LDO - Low Drop Out Operation

The LM3670 can operate at 100% duty cycle (no switching,

PMOS switch is completely on) for low drop out support of

the output voltage. In this way the output voltage is con-

trolled down to the lowest possible input voltage.

The minimum input voltage needed to support the output

voltage is

LOAD

Load current

DSON,

PFET

Drain to source resistance of PFET switch

in the triode region

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

0.5V. The resistor from V

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

Ω, and V

resistor feedback network is 2.5µA ( I

output voltage formula is:

•

V

•

V

•

R

Ω)

•

R

Ω)

For any output voltage greater than or equal to 0.7V a

frequency zero must be added at 10kHz for stability. The

formula is:

For any output voltages below 0.7 and above or equal to

2.5V, a pole must also be placed at 10kHz as well. The

lowest output voltage possible is 0.7V. At low output voltages

the duty cycle is very small and, as the input voltage in-

creases, the duty cycle decreases even further. Since the

duty cycle is so low any change due to noise is an appre-

ciable percentage. In other words, it is susceptible to noise.

Capacitors C

quency poles and zeros. If the pole and zero are at the same

frequency the formula is:

A pole can also be used at higher output voltages. For

example, in the table 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

•

I

•

V

•

L inductor

•

f switching frequency

•

I

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

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

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