Datasheet

LMR64010
SW
FB
GND
V
IN
SHDN
U1
R3
51k
SHDN
GND
5V
IN
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:
V
IN
= 5V, V
OUT
= 12V, V
DIODE
= 0.5V, V
SW
= 0.5V
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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