=

1.225V

10

ON

LM5018

SNVS787E – JANUARY 2012 – REVISED DECEMBER 2013

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Functional Description

The LM5018 step-down switching regulator features all the functions needed to implement a low cost, efficient,

buck converter capable of supplying up to 300 mA to the load. This high voltage regulator contains 100 V, N-

channel buck and synchronous switches, is easy to implement, and is provided in thermally enhanced SO

PowerPAD-8 and WSON-8 packages. The regulator operation is based on a constant on-time control scheme

circuit protection while providing minimum foldback. The simplified block diagram of the LM5018 is shown in

Figure 12.

The LM5018 can be applied in numerous applications to efficiently regulate down higher voltages. This regulator

is well suited for 48 V telecom and 42 V automotive power bus ranges. Protection features include: thermal

shutdown, undervoltage lockout, minimum forced off-time, and an intelligent current limit.

Control Overview

The LM5018 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the

output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the

reference the internal buck switch is turned on for the one-shot timer period, which is a function of the input

falls below the reference, but never before the minimum off-time forced by the minimum off-time one-shot timer.

When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck

switch is turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB

voltage is approximately equal to 1.225 V (typ).

In a synchronous buck converter, the low side (sync) FET is 'on' when the high side (buck) FET is 'off.' The

inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’.

There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative

direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of

the output loading. The operating frequency remains relatively constant with load and line variations. The

operating frequency can be calculated as:

as:

This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum

required for the LM5018. In cases where the capacitor ESR is too small, additional series resistance may be

For applications where lower output voltage ripple is required the output can be taken directly from a low ESR

reaches the undervoltage lockout threshold of 4.5 V, the IC is enabled.

boot capacitor when SW pin is low.

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