LM3560 datasheet(39/48 Pages) TI1

39 / 48 page

where

OUT

(

)

- V

L =

OUT

ESR

OUT

LED

VIN

Q =

(

)

OUT

LED

- V

OUT

LM3560

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SNOSB43B – SEPTEMBER 2011 – REVISED AUGUST 2013

APPLICATIONS INFORMATION

Output Capacitor Selection

The LM3560 is designed to operate with at least a 10 µF 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 NFET on-time

and a rise in the output voltage during the NFET 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 operation the output capacitor must be at least a 10 µF ceramic. Larger capacitors such as a 22 µF or

multiple capacitors in parallel 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 the capacitors ESR (ΔVESR)

use the following equations:

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 a close approximation is to assume that 80% of the output voltage

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

capacitors and their case sizes suitable for use with the LM3560.

Input Capacitor Selection

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

switching of the LM3560’s boost converter, and reduces noise on the boost converters input terminal that can

feed through and disrupt internal analog signals. In the Typical Application Circuit a 10 µF ceramic input

capacitor works well. It is important to place the input capacitor as close as possible to the LM3560’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 20 lists various input capacitors that are recommended for use with the LM3560.

Table 20. Recommended Input/Output Capacitors (X5R Dielectric)

Manufacturer

Part Number

Value

Case Size

Voltage Rating

TDK Corporation

C1608JB0J106M

10 µF

0603 (1.6mm×0.8mm×0.8mm)

6.3V

TDK Corporation

C2012JB1A106M

10 µF

0805 (2mm×1.25mm×1.25mm)

10V

Murata

GRM21BR61A106KE19

10 µF

0805 (2mm×1.25mm×1.25mm)

10V

Inductor Selection

The LM3560 is designed to use a 1 µH or 2.2 µH inductor. Table 21 lists various inductors and their

manufacturers that can work well with the LM3560. When the device is boosting (VOUT > VIN) the inductor will

typically be 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 should be greater than

the maximum operating peak current of the LM3560. 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 LM3560 are greater than IPEAK in the following

calculation:

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