LM5575 datasheet(16/28 Pages) TI

16 / 28 page

REF LEVEL

0.000 dB

0.0 deg

100

START 100.000 Hz

10k

STOP 100 000.000 Hz

/DIV

10.000 dB

45.000 deg

GAIN

PHASE

100k

LM5575

SNVS471G – JANUARY 2007 – REVISED APRIL 2013

www.ti.com

Capacitor C12 provides filtering for the divider. The voltage at the SD pin should never exceed 8V, when using

an external set-point divider it may be necessary to clamp the SD pin at high input voltage conditions. The

reference design utilizes the full range of the LM5575 (6V to 75V); therefore these components can be omitted.

With the SD pin open circuit the LM5575 responds once the Vcc UVLO threshold is satisfied.

R7, C11

A snubber network across the power diode reduces ringing and spikes at the switching node. Excessive ringing

and spikes can cause erratic operation and couple spikes and noise to the output. Voltage spikes beyond the

rating of the LM5575 or the re-circulating diode can damage these devices. Selecting the values for the snubber

is best accomplished through empirical methods. First, make sure the lead lengths for the snubber connections

are very short. For the current levels typical for the LM5575 a resistor value between 5 and 20 Ohms is

adequate. Increasing the value of the snubber capacitor results in more damping but higher losses. Select a

minimum value of C11 that provides adequate damping of the SW pin waveform at high load.

R4, C5, C6

These components configure the error amplifier gain characteristics to accomplish a stable overall loop gain. One

advantage of current mode control is the ability to close the loop with only two feedback components, R4 and C5.

The overall loop gain is the product of the modulator gain and the error amplifier gain. The DC modulator gain of

the LM5575 is as follows:

DC Gain(MOD) = Gm(MOD) x RLOAD = 1 x RLOAD

(13)

The dominant low frequency pole of the modulator is determined by the load resistance (RLOAD,) and output

capacitance (COUT). The corner frequency of this pole is:

fp(MOD) = 1 / (2π RLOAD COUT)

(14)

For RLOAD = 5Ω and COUT = 130µF then fp(MOD) = 245Hz

DC Gain(MOD) = 1 x 5 = 14dB

For the design example of Figure 9 the following modulator gain vs. frequency characteristic was measured as

shown in Figure 17.

Figure 17. Gain and Phase of Modulator

RLOAD = 5 Ohms and COUT = 130µF

Submit Documentation Feedback

Product Folder Links: LM5575

LM5575 Datasheet(PDF) 16 Page - Texas Instruments

LM5575 Datasheet(HTML) 16 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL