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TPS61165

SLVS790A – NOVEMBER 2007 – REVISED MAY 2010

www.ti.com

APPLICATION INFORMATION

MAXIMUM OUTPUT CURRENT

The overcurrent limit in a boost converter limits the maximum input current and thus maximum input power for a

given input voltage. Maximum output power is less than maximum input power due to power conversion losses.

Therefore, the current limit setting, input voltage, output voltage and efficiency can all change maximum current

output. The current limit clamps the peak inductor current; therefore, the ripple has to be subtracted to derive

maximum DC current. The ripple current is a function of switching frequency, inductor value and duty cycle. The

following equations take into account of all the above factors for maximum output current calculation.

(3)

Where:

L = inductor value

Fs = switching frequency

(4)

where

h = efficiency

For instance, when VIN is 3V, 8 LEDs output equivalent to VOUT of 26V, the inductor is 22mH, the Schottky

forward voltage is 0.2V; and then the maximum output current is 110mA in typical condition. When VIN is 5V, 10

LEDs output equivalent to VOUT of 32V, the inductor is 22mH, the Schottky forward voltage is 0.2V; and then the

maximum output current is 150mA in typical condition.

INDUCTOR SELECTION

The selection of the inductor affects steady state operation as well as transient behavior and loop stability. These

factors make it the most important component in power regulator design. There are three important inductor

specifications, inductor value, DC resistance and saturation current. Considering inductor value alone is not

enough.

The inductor value determines the inductor ripple current. Choose an inductor that can handle the necessary

peak current without saturating, according to half of the peak-to-peak ripple current given by Equation 3, pause

the inductor DC current given by:

(5)

Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation

level, its inductance can decrease 20% to 35% from the 0A value depending on how the inductor vendor defines

saturation current. Using an inductor with a smaller inductance value forces discontinuous PWM when the

inductor current ramps down to zero before the end of each switching cycle. This reduces the boost converter’s

maximum output current, causes large input voltage ripple and reduces efficiency. Large inductance value

provides much more output current and higher conversion efficiency. For these reasons, a 10mH to 22mH

inductor value range is recommended. A 22mH inductor optimized the efficiency for most application while

maintaining low inductor peak to peak ripple. Table 3 lists the recommended inductor for the TPS61165. When

recommending inductor value, the factory has considered –40% and +20% tolerance from its nominal value.

14

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