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

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:

20165801

FIGURE 1.

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