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LM3151, LM3152, LM3153

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SNVS562G – SEPTEMBER 2008 – REVISED MARCH 2011

THEORY OF OPERATION

The LM3151/2/3 synchronous step-down SIMPLE SWITCHER Controller employs a Constant On-Time (COT)

architecture which is a derivative of the hysteretic control scheme. COT relies on a fixed switch on-time to

automatically adjusts the on-time inversely with the input voltage to maintain a constant frequency. Assuming an

where

•

K = 100 pC

•

Control is based on a comparator and the on-timer, with the output voltage feedback (FB) attenuated and then

compared with an internal reference of 0.6V. If the attenuated FB level is below the reference, the high-side

Characteristics table or until the attenuated FB voltage is less than 0.6V. This switching cycle will continue while

maintaining regulation. During continuous conduction mode (CCM), the switching frequency depends only on

duty cycle and on-time. The duty cycle can be calculated as:

Where the switching frequency of a COT regulator is:

Typical COT hysteretic controllers need a significant amount of output capacitor ESR to maintain a minimum

amount of ripple at the FB pin in order to switch properly and maintain efficient regulation. The LM3151/2/3

however utilizes proprietary, Emulated Ripple Mode Control Scheme (ERM) that allows the use of ceramic output

capacitors without additional equivalent series resistance (ESR) compensation. Not only does this reduce the

need for output capacitor ESR, but also significantly reduces the amount of output voltage ripple seen in a typical

hysteretic control scheme. The output ripple voltage can become so low that it is comparable to voltage-mode

and current-mode control schemes.

Regulation Comparator

The output voltage is sampled through the FB pin and then divided down by two internal resistors and compared

to the internal reference voltage of 0.6V by the error comparator. In normal operation, an on-time period is

initiated when the sampled output voltage at the input of the error comparator falls below 0.6V. The high-side

switch stays on for the specified on-time, causing the sampled voltage on the error comparator input to rise

above 0.6V. After the on-time period, the high-side switch stays off for the greater of the following:

1. Minimum off time as specified in the electrical characteristics table

2. The error comparator sampled voltage falls below 0.6V

Over-Voltage Comparator

The over-voltage comparator is provided to protect the output from over-voltage conditions due to sudden input

line voltage changes or output loading changes. The over-voltage comparator continuously monitors the

attenuated FB voltage versus a 0.72V internal reference. If the voltage at FB rises above 0.72V the on-time pulse

is immediately terminated. This condition can occur if the input or the output load changes suddenly. Once the

over-voltage protection is activated, the HG and LG signals remain off until the attenuated FB voltage falls below

0.72V.

Copyright © 2008–2011, Texas Instruments Incorporated

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