LM3559TLESLASHNOPB datasheet(37/50 Pages) TI1

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where

OUT

(

)

- V

L =

OUT

ESR

OUT

LED

VIN

Q =

(

)

OUT

LED

- V

OUT

LM3559

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SNVS624B – JUNE 2011 – REVISED JUNE 2016

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Typical Applications (continued)

9.2.1.1 Design Requirements

For typical LED flash driver applications, use the parameters listed in Table 19.

Table 19. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Minimum input voltage

2.5 V

Minimum output voltage

1.8 V

Maximum output voltage

5 V

Maximum output current

1.8 A

Switching frequency

2 MHz

9.2.1.2 Detailed Design Procedure

9.2.1.2.1

Output Capacitor Selection

The LM3559 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 on time and a

rise in the output voltage during the off time. Therefore, choose the output capacitor 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

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 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 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 LM3559.

9.2.1.2.2

Input Capacitor Selection

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

of the device boost converter, and reduces noise on the input terminal of the boost converter that can feed

through and disrupt internal analog signals. In the Figure 43 a 10-µF ceramic input capacitor works well. It is

important to place the input capacitor as close as possible to the device input (IN) pin. 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 TI recommends for use with the LM3559.

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