LM3370 datasheet(20/23 Pages) NSC

20 / 23 page

Application Information

SETTING OUTPUT VOLTAGE VIA I

2C Compatible

The outputs of the LM3370 can be programmed through

Buck 1 & Buck 2 registers via I

2C. Buck 1 output voltage can

be dynamically adjusted between 1V to 2V in 50mV steps

and Buck 2 output voltage can be adjusted between 2.3V to

3.3V in 100mV steps. Please refer to voltage selection table

on page 19 for programming the desire output voltage. If the

2C compatible feature is not used, the default output voltage

will be the pre-trimmed voltage. For example, LM3370SD-

3021 refers to 1.2V for Buck 1 and 3.3V for Buck 2.

Micro-Stepping:

The Micro-Stepping feature minimizes output voltage

overshoot/undershoot during large output transients or large

iumps in output voltage. If Micro-stepping is enabled through

2C, the output voltage automatically ramps at 50mV per

step for Buck 1 and 100mV per step for Buck 2. The steps

are summarized as follow:

Buck 1: 50mV/step and 32µs/step

Buck 2: 100mV/step and 32µs/step

For example if changing Buck 1 voltage from 1V to 1.8V

yields 20 steps [(1.8 - 1)/ 0.05 = 20]. This translates to 640µs

[(20 x 32µs) = 640µs] needed to reach the final target

voltage.

INDUCTOR SELECTION

There are two main considerations when choosing an induc-

tor; the inductor should not saturate, and the inductor current

ripple is small enough to achieve the desired output voltage

ripple.

There are two methods to choose the inductor current rating.

method 1:

The total current is the sum of the load and the inductor

ripple current. This can be written as

•

LOAD load current

•

IN input voltage

•

L inductor

•

f switching frequency

method 2:

A more conservative approach is to choose an inductor that

can handle the maximum current limit of 1400mA.

Given a peak-to-peak current ripple (I

PP) the inductor needs

to be at least

A 2.2µH inductor with a saturation current rating of at least

1400mA is recommended for most applications. The induc-

tor’s resistance should be less than around 0.2

Ω for good

efficiency. Table 1 lists suggested inductors and suppliers.

For low-cost applications, an unshielded bobbin inductor is

suggested. For noise critical applications, a toroidal or

shielded-bobbin inductor should be used. A good practice is

to lay out the board with overlapping footprints of both types

for design flexibility. This allows substitution of a low-noise

toroidal inductor, in the event that noise from low-cost bobbin

models is unacceptable.

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 4.7µF, 6.3V is sufficient for most

applications. A larger value may be used for improved input

voltage filtering. The input filter capacitor supplies current to

the PFET switch of the LM3370 in the first half of each cycle

and reduces voltage ripple imposed on the input power

source. A ceramic capacitor’s low ESR provides the best

noise filtering of the input voltage spikes due to this rapidly

changing current. Select an input filter capacitor with a surge

current rating sufficient for the power-up surge from the input

power source. The power-up surge current is approximately

the capacitor’s value (µF) times the voltage rise rate (V/µs).

The input current ripple can be calculated as:

Below are some suggested inductor manufacturers include

but are not limited to:

TABLE 1. Suggested Inductors and Suppliers

Model

Vendor

Dimensions

(mm)

SAT

DO3314-222

Coilcraft

3.3 x 3.3 x 1.4

1.6A

LPO3310-222

3.3 x 3.3 x 1.0

1.1A

SD3114-2R2

Cooper

3.1 x 3.1 x 1.4

1.48A

NR3010T2R2M

Taiyo Yuden

3.0 x 3.0 x 1.0

1.1A

NR3015T2R2M

3.0 x 3.0 x 1.5

1.48A

VLF3010AT-

2R2M1R0

TDK

2.6 x 2.8 x 1.0

1.0A

OUTPUT CAPACITOR SELECTION

DC bias characteristics of ceramic capacitors must be con-

sidered when selecting case sizes like 0805 and 0603. DC

bias characteristics vary from manufacturer to manufacturer

and dc bias curves should be requested from them as part of

the capacitor selection process.

The output filter capacitor smoothes out current flow from the

inductor to the load, helps maintain a steady output voltage

during transient load changes and reduces output voltage

ripple. These capacitors must be selected with sufficient

capacitance and sufficiently low ESR to perform these func-

tions. The output ripple current can be calculated as:

Voltage peak-to-peak ripple due to capacitance =

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