LM3561 Datasheet, PDF(30/36 Page) Texas Instruments – LM3561 Synchronous Boost Converter with 600mA High Side LED Driver and I 2 C-Compatible Interface

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LM3561 Datasheet, PDF (30/36 Pages) Texas Instruments – LM3561 Synchronous Boost Converter with 600mA High Side LED Driver and I 2 C-Compatible Interface

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Applications Information

OUTPUT CAPACITOR SELECTION

The LM3561 is designed to operate with a at least a 10ÂµF

ceramic output capacitor. When the boost converter is run-

ning 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 dur-

ing 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 10ÂµF ceramic. Larger capacitors such as 22ÂµF can be

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

output voltage ripple considering the ripple due to capacitor

discharge (ÎVQ) and the ripple due to the capacitors ESR

(ÎVESR) use the following equations:

For continuous conduction mode, the output voltage ripple

due to the capacitor discharge is:

The output voltage ripple due to the output capacitors ESR is

found by:

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 12 lists different manufacturers for var-

ious 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

LM3561âs boost converter and reduces noise on the devices

input terminal that can feed through and disrupt internal ana-

log signals. In the Typical Application Circuit a 10ÂµF ceramic

input capacitor works well. It is important to place the input

capacitor as close as possible to the LM3561âs input (IN) ter-

minals. 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 recommend-

ed for use with the LM3561.

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

Manufacturer

TDK Corporation

Murata

Part Number

C1608JB0J106M

C2012JB1A106M

C2012JB0J226M

GRM21BR61A106KE19

GRM21BR60J226ME39L

Value

10ÂµF

22ÂµF

10ÂµF

22ÂµF

Case Size

0603(1.6mmÃ0.8mmÃ0.8mm)

0805(2mmÃ1.25mmÃ1.25mm)

Voltage Rating

6.3V

10V

6.3V

10V

6.3V

INDUCTOR SELECTION

The LM3561 is designed to use a 1ÂµH to 2.2ÂµH inductor.

Table 13 lists various inductors that can work well with the

LM3561. When the device is boosting (VOUT > VIN) the induc-

tor will typically be the biggest area of efficiency loss in the

circuit. Therefore, choosing an inductor with the lowest pos-

sible series resistance is important. Additionally, the satura-

tion 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 perfor-

mance ensure that the inductor saturation and the peak cur-

rent limit setting of the LM3561 is greater than IPEAK. This can

be calculated by:

ÆSW = 2MHz, and Î· can be found in the Typical Performance

Characteristics plots.

TABLE 13. Recommended Inductors

Manufacturer

Coilcraft

1ÂµH

TDK

1ÂµH

TDK

1ÂµH

TDK

1ÂµH

TDK

1ÂµH

TDK

1ÂµH

Part Number

XPL2010-102ML

VLS252012T-1R0N

VLS2010-1R0N

VLS2012ET-1R0N

VLS20160ET-1R0N

VLS252010ET-1R0N

Dimensions (LÃWÃH)

2mmÃ1.9mmÃ1mm

2mmÃ2.5mmÃ1.2mm

2mm x 2mm x 1mm

2mm x 2mm x 1.2mm

2mm x 1.6mm x 0.95mm

2.5mm x 2mm x 1mm

RDC

81mâ¦

73mâ¦

90mâ¦

71mâ¦

100mâ¦

70mâ¦

ISAT

1.6A

2.7A

1.65A

1.5A

1.9A

NTC THERMISTOR SELECTION

Programming bit [4] of Configuration Register 1 with a (1) se-

lects Thermal Comparator mode, making the LEDI/NTC pin

a comparator input for flash LED thermal sensing. The ther-

mal sensing circuit consists of a negative temperature coef-

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