LP2975 datasheet(12/19 Pages) NSC

12 / 19 page

Application Hints (Continued)

For maximum accuracy, the

INPUT and CURRENT LIMIT

pins must be Kelvin connected to R

SC,

to avoid errors

caused by voltage drops along the traces carrying the cur-

rent from the input supply to the

Source pin of the FET.

EXTERNAL CAPACITORS

The best capacitors for use in a specific design will depend

on voltage and load current (examples of tested circuits for

several different output voltages and currents are provided in

a previous section.)

Information in the next sections is provided to aid the de-

signer in the selection of the external capacitors.

INPUT CAPACITOR: Although not always required, an input

capacitor is recommended. Good bypassing on the input as-

sures that the regulator is working from a source with a low

impedance, which improves stability. A good input capacitor

can also improve transient response by providing a reservoir

of stored energy that the regulator can utilize in cases where

the load current demand suddenly increases. The value

used for C

IN may be increased without limit. Refer to the Ref-

erence Designs section for examples of input capacitors.

OUTPUT CAPACITOR: The output capacitor is required for

loop stability (compensation) as well as transient response.

During sudden changes in load current demand, the output

capacitor must source or sink current during the time it takes

the control loop of the LP2975 to adjust the gate drive to the

pass FET. As a general rule, a larger output capacitor will im-

prove both transient response and phase margin (stability).

The value of C

OUT may be increased without limit.

OUTPUT CAPACITOR AND COMPENSATION: Loop com-

pensation for the LP2975 is derived from C

OUT and, in some

cases, the feed-forward capacitor C

F (see next section).

OUT forms a pole (referred to as fp) in conjuction with the

load resistance which causes the loop gain to roll off (de-

crease) at an additional −20 dB/decade. The frequency of

the pole is:

p =0.16/[(RL +ESR)xCOUT]

Where:

L is the load resistance.

OUT is the value of the output capacitor.

ESR is the equivalent series resistance of C

OUT.

As a general guideline, the frequency of f

p should be ≤ 200

Hz. It should be noted that higher load currents correspond

to lower values of R

L, which requires that COUT be increased

to keep f

p at a given frequency.

DESIGN EXAMPLE: Select the minimum required output

capacitance for a design whose output specifications are 5V

@ 1A:

p =0.16/[(RL +ESR)xCOUT]

Re-written:

OUT =0.16/[fp x(RL + ESR) ]

Values used for the calculation:

p = 200 Hz, RL =5Ω, ESR = 0.1Ω (assumed).

Solving for C

OUT,weget 157 µF (nearest standard size

would be 180 µF).

The ESR of the output capacitor is very important for stabil-

ity, as it creates a zero (f

z) which cancels much of the phase

shift resulting from one of the poles present in the loop. The

frequency of the zero is calculated from:

z = 0.16 / (ESR x COUT)

For best results in most designs, the frequency of f

z should

fall between 5 kHz and 50 kHz. It must be noted that the val-

ues of C

OUT and ESR usually vary with temperature (se-

verely in the case of aluminum electrolytics), and this must

be taken into consideration.

For the design example (V

OUT =5V @ 1A), select a capacitor

which meets the f

z requirements. Solving the equation for

ESR yields:

ESR = 0.16 / (f

z xCOUT)

Assuming f

z = 5 kHz and 50 kHz, the limiting values of ESR

for the 180 µF capacitor are found to be:

18 m

Ω≤ ESR ≤ 0.18Ω

A good-quality, low-ESR capacitor type such as the Pana-

sonic HFQ is a good choice. However, the 10V/180 µF ca-

pacitor (#ECA-1AFQ181) has an ESR of 0.3

Ω which is not in

the desired range.

To assure a stable design, some of the options are:

1) Use a different type capacitor which has a lower ESR

such as an organic-electrolyte OSCON.

2) Use a higher voltage capacitor. Since ESR is inversely

proportional to the physical size of the capacitor, a higher

voltage capacitor with the same C value will typically have a

lower ESR (because of the larger case size). In this ex-

ample, a Panasonic ECA-1EFQ181 (which is a 180 µF/25V

part) has an ESR of 0.17

Ω and would meet the desired ESR

range.

3) Use a feed-forward capacitor (see next section).

FEED-FORWARD CAPACITOR: Although not required in

every application, the use of a feed-forward capacitor (C

can yield improvements in both phase margin and transient

response in most designs.

The added phase margin provided by C

F can prevent oscil-

lations in cases where the required value of C

OUT and ESR

can not be easily obtained (see previous section).

F can also reduce the phase shift due to the pole resulting

from the Gate capacitance, stabilizing applications where

this pole occurs at a low frequency (before cross-over) which

would cause oscillations if left uncompensated (see later

section

GATE CAPACITANCE POLE FREQUENCY).

Even in a stable design, adding C

F will typically provide more

optimal loop response (faster settling time). For these rea-

sons,

the use of a feed-forward capacitor is always rec-

ommended.

F is connected across the top resistor in the divider used to

set the output voltage (see Typical Application Circuit). This

forms a zero in the loop response (defined as f

zf), whose fre-

quency is:

zf =6.6x10

−6 /[C

F x(VOUT /1.24−1)]

When solved for C

F, the fzf equation is:

F =6.6x10

−6 /[f

zf x(VOUT /1.24−1)]

For most applications, f

zf should be set between 5 kHz and

50 kHz.

ADJUSTING THE OUTPUT VOLTAGE

If an output voltage is required which is not available as a

standard voltage, the LP2975 can be used as an adjustable

regulator (see Typical Application circuit). The external resis-

tors R1 and R2 (along with the internal 24 k

Ω resistor) set

the output voltage.

It is important to note that R2 is connected in parallel with the

internal 24 k

Ω resistor. If we define R

EQ as the total resis-

tance between the COMP pin and ground, then its value

will be the parallel combination of R2 and 24 k

Ω:

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