LM25005 datasheet(15/21 Pages) NSC

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Application Information

EXTERNAL COMPONENTS

The procedure for calculating the external components is

illustrated with the following design example. The Bill of

Materials for this design is listed in Table1. The circuit shown

in Figure 1 is configured for the following specifications:

•

OUT =5V

•

IN =7Vto42V

•

Fs = 300 KHz

•

Minimum load current (for CCM) = 250 mA

•

Maximum load current = 2.5A

R3 (R

T sets the oscillator switching frequency. Generally, higher

frequency applications are smaller but have higher losses.

Operation at 300KHz was selected for this example as a

reasonable compromise for both small size and high effi-

ciency. The value of R

T for 300KHz switching frequency can

be calculated as follows:

The nearest standard value of 21 k

Ω was chosen for RT.

The inductor value is determined based on the operating

frequency, load current, ripple current, and the minimum and

maximum input voltage (V

IN(min),VIN(max)).

To keep the circuit in continuous conduction mode (CCM),

the maximum ripple current I

RIPPLE should be less than twice

the minimum load current, or 0.5 Ap-p. Using this value of

ripple current, the value of inductor (L1) is calculated using

the following:

This procedure provides a guide to select the value of L1.

The nearest standard value (33 µH) will be used. L1 must be

rated for the peak current (I

PK+) to prevent saturation. During

normal loading conditions, the peak current occurs at maxi-

mum load current plus maximum ripple. During an overload

condition the peak current is limited to 3.5A nominal (4.25A

maximum). The selected inductor (see Table 1) has a con-

servative 6.2 Amp saturation current rating. For this manu-

facturer, the saturation rating is defined as the current nec-

essary for the inductance to reduce by 30%, at 20˚C.

C3 (C

RAMP)

With the inductor value selected, the value of C3 (C

RAMP)

necessary for the emulation ramp circuit is:

RAMP =L x10

-5

Where L is in Henrys

With L1 selected for 33µH the recommended value for C3 is

330pF.

C9, C10

The output capacitors C9, and C10, smooth the inductor

ripple current and provide a source of charge for transient

loading conditions. For this design a 22µF ceramic capacitor

and a 150µF SP organic capacitor were selected. The ce-

ramic capacitor provides ultra low ESR to reduce the output

ripple voltage and noise spikes, while the SP capacitor pro-

vides a large bulk capacitance in a small volume for transient

loading conditions. An approximation for the output ripple

voltage is:

A Schottky type re-circulating diode is required for all

LM25005 applications. Ultra-fast diodes are not recom-

mended and may result in damage to the IC due to reverse

recovery current transients. The near ideal reverse recovery

characteristics and low forward voltage drop are particularly

important diode characteristics for high input voltage and low

output voltage applications common to the LM25005. The

reverse recovery characteristic determines how long the

current surge lasts each cycle when the buck switch is

turned on. The reverse recovery characteristics of Schottky

diodes minimize the peak instantaneous power in the buck

switch occurring during turn-on each cycle. The resulting

switching losses of the buck switch are significantly reduced

when using a Schottky diode. The reverse breakdown rating

should be selected for the maximum V

IN, plus some safety

margin.

The forward voltage drop has a significant impact on the

conversion efficiency, especially for applications with a low

output voltage. “Rated” current for diodes vary widely from

various manufactures. The worst case is to assume a short

circuit load condition. In this case the diode will carry the

output current almost continuously. For the LM25005 this

current can be as high as 3.5A. Assuming a worst case 1V

drop across the diode, the maximum diode power dissipation

can be as high as 3.5W. For the reference design a 60V

Schottky in a DPAK package was selected.

C1, C2

The regulator supply voltage has a large source impedance

at the switching frequency. Good quality input capacitors are

necessary to limit the ripple voltage at the VIN pin while

supplying most of the switch current during the on-time.

20170010

FIGURE 7. Inductor Current Waveform

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