LM5574MTSLASHNOPB datasheet(14/29 Pages) TI1

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'VOUT = 'IL x

8 x FS x COUT

ESR +

L1 =

5V x (75V ± 5V)

0.2A x 300 kHz x 75V

= 78 PH

L1 =

VOUT x (VIN(max) ± VOUT)

IRIPPLE x FS x VIN(max)

IPK+

0 mA

IPK-

IRIPPLE

1/Fs

LM5574, LM5574-Q1

SNVS478F – JANUARY 2007 – REVISED APRIL 2013

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The inductor value is determined based on the operating frequency, load current, ripple current, and the

minimum and maximum input voltage (VIN(min), VIN(max)).

Figure 16. Inductor Current Waveform

To keep the circuit in continuous conduction mode (CCM), the maximum ripple current IRIPPLE should be less

than twice the minimum load current, or 0.2Ap-p. Using this value of ripple current, the value of inductor (L1) is

calculated using the following:

(6)

(7)

This procedure provides a guide to select the value of L1. The nearest standard value (100µH) will be used. L1

must be rated for the peak current (IPK+) to prevent saturation. During normal loading conditions, the peak current

occurs at maximum load current plus maximum ripple. During an overload condition the peak current is limited to

0.7A nominal (0.85A maximum). The selected inductor (see Table 1) has a conservative 1.0 Amp saturation

current rating. For this manufacturer, the saturation rating is defined as the current necessary for the inductance

to reduce by 30%, at 20°C.

C3 (CRAMP)

With the inductor value selected, the value of C3 (CRAMP) necessary for the emulation ramp circuit is:

CRAMP = L x 5 x 10

-6

(8)

Where L is in Henrys

With L1 selected for 100µH the recommended value for C3 is 470pF (nearest standard value).

The output capacitor, C9 smoothes the inductor ripple current and provides a source of charge for transient

loading conditions. For this design a 22µF ceramic capacitor was selected. The ceramic capacitor provides ultra

low ESR to reduce the output ripple voltage and noise spikes. An approximation for the output ripple voltage is:

(9)

A Schottky type re-circulating diode is required for all LM5574 applications. Ultra-fast diodes are not

recommended 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 LM5574. 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 VIN, plus some

safety margin.

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