LM5009EVAL/NOPB National Semiconductor, LM5009EVAL/NOPB Datasheet - Page 7

• Manufacturer Part Number: LM5009EVAL/NOPB
• Description: EVAL BOARD FOR LM5009
• Manufacturer: National Semiconductor

Specifications of LM5009EVAL/NOPB

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Typical Application Circuit and Block Diagram
Functional Description
The LM5009 Step Down Switching Regulator features all the
functions needed to implement a low cost, efficient, Buck bias
power converter. This high voltage regulator contains a 100
V N-Channel Buck Switch, is easy to implement and is pro-
vided in the MSOP-8 and the thermally enhanced LLP-8
packages. The regulator is based on a control scheme using
an on-time inversely proportional to V
requires no loop compensation. Current limit is implemented
with forced off-time, which is inversely proportional to V
This scheme ensures short circuit protection while providing
minimum foldback. The Functional Block Diagram of the
LM5009 is shown in
The LM5009 can be applied in numerous applications to ef-
ficiently regulate down higher voltages. This regulator is well
suited for 48 Volt Telecom and the 42V Automotive power bus
ranges. Additional features include: Thermal Shutdown, V
under-voltage lockout, Gate drive under-voltage lockout, Max
Duty Cycle limit timer and the intelligent current limit off timer.
Control Circuit Overview
The LM5009 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 will remain off for at least the minimum
off-timer period of 300 ns. If FB is still below the reference at
IN
). Control is based on a comparator and the on-time one-
Figure
1.
IN
. The control scheme
ON
). Following the
OUT
CC
FIGURE 1.
.
7
that time the switch will turn on again for another on-time pe-
riod. This will continue until regulation is achieved, at which
time the off-time increases based on the required duty cycle.
The LM5009 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-
duce with the reduction in load and frequency. The discon-
tinuous operating frequency can be calculated as follows:
where R
In continuous conduction mode, current flows continuously
through the inductor and never ramps down to zero. In this
mode the operating frequency is greater than the discontinu-
ous mode frequency and remains relatively constant with load
and line variations. The approximate continuous mode oper-
ating frequency can be calculated as follows:
L
= the load resistance
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