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LM3150

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SNVS561D – SEPTEMBER 2008 – REVISED MARCH 2011

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 voltage

at the FB pin and compares it to 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 voltage at FB pin falls below

0.72V.

Current Limit

Current limit detection occurs during the off-time by monitoring the current through the low-side switch using an

limit value, the next on-time cycle is immediately terminated. Current sensing is achieved by comparing the

on-time cycle.

(6)

where

•

•

•

(7)

Figure 12 illustrates the inductor current waveform. During normal operation, the output current ripple is dictated

where

•

•

(8)

If an overload condition occurs, the current through the low-side switch will increase which will cause the current

limit comparator to trigger the logic to skip the next on-time cycle. The IC will then try to recover by checking the

low-side FET on and allow the inductor current to further decay.

Throughout the whole process, regardless of the load current, the on-time of the controller will stay constant and

thereby the positive ripple current slope will remain constant. During each on-time the current ramps-up an

amount equal to:

(9)

The valley current limit feature prevents current runaway conditions due to propagation delays or inductor

saturation since the inductor current is forced to decay following any overload conditions.

Current sensing is achieved by either a low value sense resistor in series with the low-side FET or by utilizing the

FET’s temperature increases. The LM3150 controller will maintain a more stable current limit that is closer to the

This does not provide an exact temperature compensation but allows for a more tightly controlled current limit

room to maximum temperature and cause other components to be over-designed. The temperature

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