LM5009MM National Semiconductor, LM5009MM Datasheet - Page 7

• Manufacturer Part Number: LM5009MM
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,TSSOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM5009MM

• Rohs Compliant: NO

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
provided in the MSOP-8 and the thermally enhanced LLP-8
packages. The regulator is based on a hysteretic control
scheme using an on-time inversely proportional to V
hysteretic control requires no loop compensation. Current
limit is implemented with forced off-time, which is inversely
proportional to V
tection while providing minimum foldback. The Functional
Block Diagram of the LM5009 is shown in Figure 1.
The LM5009 can be applied in numerous applications to
efficiently 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
voltage lockout, Max Duty Cycle limit timer and the intelligent
current limit off timer.
Hysteretic 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 volt-
age (V
one-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
). Following the ON period the switch will remain off for
IN
). Control is based on a comparator and the on-time
CC
OUT
under-voltage lockout, Gate drive under-
. This scheme ensures short circuit pro-
IN
. The
FIGURE 1.
7
at least the minimum off-timer period of 300 ns. If FB is still
below the reference at that time the switch will turn on again
for another on-time period. 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, cur-
rent 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 frequency is lower than in continuous conduction
mode, and varies with load current. Therefore at light loads
the conversion efficiency is maintained, since the switching
losses reduce with the reduction in load and frequency. The
discontinuous operating frequency can be calculated as fol-
lows:
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
operating frequency can be calculated as follows:
L
= the load resistance
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