L5987 datasheet(15/42 Pages) STMICROELECTRONICS

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L5987 - L5987A

Functional description

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5.7

Maximum DC output current L5987A (HSOP8)

The L5987A can manage DC output currents up to 3 A and the rated RMS current of its

internal power switch is 3 A. So the L5987A can deliver 3 A with 100 % of duty cycle.

5.8

Maximum DC output current L5987 (VFQFPN)

The L5987 can manage DC output currents up to 3 A. However the rated RMS current of its

internal power switch is 2.5 A.

Since the current flows through the integrated power element only during the on time, the

RMS value is given by:

Equation 3

Where D is the duty cycle (VO/VIN).

Considering IO = 3 A, the maximum duty cycle that can be managed is:

Equation 4

In Figure 8 the maximum DC output current is reported as a function of the duty cycle. For

duty cycles lower than 69 %, the RMS current does not limit the maximum DC output current

of 3 A. For duty cycles higher than the 69 % the maximum DC output current is limited by

the RMS current to:

Equation 5

In order to have a more accurate calculation of the maximum DC output current, the

complete expression for the duty cycle can be adopted, considering the voltage drop across

the power MOSFET, the series resistance of the inductor and the forward voltage of the

rectification diode. The duty cycle results:

Equation 6

where IO is the desired DC output current.

For example with VIN = 5 V, VOUT = 3.3 V, IO = 2.6 A, RDS(on) = 220 mΩ, VF = 0.35 V and

DCR = 30 m

Ω, the duty results D = 78 %, so according to Equation 5 the maximum DC

output current is 2.83 A, which is higher than desired current.

RMS

⋅

RMS

------------

69%

OMAX

2.5

--------

[]

if D

69%

≥

OUT

DCR I

⋅

DSON

⋅

–

----------------------------------------------------------

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