LM34914EVAL National Semiconductor, LM34914EVAL Datasheet - Page 8

• Manufacturer Part Number: LM34914EVAL
• Description: BOARD EVALUATION LM34914
• Manufacturer: National Semiconductor

Specifications of LM34914EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 1A
• Voltage - Input: 8 ~ 40V
• Regulator Topology: Buck
• Frequency - Switching: 275kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34914
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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Functional Description
The LM34914 Step Down Switching Regulator features all the
functions needed to implement a low cost, efficient buck bias
power converter capable of supplying at least 1.25A to the
load. This high voltage regulator contains an N-Channel buck
switch, is easy to implement, and is available in the thermally
enhanced 3mm x 3mm LLP-10 package. The regulator’s op-
eration is based on a constant on-time control scheme where
the on-time is determined by V
operating frequency remaining relatively constant with load
and input voltage variations. The feedback control scheme
requires no loop compensation resulting in very fast load tran-
sient response. The valley current limit scheme protects
against excessively high currents if the output is short circuit-
ed when V
current due to a possible saturating inductor the valley current
limit threshold changes with input and output voltages, and
the on-time is reduced by approximately 50% when current
limit is detected. The LM34914 can be applied in numerous
applications to efficiently regulate down higher voltages. Ad-
IN
is high. To aid in controlling excessive switch
IN
. This feature results in the
FIGURE 1. Startup Sequence
8
ditional features include: Thermal shutdown, V
age lock-out, gate drive under-voltage lock-out, and maxi-
mum duty cycle limit.
Control Circuit Overview
The LM34914 buck DC-DC regulator employs a control
scheme based on a comparator and a one-shot on-timer, with
the output voltage feedback (FB) compared to an internal ref-
erence (2.5V). If the FB voltage is below the reference the
buck switch is switched on for a time period determined by
the input voltage and a programming resistor (R
the on-time the switch remains off until the FB voltage falls
below the reference, but not less than the minimum off-time
forced by the LM34914. The buck switch is then turned on for
another on-time period.
When in regulation, the LM34914 operates in continuous con-
duction mode at heavy load currents and discontinuous con-
duction mode at light load currents. In continuous conduction
mode the inductor’s current is always greater than zero, and
the operating frequency remains relatively constant with load
20197310
CC
ON
). Following
under-volt-