LM34923EVAL/NOPB National Semiconductor, LM34923EVAL/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM34923EVAL/NOPB
• Description: BOARD EVALUATION FOR LM34923
• Manufacturer: National Semiconductor
• Series: -r

Specifications of LM34923EVAL/NOPB

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Power - Output: -
• Voltage - Output: 5V
• Current - Output: 500mA
• Voltage - Input: 6 ~ 75 V
• Regulator Topology: Buck
• Frequency - Switching: 200kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34923
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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Block Diagram
Functional Description
The LM34923 Step Down Switching Regulator features all the
functions needed to implement a low cost, efficient, Buck bias
power converter. This high voltage regulator contains an 80
V N-Channel Buck Switch, is easy to implement and is pro-
vided in the MSOP-10 package. The regulator is based on a
control scheme using an on-time inversely proportional to
V
rent limit is implemented with forced off-time, which is in-
versely proportional to V
circuit control while providing minimum foldback.
The LM34923 can be applied in numerous applications to ef-
ficiently regulate down higher voltages. This regulator is well
suited for 48 Volt Telecom and the new 42V Automotive pow-
er bus ranges. Features include: Thermal Shutdown, V
under-voltage lockout, Gate drive under-voltage lockout, Max
Duty Cycle limit timer, intelligent current limit off timer, a pre-
charge switch, and a programmable under voltage detector
with status flag.
IN
. The control scheme requires no loop compensation. Cur-
OUT
. This scheme ensures short
CC
10
Control Circuit Overview
The LM34923 is a Buck DC-DC regulator that uses a control
scheme in which the on-time varies inversely with line voltage
(V
shot, with the output voltage feedback (FB) compared to an
internal reference (2.5V). If the FB level is below the reference
the buck switch is turned on for a fixed time determined by the
line voltage and a programming resistor (R
ON period the switch remains off for at least the minimum off-
timer period of 260 ns. If FB is still below the reference at that
time the switch turns on again for another on-time period. This
continues until regulation is achieved.
The LM34923 operates in discontinuous conduction mode at
light load currents, and continuous conduction mode at heavy
load current. In discontinuous conduction mode, current
through the output inductor starts at zero and ramps up to a
peak during the on-time, then ramps back to zero before the
end of the off-time. The next on-time period starts when the
voltage at FB falls below the internal reference - until then the
inductor current remains zero. In this mode the operating fre-
quency is lower than in continuous conduction mode, and
varies with load current. Therefore at light loads the conver-
sion efficiency is maintained, since the switching losses re-
IN
). Control is based on a comparator and the on-time one-
T
). Following the
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