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IN

OUT

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)

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SW

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L

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+

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=

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ESR

ESR

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LED

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§

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(

)

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OUT

LED

- V

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C

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LM3561

SNOSB44C – MARCH 2011 – REVISED MAY 2013

www.ti.com

APPLICATIONS INFORMATION

OUTPUT CAPACITOR SELECTION

The LM3561 is designed to operate with a at least a 10µF ceramic output capacitor. 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 10µF ceramic. Larger capacitors such as 22µF

can be used if lower output voltage ripple is desired. To estimate the output voltage ripple considering the ripple

due to capacitor discharge (

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 so assume that 80% of the output voltage ripple is due to capacitor

discharge and 20% from ESR. Table 14 lists different manufacturers for various output capacitors and their case

sizes suitable for use with the LM3561.

INPUT CAPACITOR SELECTION

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

of the LM3561’s boost converter and reduces noise on the devices input terminal that can feed through and

disrupt internal analog signals. In the Typical Application Circuit a 10µF ceramic input capacitor works well. It is

important to place the input capacitor as close as possible to the LM3561’s input (IN) terminals. This reduces the

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

Table 14 lists various input capacitors recommended for use with the LM3561.

Table 14. Recommended Input/Output Capacitors (X5R Dielectric)

Manufacturer

Part Number

Value

Case Size

Voltage Rating

TDK Corporation

C1608JB0J106M

10µF

0603(1.6mm×0.8mm×0.8mm)

6.3V

TDK Corporation

C2012JB1A106M

10µF

0805(2mm×1.25mm×1.25mm)

10V

TDK Corporation

C2012JB0J226M

22µF

0805(2mm×1.25mm×1.25mm)

6.3V

Murata

GRM21BR61A106KE19

10µF

0805(2mm×1.25mm×1.25mm)

10V

Murata

GRM21BR60J226ME39L

22µF

0805(2mm×1.25mm×1.25mm)

6.3V

INDUCTOR SELECTION

The LM3561 is designed to use a 1µH to 2.2µH inductor. Table 15 lists various inductors that can work well with

loss in the circuit. Therefore, choosing an inductor with the lowest possible series resistance is important.

Additionally, the saturation rating of the inductor should be greater than the maximum operating peak current of

the LM3561. 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

28

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