L

L

Lmax

outmax

Vout

1

I

Vin

I

= Vout

I

= I

+

L

2

f

-

´

´

c

1

=

with L = 1.5

H, Cout = 10

F

2π L

Cout

f

m

m

´

TPS65708

SLVSAE1A

– OCTOBER 2010 – REVISED FEBRUARY 2011

www.ti.com

LED DRIVER

TPS65708 contains a LED driver for a current of up to 30mA. ISINK is an open drain current sink that regulates a

output voltage of one of the dcdc converters in TPS65708, depending on the forward voltage of the LED. The

cathode of the LED is connected to ISINK which sets a constant current to GND. ISINK is regulated internally

based on the default current set internally. In addition, the LED current can be PWM dimmed by a signal applied

to pin PWM. If pin PWM is pulled LOW, the LED driver is disabled and its output ISINK is high resistive. If PWM

is HIGH, the current sink regulates to the current defined by EEPROM (TI factory set in two ranges. 7.5mA to

15mA with 0.5mA resolution and 15mA to 30mA in 1mA resolution). The maximum PWM frequency is 50kHz

with a duty cycle range of 5% to 100%. TPS65708 is set to 7.5mA as a default. Contact TI about different default

settings.

APPLICATION INFORMATION

OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)

Inductor Selection

The converter operates typically with a 1.5

µH or 2.2µH output inductor. The selected inductor has to be rated for

its dc resistance and saturation current. The DC resistance of the inductance will influence directly the efficiency

of the converter. Therefore an inductor with lowest dc resistance should be selected for highest efficiency.

Equation 1 calculates the maximum inductor current under static load conditions. The saturation current of the

inductor should be rated higher than the maximum inductor current as calculated with Equation 1. This is

recommended because during heavy load transient the inductor current will rise above the calculated value.

(1)

With:

f = Switching Frequency (2.25MHz typical)

L = Inductor Value

The highest inductor current will occur at maximum Vin.

Open core inductors have a soft saturation characteristic and they can usually handle higher inductor currents

versus a comparable shielded inductor.

A more conservative approach is to select the inductor current rating just for the maximum switch current of the

corresponding converter. It must be considered, that the core material from inductor to inductor differs and will

have an impact on the efficiency especially at high switching frequencies.

Note that the step down converter has internal loop compensation. The internal loop compensation is designed

to work with an output filter corner frequency calculated as follows:

(2)

This leads to the fact the selection of external L-C filter has to be coped with the above equation. As a general

value.

14

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