LMR64010

SW

FB

GND

SHDN

U1

R3

51k

SHDN

GND

C1

2.2 PF

R2

13.3k

CF

220 pF

D1

MBR0520

R1/117K

L1/10 PH

C2

4.7 PF

12V

OUT

330 mA

(TYP)

LMR64010

www.ti.com

SNVS736B – SEPTEMBER 2011 – REVISED APRIL 2013

An important point to observe is that the LMR64010 will limit its switch current based on peak current. This

means that since lp(max) is fixed, increasing L will increase the maximum amount of power available to the load.

Conversely, using too little inductance may limit the amount of load current which can be drawn from the output.

Best performance is usually obtained when the converter is operated in “continuous” mode at the load current

range of interest, typically giving better load regulation and less output ripple. Continuous operation is defined as

not allowing the inductor current to drop to zero during the cycle. It should be noted that all boost converters shift

over to discontinuous operation as the output load is reduced far enough, but a larger inductor stays “continuous”

over a wider load current range.

To better understand these tradeoffs, a typical application circuit (5V to 12V boost with a 10 µH inductor) will be

analyzed. We will assume:

Since the frequency is 1.6 MHz (nominal), the period is approximately 0.625 µs. The duty cycle will be 62.5%,

which means the ON time of the switch is 0.390 µs. It should be noted that when the switch is ON, the voltage

across the inductor is approximately 4.5V.

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 18. 10 µH Inductor Current,5V–12V Boost

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.

Figure 19. Typical Application, 5V–12V Boost

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