LM2733

SNVS209F – NOVEMBER 2002 – REVISED DECEMBER 2014

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approximation can be obtained by multiplying the "ON Resistance" of the FET times the average inductor

current.

The maximum peak switch current the device can deliver is dependent on duty cycle. The minimum value is

specified to be > 1 A at duty cycle below 50%. For higher duty cycles, see Typical Characteristics.

8.2.2.13 Thermal Considerations

At higher duty cycles, the increased ON time of the FET means the maximum output current will be determined

by power dissipation within the LM2733 FET switch. The switch power dissipation from ON-state conduction is

calculated by:

(11)

There will be some switching losses as well, so some derating needs to be applied when calculating IC power

dissipation.

8.2.2.14 Minimum Inductance

In some applications where the maximum load current is relatively small, it may be advantageous to use the

smallest possible inductance value for cost and size savings. The converter will operate in discontinuous mode in

such a case.

The minimum inductance should be selected such that the inductor (switch) current peak on each cycle does not

reach the 1-A current limit maximum. To understand how to do this, an example will be presented.

In the example, the LM2733X will be used (nominal switching frequency 1.6 MHz, minimum switching frequency

1.15 MHz). This means the maximum cycle period is the reciprocal of the minimum frequency:

(12)

Duty Cycle = 60.3%

Therefore, the maximum switch ON time is 0.524 µs. An inductor should be selected with enough inductance to

prevent the switch current from reaching 1A in the 0.524 µs ON time interval (see below):

Figure 30. Discontinuous Design, 5V–12V Boost (LM2733X)

The voltage across the inductor during ON time is 4.8V. Minimum inductance value is found by:

V = L X dl/dt, L = V X (dt/dl) = 4.8 (0.524µ/1) = 2.5 µH

(13)

In this case, a 2.7 µH inductor could be used assuming it provided at least that much inductance up to the 1A

current value. This same analysis can be used to find the minimum inductance for any boost application. Using

the slower switching “Y” version requires a higher amount of minimum inductance because of the longer

switching period.

8.2.2.15 Inductor Suppliers

Some of the recommended suppliers of inductors for this product include, but not limited to are Sumida, Coilcraft,

Panasonic, TDK and Murata. When selecting an inductor, make certain that the continuous current rating is high

enough to avoid saturation at peak currents. A suitable core type must be used to minimize core (switching)

losses, and wire power losses must be considered when selecting the current rating.

16

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