LM3151 datasheet(10/27 Pages) TI1

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ICL

IPK

IOCL

IOUT

Load Current

Increases

Normal Operation

Current Limited

'I =

(VIN - VOUT) x tON

Ivalley = IOUT - '

VCL (Tj) = VCL x [1 + 3.3 x 10

-3

x (Tj - 27)]

ICL (Tj) =

VCL (Tj)

RDS(ON)max

LM3151, LM3152, LM3153

SNVS562G – SEPTEMBER 2008 – REVISED MARCH 2011

www.ti.com

Current Limit

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

the off-time the current in the low-side switch exceeds the user defined current limit value, the next on-time cycle

is immediately terminated. Current sensing is achieved by comparing the voltage across the low-side switch

against an internal reference value, VCL, of 200 mV. If the voltage across the low-side switch exceeds 200 mV,

the current limit comparator will trigger logic to terminate the next on-time cycle. The current limit ICL, can be

determined as follows:

where

•

IOCL is the user-defined average output current limit value

•

RDS(ON)max is the resistance value of the low-side FET at the expected maximum FET junction temperature

•

VCL is the internal current limit reference voltage

•

Tj is the junction temperature of the LM3151/2/3

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

by the switching of the FETs. The current through the low-side switch, Ivalley, is sampled at the end of each

switching cycle and compared to the current limit threshold voltage, VCL. The valley current can be calculated as

follows:

where

•

IOUT is the average output current

•

ΔIL is the peak-to-peak inductor ripple current

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

valley current during each off-time. If the valley current is greater than or equal to ICL, then the IC will keep 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:

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.

Figure 12. Inductor Current - Current Limit Operation

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