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LM3150

SNVS561D – SEPTEMBER 2008 – REVISED MARCH 2011

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THEORY OF OPERATION

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

regulate the output. The on-time of the high-side switch can be set manually by adjusting the size of an external

the following approximations can be made:

where

•

constant K = 100 pC

(1)

where

•

(2)

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

internal reference of 0.6V. If the FB level is below the reference, the high-side switch is turned on for a fixed time,

the reference, then the switch turns on again for another on-time period. The switching will continue in this

fashion to maintain regulation. During continuous conduction mode (CCM), the switching frequency ideally

depends on duty-cycle and on-time only. In a practical application however, there is a small delay in the time that

the HG goes low and the SW node goes low that also affects the switching frequency that is accounted for in the

typical application curves. The duty-cycle and frequency can be approximated as:

(3)

(4)

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 LM3150 however,

utilizes a proprietary Emulated Ripple Mode control scheme (ERM) that allows the use of low ESR output

capacitors. Not only does this reduce the need for high 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.

Programming the Output Voltage

follows:

where

•

•

(5)

Regulation Comparator

The feedback voltage at FB is compared to the internal reference voltage of 0.6V. In normal operation (the output

voltage is regulated), an on-time period is initiated when the voltage at FB falls below 0.6V. The high-side switch

stays on for the on-time, causing the FB voltage to rise above 0.6V. After the on-time period, the high-side switch

stays off until the FB voltage falls below 0.6V.

8

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