Datasheet

LM2733
SNVS209E –NOVEMBER 2002–REVISED APRIL 2013 www.ti.com
The maximum peak switch current the device can deliver is dependent on duty cycle. The minimum value is
specified to be > 1A at duty cycle below 50%. For higher duty cycles, 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
(SW)
= DC x I
IND
(AVE)
2
x R
DS
ON (11)
There will be some switching losses as well, so some derating needs to be applied when calculating IC power
dissipation.
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 switching frequency 1.6 MHz, minimum switching frequency
1.15 MHz). This means the maximum cycle period is the reciprocal of the minimum frequency:
T
ON(max)
= 1/1.15M = 0.870 µs (12)
We will assume the input voltage is 5V, V
OUT
= 12V, V
SW
= 0.2V, V
DIODE
= 0.3V. The duty cycle is:
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 32. 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.
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
IN
. If the SHDN function will be needed, a pull-up resistor must be used to V
IN
(approximately
50k-100kΩ recommended). The SHDN pin must not be left unterminated.
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