L =

VO + VD

IO x r x fS

x (1-D)

r =

iL

lO

D =

VO + VD

VIN + VD - VSW

D =

VO

VIN

LM2736

SNVS316H – SEPTEMBER 2004 – REVISED DECEMBER 2014

www.ti.com

Typical Applications (continued)

Table 1. Bill of Materials for Figure 15 (continued)

PART ID

PART VALUE

PART NUMBER

MANUFACTURER

L1

4.7-µH, 1.7 A,

VLCF4020T- 4R7N1R2

TDK

R1

2 k

Ω, 1%

CRCW06032001F

Vishay

R2

10 k

Ω, 1%

CRCW06031002F

Vishay

R3

100 k

Ω, 1%

CRCW06031003F

Vishay

8.2.1.2.1

Inductor Selection

The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (VO) to input voltage (VIN) as

shown in Equation 14:

(14)

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. Use Equation 15 to Calculate D.

(15)

VSW can be approximated by:

VSW = IO x RDS(ON)

(16)

The diode forward drop (VD) can range from 0.3 V to 0.7 V depending on the quality of the diode. The lower VD

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 (

ΔiL) to output current (IO) is optimized when it is set between 0.3 and 0.4 at 750 mA.

The ratio r is defined in .

(17)

One must also ensure that the minimum current limit (1.0 A) is not exceeded, so the peak current in the inductor

must be calculated. Use Equation 18 to calculate the peak current (ILPK) in the inductor.

ILPK = IO + ΔIL/2

(18)

If r = 0.7 at an output of 750 mA, the peak current in the inductor will be 1.0125 A. The minimum ensured current

limit over all operating conditions is 1.0 A. One can either reduce r to 0.6 resulting in a 975 mA peak current, or

make the engineering judgement that 12.5 mA over will be safe enough with a 1.5 A 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

0.1 A, 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. Equation 19 is

empirically developed for the maximum ripple ratio at any current below 2 A.

r = 0.387 x IOUT

-0.3667

(19)

Note that this is just a guideline.

The LM2736 device 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 using Equation 20

(20)

14

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