LM3102 datasheet(17/30 Pages) TI

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L =

ILR x fSW x VIN

VOUT x (VIN - VOUT)

1.3 x 10

-10

VIN(MAX) x 150 ns

RON J

LM3102, LM3102-Q1

www.ti.com

SNVS515H – SEPTEMBER 2007 – REVISED JUNE 2015

Typical Application (continued)

8.2.2.2 External Components

The following guidelines can be used to select external components.

RFB1 and RFB2: These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ,

satisfying the following ratio:

RFB1/RFB2 = (VOUT/0.8 V) – 1

(7)

For VOUT = 0.8 V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than

20 µA. It is because the converter operation needs a minimum inductor current ripple to maintain good regulation

when no load is connected.

RON: Equation 2 can be used to select RON if a desired operating frequency is selected. But the minimum value

of RON is determined by the minimum ON-time. It can be calculated as follows:

(8)

If RON calculated from Equation 2 is smaller than the minimum value determined in Equation 8, a lower frequency

should be selected to recalculate RON by Equation 2. Alternatively, VIN(MAX) can also be limited to keep the

frequency unchanged. The relationship of VIN(MAX) and RON is shown in Figure 25.

On the other hand, the minimum OFF-time of 260 ns can limit the maximum duty ratio. Larger RON should be

selected in any application requiring large duty ratio.

Figure 25. Maximum VIN for Selected RON

L: The main parameter affected by the inductor is the amplitude of inductor current ripple (ILR). Once ILR is

selected, L can be determined by:

where

•

VIN is the maximum input voltage

•

fSW is determined from Equation 2

(9)

If the output current IOUT is determined, by assuming that IOUT = IL, the higher and lower peak of ILR can be

determined. Beware that the higher peak of ILR should not be larger than the saturation current of the inductor

and current limits of the main and synchronous MOSFETs. Also, the lower peak of ILR must be positive if CCM

operation is required.

Figure 26 and Figure 27 show curves on inductor selection for various VOUT and RON. For small RON, according

to (8), VIN is limited. Some curves are therefore limited as shown in the figures.

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