LM3670 datasheet(13/17 Pages) NSC

13 / 17 page

Application Information

OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE

LM3670

The output voltage of the adjustable parts can be pro-

grammed through the resistor network connected from V

OUT

to V

FB the to GND. VOUT will be adjusted to make VFB equal

to .5V. The resistor from V

FB to GND (R2) should be at least

100K

Ω to keep the current sunk through this network well

below the 15µA quiescent current level (PFM mode with no

switching) but large enough that it is not susceptible to noise.

If R

2 is 200K

Ω, and given the V

FB is .5V, then the current

through the resistor feedback network will be 2.5µA ( I

=.5V/R

2). The output voltage formula is:

•

OUT Output Voltage (V)

•

FB Feedback Voltage (.5V typ)

•

1 Resistor from VOUT to VFB (

Ω)

•

2 Resistor from VOUT to GND (

Ω)

For any output voltage greater than or equal to .8V a fre-

quency zero must be added at 10KHz for stability. The

formula is:

For output voltages between .7 and .8V a pole must also be

placed at 10KHz as well. The lowest output voltage possible

is .7V. At the low voltages the duty cycle is very small. In

addition, as the input voltage increases the duty cycle de-

creases even further. Since the duty cycle is so low any

change due to noise is an appreciable percentage. In other

words, it is susceptible to noise. The C

1 and C2 act as noise

filters at this point rather than frequency poles and zeroes. If

tghe pole and zero are at the sasme frequency the formula

is:

A pole can be usesd at higher output voltages too. For

example, in the table "Adjustable LM3670 Configurations for

Various VOUT"Table 3 there is an entry for 1.24V with both a

pole and zero at approximately 10KHz for noise rejection.

INDUCTOR SELECTION

There are two main considerations when choosing an induc-

tor; the inductor current 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

•

RIPPLE peak-to-peak

Method 2:

A more conservative approach is to choose an inductor that

can handle the current limit of 700 mA.

Given a peak-to-peak current ripple (I

PP) the inductor needs

to be at least

A 10 µH inductor with a saturation current rating of at least

800 mA is recommended for most applications. The induc-

tor’s resistance should be less than around 0.3

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 4.7 µF 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 LM3670 in the first half of each cycle

and reduces voltage ripple imposed on the input power

source. A ceramic capcitor’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:

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