The maximum peak switch current the device can deliver is

dependent on duty cycle. The minimum value is guaranteed

see Typical performance Characteristics curves.

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:

P

There will be some switching losses as well, so some derat-

ing needs to be applied when calculating IC power dissipa-

tion.

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 1A current limit maximum. To understand how to do this,

an example will be presented.

In the example, the LM2733X will be used (nominal switch-

ing frequency 1.6 MHz, minimum switching frequency

1.15 MHz). This means the maximum cycle period is the

reciprocal of the minimum frequency:

T

We will assume the input voltage is 5V, V

0.2V, V

Duty Cycle = 60.3%

Therefore, the maximum switch ON time is 0.524 µs. An

inductor should be selected with enough inductance to pre-

vent the switch current from reaching 1A in the 0.524 µs ON

time interval (see below):

20055413

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

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.

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.

SHUTDOWN PIN OPERATION

The device is turned off by pulling the shutdown pin low. If

this function is not going to be used, the pin should be tied

directly to V

resistor must be used to V

Ω rec-

ommended). The SHDN pin must not be left unterminated.

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