LM3554TMXSLASHNOPB datasheet(30/47 Pages) TI1

30 / 47 page

where

OUT

(

)

- V

L =

OUT

ESR

OUT

LED

VIN

Q =

(

)

OUT

LED

- V

OUT

LM3554

SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016

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8.2.2 Detailed Design Procedure

8.2.2.1 Output Capacitor Selection

The LM3554 is designed to operate with a at least a 4.7-µF ceramic output capacitor in LED mode and a 10-µF

output capacitor in voltage-output mode. When the boost converter is running the output capacitor supplies the

load current during the boost converters on-time. When the NMOS switch turns off the inductor energy is

discharged through the internal PMOS switch supplying power to the load and restoring charge to the output

capacitor. This causes a sag in the output voltage during the on time and a rise in the output voltage during the

off time. The output capacitor is therefore chosen to limit the output ripple to an acceptable level depending on

load current and input/output voltage differentials and also to ensure the converter remains stable.

For proper LED operation the output capacitor must be at least a 4.7-µF ceramic (10-µF in voltage-output mode).

Larger capacitors such as 10 µF or 22 µF can 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 equivalent

series resistance (ESR) of the capacitor (

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 output capacitors ESR is found by:

(2)

In ceramic capacitors the ESR is very low, thus the assumption is that that 80% of the output voltage ripple is

due to capacitor discharge and 20% from ESR. Table 12 lists different manufacturers for various output

capacitors and their case sizes suitable for use with the LM3554.

8.2.2.2 Input Capacitor Selection

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

boost converter switching and reduces noise on the devices input terminal that can feed through and disrupt

internal analog signals. In the Figure 41 a 4.7-µF ceramic input capacitor works well. It is important to place the

input capacitor as close to the device input (IN) terminals as possible. This reduces the series resistance and

inductance that can inject noise into the device due to the input switching currents. Table 12 lists various input

capacitors that or recommended for use with the LM3554.

Table 12. Recommended Input/Output Capacitors (X5R Dielectric)

MANUFACTURER

PART NUMBER

VALUE

CASE SIZE (mm)

VOLTAGE RATING

TDK Corporation

C1608JB0J475K

4.7 µF

0603 (1.6 × 0.8 × 0.8 )

6.3 V

TDK Corporation

C1608JB0J106M

10 µF

0603 (1.6 × 0.8 × 0.8 )

6.3 V

TDK Corporation

C2012JB1C475K

4.7 µF

0805 (2 ×1.25 ×1.25)

16 V

TDK Corporation

C2012JB1A106M

10 µF

0805 (2 ×1.25 ×1.25)

10 V

TDK Corporation

C2012JB0J226M

22 µF

0805 (2 ×1.25 ×1.25)

6.3 V

Murata

GRM188R60J475KE19

4.7 µF

0603 (1.6 × 0.8 × 0.8 )

6.3 V

Murata

GRM21BR61C475KA88

4.7 µF

0805 (2 ×1.25 ×1.25)

16 V

Murata

GRM21BR61A106KE19

10 µF

0805 (2 ×1.25 ×1.25)

10 V

Murata

GRM21BR60J226ME39L

22 µF

0805 (2 ×1.25 ×1.25)

6.3 V

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