator to ensure noise
free operation.
When the FB input to the comparator falls below the reference
voltage, the output of the comparator moves to a low state.
This results in the driver output, PGATE, pulling the gate of
the PFET low and turning on the PFET. With the PFET on,
the input supply charges Cout and supplies current to the load
via the series path through the PFET and the inductor. Current
through the Inductor ramps up linearly and the output voltage
increases. As the FB voltage reaches the upper threshold,
which is the internal reference voltage plus 10mV, the output
of the comparator changes from low to high, and the PGATE
responds by turning the PFET off. As the PFET turns off, the
IN
.
8
inductor voltage reverses, the catch diode turns on, and the
current through the inductor ramps down. Then, as the output
voltage reaches the internal reference voltage again, the next
cycle starts.
The LM3485 operates in discontinuous conduction mode at
light load current or continuous conduction mode at heavy
load current. In discontinuous conduction mode, current
through the inductor starts at zero and ramps up to the peak,
then ramps down to zero. Next cycle starts when the FB volt-
age reaches the internal voltage. Until then, the inductor
current remains zero. Operating frequency is lower and
switching losses reduce. In continuous conduction mode, cur-
rent always flows through the inductor and never ramps down
to zero.
The output voltage (V
resistors. It can be calculated as follows:
V
OUT
= 1.242* ( R1 + R2 ) / R2
OUT
) can be programmed by 2 external
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