LM2647 datasheet(21/42 Pages) TI

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Ipeak = Io x (1+

)

r =

x (1-0.18) x 10

6 = 0.45

3 x 10 x 300000

r =

x (1-D) x 10

Io x L x f

Ipeak = Io x (1+

)

VILIM = -v + 'V (= 0 at CLIM)

v = Rds x Ilowside

'V = RLIM x IILIM

IILIM= 46 PA

VLIM

RLIM

GND

CURRENT

VOLTAGE

-v

DETECTOR

Ilowside

LM2647

www.ti.com

SNVS210F – JUNE 2003 – REVISED APRIL 2013

Figure 19. Understanding Current Sensing

The detector sets the overcurrent latch as soon the voltage on the ILIM pin crosses below zero. Therefore the

basic design equation for calculating RLIM is:

ΔV = v

(1)

(see Figure 19)

At the point where current limiting occurs (peak inductor current becomes equal to current limit) the resistor for

setting the current limit can be calculated.

But what (peak) current limit value should actually be set? This depends on two factors:

a) There is a natural steady state peak current in the inductor with the converter delivering maximum rated load.

This should be calculated at VINMAX (the maximum of the input voltage range):

b) Over and above this steady state value we need to provide an ‘overload margin’. This margin will depend on

the step loads likely to be seen in the application and the response expected.

The equation for calculating the steady state peak current is:

(2)

where ‘r’ is the current ripple ratio (refer to Application Note AN-1197 for a detailed understanding of how ‘r’

affects all the power components). ‘r’ is given by:

(3)

where L is in µH, f is in Hz.

Example: Let VIN range from 5.5V to 28V, Vo=5V, Io=3A, L=10µH, f=300kHz. What is the peak current under

normal operation?

Only the highest input voltage must be used for any peak current calculation. At VINMAX the duty cycle is

D=Vo/Vin=5/28=18%. So

(4)

NOTE

In general, as discussed in AN-1197, the optimum value of ‘r’ is between 0.3 to 0.5. Large

inductances (higher than ‘optimum’) may be selected if the output voltage ripple needs to

be decreased but it is not desirable to achieve this by adding more (expensive?) output

specialty caps.

The peak current under normal operation is

(5)

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