Datasheet

20 30 40 50 60 70 80 90 100
DUTY CYCLE (%) = [1 - EFF*(V
IN
/ V
OUT
)]
0
500
1000
1500
2000
2500
3000
SW CURRENT LIMIT (mA)
V
IN
= 5V
V
IN
= 3.3V
V
IN
= 2.7V
V
IN
= 3V
20 30 40 50 60 70 80 90 100
DUTY CYCLE (%) = [1 - EFF*(V
IN
/ V
OUT
)]
0
500
1000
1500
2000
2500
3000
SW CURRENT LIMIT (mA)
V
IN
= 5V
V
IN
= 3.3V
V
IN
= 2.7V
V
IN
= 3V
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"
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