LT1375CS8 datasheet(21/28 Pages) LINER

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LT1375/LT1376

APPLICATIONS INFORMATION

77dB at low frequency, rolling off to unity-gain at 20kHz.

Phase shows a two-pole characteristic until the ESR of the

output capacitor brings it back above 10kHz. Phase mar-

gin is about 60

° at unity-gain.

Analog experts will note that around 1kHz, phase dips very

close to the zero phase margin line. This is typical of switch-

ing regulators, especially those that operate over a wide

range of loads. This region of low phase is not a problem

as long as it does not occur near unity-gain. In practice, the

variability of output capacitor ESR tends to dominate all

other effects with respect to loop response. Variations in

ESR

will cause unity-gain to move around, but at the same

time phase moves with it so that adequate phase margin

is maintained over a very wide range of ESR (

≥ ±3:1).

What About a Resistor in the Compensation Network?

It is common practice in switching regulator design to add

a “zero” to the error amplifier compensation to increase

loop phase margin. This zero is created in the external

network in the form of a resistor (RC) in series with the

compensation capacitor. Increasing the size of this resis-

tor generally creates better and better loop stability, but

there are two limitations on its value. First, the combina-

tion of output capacitor ESR and a large value for RC may

cause loop gain to stop rolling off altogether, creating a

gain margin problem. An approximate formula for RC

where gain margin falls to zero is:

R Loop

ESR

OUT

Gain = 1

()=()( )( )( )

242

GMP = Transconductance of power stage = 2A/V

GMA = Error amplifier transconductance = 2 × 10–3

ESR = Output capacitor ESR

2.42 = Reference voltage

With VOUT = 5V and ESR = 0.1Ω, a value of 5.17k for RC

would yield zero gain margin, so this represents an upper

limit. There is a second limitation however which has

nothing to do with theoretical small signal dynamics. This

resistor sets high frequency gain of the error amplifier,

including the gain at the switching frequency. If switching

frequency gain is high enough, output ripple voltage will

appear at the VC pin with enough amplitude to muck up

proper operation of the regulator. In the marginal case,

subharmonic switching occurs, as evidenced by alternat-

ing pulse widths seen at the switch node. In more severe

cases, the regulator squeals or hisses audibly even though

the output voltage is still roughly correct. None of this will

show on a theoretical Bode plot because Bode is an

amplitude insensitive analysis.

Tests have shown that if

ripple voltage on the VC is held to less than 100mVP-P, the

LT1376 will be well behaved. The formula below will give

an estimate of VC ripple voltage when RC is added to the

loop, assuming that RC is large compared to the reactance

of CC at 500kHz.

ESR

VL f

C RIPPLE

OUT

(

) =

()( ) −

()( )( )

GMA = Error amplifier transconductance (2000µMho)

If a computer simulation of the LT1376 showed that a

series compensation resistor of 3k gave best overall loop

response, with adequate gain margin, the resulting VC pin

ripple voltage with VIN = 10V, VOUT = 5V, ESR = 0.1Ω,

L = 10

µH, would be:

C RIPPLE

(

)

−

()





−

()( )( )

()













2 10

10 5 0 1 2 4

10 10 10

500 10

0 144

•.

••

This ripple voltage is high enough to possibly create

subharmonic switching. In most situations a compromise

value (< 2k in this case) for the resistor gives acceptable

phase margin and no subharmonic problems. In other

cases, the resistor may have to be larger to get acceptable

phase response, and some means must be used to control

ripple voltage at the VC pin. The suggested way to do this

is to add a capacitor (CF) in parallel with the RC/CC network

on the VC pin. Pole frequency for this capacitor is typically

set at one-fifth of switching frequency so that it provides

significant attenuation of switching ripple, but does not

add unacceptable phase shift at loop unity-gain frequency.

With RC = 3k,

= ()( )( )= 





 ()

500 10

531

•

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