'I =

(VIN - VOUT) x tON

L1

tON =

1.18 x 10

-10 x (RON + 1.4k)

VIN - 1.4V

+ 67 ns

12

LM5010

SNVS307G – SEPTEMBER 2004 – REVISED APRIL 2016

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Feature Description (continued)

7.3.4 Overvoltage Comparator

The feedback voltage at FB is compared to an internal 2.9-V reference. If the voltage at FB rises above 2.9 V,

the ON-time is immediately terminated. This condition can occur if the input voltage or the output load changes

suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5 V.

7.3.5 ON-Time Control

The ON-time of the internal switch (see Figure 4) is determined by the RON resistor and the input voltage (VIN),

calculated with Equation 5.

(5)

The inverse relationship of tON vs VIN results in a nearly constant frequency as VIN is varied. If the application

requires a high frequency, the minimum value for tON, and consequently RON, is limited by the OFF-time (265 ns,

±15%) which limits the maximum duty cycle at minimum VIN. The tolerance for Equation 5 is ±25%. Frequencies

in excess of 1 MHz are possible with the LM5010.

7.3.6 Current Limit

Current limit detection occurs during the OFF-time by monitoring the recirculating current through the free-

wheeling diode (D1). The detection threshold is 1.25 A, ±0.25 A. Referring to Functional Block Diagram, when

the buck switch is off the inductor current flows through the load, into SGND, through the sense resistor, out of

ISEN and through D1. If that current exceeds the threshold the current limit comparator output switches to delay

the start of the next ON-time period. The next ON-time starts when the current out of ISEN is below the threshold

and the voltage at FB is below 2.5 V. If the overload condition persists causing the inductor current to exceed the

threshold during each ON-time, that is detected at the beginning of each OFF-time. The operating frequency is

lower due to longer-than-normal OFF-times.

Figure 10 illustrates the inductor current waveform. During normal operation the load current is IO, the average of

the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak attempts to

exceed the threshold. During the Current Limited portion of Figure 10, the current ramps down to the threshold

during each OFF-time, initiating the next ON-time (assuming the voltage at FB is < 2.5 V). During each ON-time

the current ramps up an amount equal to Equation 6.

(6)

During this time the LM5010 is in a constant current mode, with an average load current (IOCL) equal to the

threshold +

ΔI / 2.

The valley current limit technique allows the load current to exceed the current limit threshold as long as the

lower peak of the inductor current is less than the threshold.