0

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0

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DUTY CYCLE (%) = [1 - EFF*(VIN/VOUT))]

VIN = 5V

VIN = 3.3V

VIN = 2.7V

LM27313, LM27313-Q1

www.ti.com

SNVS487E – DECEMBER 2006 – REVISED JANUARY 2015

Typical Applications (continued)

Figure 14. 10 µH Inductor Current, 5 V – 12 V Boost

During the 0.390-µs ON time, the inductor current ramps up 0.176 A 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.

8.2.1.2.9

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 Figure 15 which shows typical values of switch current as a function of effective

(actual) duty cycle:

Figure 15. Switch Current Limit vs Duty Cycle

8.2.1.2.10

Calculating Load Current

As shown in Figure 14 which depicts inductor current, the load current is related to the average inductor current

by the relation:

ILOAD = IIND(AVG) x (1 - DC)

where

•

DC is the duty cycle of the application.

(9)

The switch current can be found by:

ISW = IIND(AVG) + ½ (IRIPPLE)

(10)

Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency:

IRIPPLE = DC x (VIN - VSW) / (fSW x L)

(11)

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