Datasheet

L =
V
O
+ V
D
I
O
x r x f
S
x (1-D)
r =
'i
L
l
O
D =
V
O
+ V
D
V
IN
+ V
D
- V
SW
D =
V
O
V
IN
LM2736
www.ti.com SNVS316F –SEPTEMBER 2004–REVISED APRIL 2013
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
O
) to input voltage (V
IN
):
(12)
The catch diode (D1) forward voltage drop and the voltage drop across the internal NMOS must be included to
calculate a more accurate duty cycle. Calculate D by using the following formula:
(13)
V
SW
can be approximated by:
V
SW
= I
O
x R
DS(ON)
(14)
The diode forward drop (V
D
) can range from 0.3V to 0.7V depending on the quality of the diode. The lower V
D
is,
the higher the operating efficiency of the converter.
The inductor value determines the output ripple current. Lower inductor values decrease the size of the inductor,
but increase the output ripple current. An increase in the inductor value will decrease the output ripple current.
The ratio of ripple current (Δi
L
) to output current (I
O
) is optimized when it is set between 0.3 and 0.4 at 750mA.
The ratio r is defined as:
(15)
One must also ensure that the minimum current limit (1.0A) is not exceeded, so the peak current in the inductor
must be calculated. The peak current (I
LPK
) in the inductor is calculated by:
I
LPK
= I
O
+ ΔI
L
/2 (16)
If r = 0.7 at an output of 750mA, the peak current in the inductor will be 1.0125A. The minimum ensured current
limit over all operating conditions is 1.0A. One can either reduce r to 0.6 resulting in a 975mA peak current, or
make the engineering judgement that 12.5mA over will be safe enough with a 1.5A typical current limit and 6
sigma limits. When the designed maximum output current is reduced, the ratio r can be increased. At a current of
0.1A, r can be made as high as 0.9. The ripple ratio can be increased at lighter loads because the net ripple is
actually quite low, and if r remains constant the inductor value can be made quite large. An equation empirically
developed for the maximum ripple ratio at any current below 2A is:
r = 0.387 x I
OUT
-0.3667
(17)
Note that this is just a guideline.
The LM2736 operates at frequencies allowing the use of ceramic output capacitors without compromising
transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.
See the OUTPUT CAPACITOR section for more details on calculating output voltage ripple.
Now that the ripple current or ripple ratio is determined, the inductance is calculated by:
(18)
where f
s
is the switching frequency and I
O
is the output current. When selecting an inductor, make sure that it is
capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden
reduction in inductance and prevent the regulator from operating correctly. Because of the speed of the internal
current limit, the peak current of the inductor need only be specified for the required maximum output current. For
example, if the designed maximum output current is 0.5A and the peak current is 0.7A, then the inductor should
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