Current

Limit Threshold

Normal

Operation

Load

Current

Increases

Current

Limited

'I

Inductor

Current

Decreases

Load Current

Normal

Operation

Voltage at the FB Pin

2.51V

Voltage at the CS Pin

0V

IPK

IO1

IO2

IOCL

12

LM25011, LM25011-Q1, LM25011A, LM25011A-Q1

SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014

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Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1

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Copyright © 2009–2014, Texas Instruments Incorporated

Feature Description (continued)

output voltage resumes its normal regulated value. The maximum continuous current into the RT pin must be

less than 2 mA. For high-frequency applications, the maximum switching frequency is limited at the maximum

input voltage by the minimum on-time one-shot period (90 ns). At minimum input voltage the maximum switching

frequency is limited by the minimum off-time one-shot period which, if reached, prevents achievement of the

proper duty cycle.

7.3.3 Current Limit

Current limit detection occurs during the off-time by monitoring the voltage across the external current sense

resistor RS. Referring to the Functional Block Diagram , during the off-time the recirculating current flows through

the inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across

the sense resistor exceeds the threshold (VILIM), the current limit comparator output switches to delay the start of

the next on-time period. The next on-time starts when the recirculating current decreases such that the voltage

across RS reduces to the threshold and the voltage at FB is below 2.51 V. The operating frequency is typically

lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by

approximately 40% (only in LM25011) if the voltage at the FB pin is below its threshold when the voltage across

RS reduces to its threshold (VOUT is low due to current limiting).

Figure 15 illustrates the inductor current waveform during normal operation and in current limit. During the first

normal operation, the load current is I01, the average of the inductor current waveform. As the load resistance is

reduced, the inductor current increases until the lower peak of the inductor ripple current exceeds the threshold.

During the current limited portion of Figure 15, each on-time is reduced by approximately 40%, resulting in lower

ripple amplitude for the inductor current. During this time the LM25011 is in a constant-current mode with an

average load current equal to the current limit threshold plus half the ripple amplitude (IOCL), and the output

voltage is below the normal regulated value. Normal operation resumes when the load current is reduced (to IO2),

allowing VOUT and the on-time to return to their normal values. Note that in the second period of normal

operation, even though the peak current of the inductor exceeds the current limit threshold during part of each

cycle, the circuit is not in current limit because the inductor current falls below the current limit threshold during

each off-time. The peak current allowed through the buck switch is 3.5 A and the maximum allowed average

current is 2.0 A.

Figure 15. Normal and Current Limit Operation