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17

LM3241

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SNOSB38C – JANUARY 2009 – REVISED NOVEMBER 2017

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Copyright © 2009–2017, Texas Instruments Incorporated

8.2.2.1 Setting the Output Voltage

The LM3241 features a pin-controlled adjustable output voltage to eliminate the need for external feedback

resistors. The output voltage can be programmed for an output voltage from 0.6 V to 3.4 V by setting the voltage

on the VCON pin, as shown in Equation 1.

VOUT = 2.5 × VCON

(1)

When the VCON voltage is between 0.24 V and 1.36 V, the output voltage follows proportionally by 2.5 times of

VCON.

If the VCON voltage is less than 0.24 V (VOUT = 0.6 V), the output voltage may be regulated (for details see

Figure 17). Figure 17 exhibits the characteristics of a typical part, and the performance cannot be ensured as a

part-to-part variation could occur for output voltages less than 0.6 V. For VOUT lower than 0.6 V, the converter

could suffer from larger output ripple voltage and higher current limit operation.

8.2.2.2 Inductor Selection

Two main considerations must be considered when choosing an inductor: the inductor should not saturate, and

the inductor current ripple should be small enough to achieve the desired output voltage ripple. Different

manufacturers follow different saturation current rating specifications, so attention must be given to details.

Saturation current ratings are typically specified at 25°C so ratings over the ambient temperature of application

should be requested from manufacturer.

The minimum value of inductance to ensure good performance is 0.3 µH at bias current (ILIM, typical) over the

ambient temperature range. Shielded inductors radiate less noise and should be preferred. Two methods are

avaialble to choose the inductor saturation current rating.

8.2.2.2.1

Method 1

The saturation current should be greater than the sum of the maximum load current and the worst case average

to peak inductor current. Use Equation 2 to find the saturation current (ISAT).

ISAT > IOUT_MAX + IRIPPLE

where

•

IOUT_MAX is the maximum load current (750 mA).

•

IRIPPLE is the average-to-peak inductor current. Use Equation 3 to calculate the IRIPPLE value.

(2)

where

•

VIN is the maximum input voltage in application.

•

VOUT is the output voltage

•

L is the minimum inductor value including worst-case tolerances (30% drop can be considered for Method 1)

•

f is the minimum switching frequency (5.7 MHz)

(3)

8.2.2.2.2

Method 2

A more conservative and recommended approach is to choose an inductor that can support the maximum

current limit of 1600 mA.

The resistance of the inductor should be less than approximately 0.1 Ω for good efficiency. Table 3 lists

recommended inductors and suppliers.

Table 3. Recommended Inductors

Model

Size (W x L x H) (mm)

Vendor

MIPSZ2012D0R5

2.0 x 1.2 x 1.0

FDK

LQM21PNR54MG0

2.0 x 1.25 x 0.9

Murata

LQM2MPNR47NG0

2.0 x 1.6 x 0.9

Murata