20

30

40

50

60

70

80

90

100

0

500

1000

1500

2000

2500

3000

20

30

40

50

60

70

80

90

100

0

500

1000

1500

2000

2500

3000

TI Confidential - NDA Restrictions

LM2731

www.ti.com

SNVS217F – MAY 2004 – REVISED NOVEMBER 2012

Using the equation:

V = L (di/dt)

(6)

We can then calculate the di/dt rate of the inductor which is found to be 0.45 A/µs during the ON time. Using

these facts, we can then show what the inductor current will look like during operation:

Figure 30. 10 µH Inductor Current, 5V–12V Boost (LM2731X)

During the 0.390 µs ON time, the inductor current ramps up 0.176A and ramps down an equal amount during the

OFF time. This is defined as the inductor “ripple current”. It can also be seen that if the load current drops to

about 33 mA, the inductor current will begin touching the zero axis which means it will be in discontinuous mode.

A similar analysis can be performed on any boost converter, to make sure the ripple current is reasonable and

continuous operation will be maintained at the typical load current values.

MAXIMUM SWITCH CURRENT

The maximum FET switch current available before the current limiter cuts in is dependent on duty cycle of the

application. This is illustrated in the graphs below which show typical values of switch current for both the "X" and

"Y" versions as a function of effective (actual) duty cycle:

Figure 31. Switch Current Limit vs Duty Cycle - "X"

Figure 32. Switch Current Limit vs Duty Cycle - "Y"

Copyright © 2004–2012, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM2731