LM2591HV datasheet(14/20 Pages) NSC

14 / 20 page

Application Information (Continued)

Therefore, looking at Figure 2 we quick-select a 100µH/1A

inductor (designed for 150 kHz operation) for this applica-

tion.

2. We should confirm that it is rated to handle 100 µJ (see

Figure 4) by the procedure shown in AN-1197 and that the

losses are acceptable. (If the DC Input voltage had been

greater than 40V we would need to consider e

CLIM as in

Example 2 above).

Note that we have taken V

SAT as 1.5V which includes an

estimated resistive drop across the inductor.

This completes the simplified inductor selection procedure.

For more general applications and better optimization, the

designer should refer to AN-1197. Figure 6 provides helpful

contact information on suggested Inductor manufacturers

who may be able to recommend suitable parts, if the require-

ments are known.

FEEDFORWARD CAPACITOR

(Adjustable Output Voltage Version)

FF - A Feedforward Capacitor CFF, shown across R2 in

Figure 1 is used when the output voltage is greater than 10V

or when C

OUT has a very low ESR. This capacitor adds lead

compensation to the feedback loop and increases the phase

margin for better loop stability.

If the output voltage ripple is large (> 5% of the nominal

output voltage), this ripple can be coupled to the feedback

pin through the feedforward capacitor and cause the error

comparator to trigger the error flag. In this situation, adding a

resistor, R

FF, in series with the feedforward capacitor, ap-

proximately 3 times R1, will attenuate the ripple voltage at

the feedback pin.

INPUT CAPACITOR

IN — A low ESR aluminum or tantalum bypass capacitor is

needed between the input pin and ground pin. It must be

located near the regulator using short leads. This capacitor

prevents large voltage transients from appearing at the in-

put, and provides the instantaneous current needed each

time the switch turns on.

The important parameters for the Input capacitor are the

voltage rating and the RMS current rating. Because of the

relatively high RMS currents flowing in a buck regulator’s

input capacitor, this capacitor should be chosen for its RMS

current rating rather than its capacitance or voltage ratings,

although the capacitance value and voltage rating are di-

rectly related to the RMS current rating. The voltage rating of

the capacitor and its RMS ripple current capability must

never be exceeded.

OUTPUT CAPACITOR

OUT — An output capacitor is required to filter the output

and provide regulator loop stability. Low impedance or low

ESR Electrolytic or solid tantalum capacitors designed for

switching regulator applications must be used. When select-

ing an output capacitor, the important capacitor parameters

are; the 100 kHz Equivalent Series Resistance (ESR), the

RMS ripple current rating, voltage rating, and capacitance

value. For the output capacitor, the ESR value is the most

important parameter. The ESR should generally not be less

than 100 m

Ω or there will be loop instability. If the ESR is too

large, efficiency and output voltage ripple are effected. So

ESR must be chosen carefully.

CATCH DIODE

Buck regulators require a diode to provide a return path for

the inductor current when the switch turns off. This must be

a fast diode and must be located close to the LM2591HV

using short leads and short printed circuit traces.

Because of their very fast switching speed and low forward

voltage drop, Schottky diodes provide the best performance,

especially in low output voltage applications (5V and lower).

Ultra-fast recovery, or High-Efficiency rectifiers are also a

good choice, but some types with an abrupt turnoff charac-

teristic may cause instability or EMI problems. Ultra-fast

recovery diodes typically have reverse recovery times of 50

ns or less. The diode must be chosen for its average/RMS

current rating and maximum voltage rating. The voltage

rating of the diode must be greater than the DC input voltage

(not the output voltage).

DELAYED STARTUP

The circuit in Figure 7 uses the the ON /OFF pin to provide

a time delay between the time the input voltage is applied

and the time the output voltage comes up (only the circuitry

pertaining to the delayed start up is shown). As the input

voltage rises, the charging of capacitor C1 pulls the ON /OFF

pin high, keeping the regulator off. Once the input voltage

reaches its final value and the capacitor stops charging, and

resistor R

2 pulls the ON /OFF pin low, thus allowing the

circuit to start switching. Resistor R

1 is included to limit the

maximum voltage applied to the ON /OFF pin (maximum of

25V), reduces power supply noise sensitivity, and also limits

the capacitor, C1, discharge current. When high input ripple

voltage exists, avoid long delay time, because this ripple can

be coupled into the ON /OFF pin and cause problems.

This delayed startup feature is useful in situations where the

input power source is limited in the amount of current it can

deliver. It allows the input voltage to rise to a higher voltage

before the regulator starts operating. Buck regulators require

less input current at higher input voltages.

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