Typical Performance

Functional Description

The LM5008 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 LM5008 is shown in Figure 1.

The LM5008 can be applied in numerous applications to

efficiently regulate down higher voltages. This regulator is

well suited for 48 Volt Telecom and the new 42V Automotive

power bus ranges. Protection features include: Thermal

Shutdown, V

voltage lockout, Max Duty Cycle limit timer and the intelligent

current limit off timer.

Hysteretic Control Circuit

Overview

The LM5008 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 300ns. 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.

The LM5008 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

20097923

FIGURE 6. Efficiency vs V

IN

(Circuit of Figure 13)

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FIGURE 7. Efficiency vs Load Current vs V

IN

(Circuit of Figure 13)

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FIGURE 8. Output Voltage vs Load Current

(Circuit of Figure 13)

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