PEAK

I

LOAD

I

=

K

x

L

I

+ '

where

L =

I

'

IN x

V

(

)

IN

OUT - V

V

OUT

SW

V

x

L

x

f

x

2

IN

OUT

V

V

where

IN

IN

OUT

(

)

- V

V

x

V

L =

I

'

OUT

SW

V

x

L

x

f

x

2

I

+

x

R

=

V

L

ESR

ESR

'

'

V

x

I

OUT

LED

VIN

¹

·

©

§

Q =

V

'

(

)

IN

OUT

LED

- V

V

x

I

OUT

OUT

SW

C

x

V

x

f

LM3556

SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015

www.ti.com

Larger capacitors (for example, a 22-µF capacitor) or capacitors in parallel may be used if lower output voltage

ripple is desired. To estimate the output voltage ripple, considering the ripple due to capacitor discharge (

ΔVQ)

and the ripple due to the capacitors equivalent series resistance (ESR) (

ΔVESR), use Equation 1 and Equation 2.

For continuous conduction mode, the output voltage ripple due to the capacitor discharge is:

(1)

The output voltage ripple due to the ESR of the output capacitor is found by:

(2)

In ceramic capacitors the ESR is very low so the assumption is that 80% of the output voltage ripple is due to

capacitor discharge and 20% from ESR. Table 16 lists different manufacturers for various output capacitors and

their case sizes suitable for use with the LM3556.

8.2.2.2 Input Capacitor Selection

Choosing the correct size and type of input capacitor helps minimize the voltage ripple caused by the switching

of the LM3556 device’s boost converter and reduces noise on the boost converter's input terminal that can feed

through and disrupt internal analog signals. In the Figure 41 a 10-µF ceramic input capacitor works well. It is

important to place the input capacitor as close as possible to the LM3556’s input (IN) pin. This reduces the series

resistance and inductance that can inject noise into the device due to the input switching currents. Table 16 lists

various input capacitors recommended for use with the LM3556.

Table 16. Recommended Input/Output Capacitors (X5R/X7R Dielectric)

MANUFACTURER

PART NUMBER

VALUE

CASE SIZE

VOLTAGE RATING

TDK Corporation

C1608JB0J106M

10 µF

0603 (1.6 mm × 0.8 mm × 0.8 mm)

6.3 V

TDK Corporation

C2012JB1A106M

10 µF

0805 (2 mm × 1.25 mm × 1.25 mm)

10 V

Murata

GRM188R60J106M

10 µF

0603 (1.6 mm x 0.8 mm x 0.8 mm)

6.3 V

Murata

GRM21BR61A106KE19

10 µF

0805 (2 mm × 1.25 mm × 1.25 mm)

10 V

8.2.2.3 Inductor Selection

The LM3556 is designed to use a 1-µH or 0.47-µH inductor. Table 17 lists various inductors and their

manufacturers that work well with the LM3556. When the device is boosting (VOUT > VIN) the inductor is typically

the largest area of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible series

resistance is important. Additionally, the saturation rating of the inductor must be greater than the maximum

operating peak current of the LM3556. This prevents excess efficiency loss that can occur with inductors that

operate in saturation. For proper inductor operation and circuit performance, ensure that the inductor saturation

and the peak current limit setting of the LM3556 are greater than IPEAK in Equation 3:

where

•

ƒSW = 4 MHz

•

Efficiency can be found in Typical Characteristics.

(3)

Table 17. Recommended Inductors

MANUFACTURER

L

PART NUMBER

DIMENSIONS (L × W × H)

ISAT

RDC

TOKO

1 µH

FDSD0312

3 mm x 3 mm x 1.2 mm

4.5 A

43 m

Ω

TOKO

1 µH

DFE252010C

2.5 mm × 2 mm × 1 mm

3.4 A

60 m

Ω

TOKO

1 µH

DFE252012C

2.5 mm × 2 mm × 1.2 mm

3.8 A

45 m

Ω

28

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