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LT3505

3505fc

APPLICATIONS INFORMATION

capacitor (CC) and a resistor (RC) in series to ground are

used. In addition, a lower value ﬁlter capacitor (CF) may be

addedinparallel.Theﬁltercapacitorisnotapartoftheloop

compensation but is used to ﬁlter noise at the switching

frequency, and is required only if a phase-lead capacitor

is used or if the output capacitor has high ESR.

Loop compensation determines the stability and transient

performance.Designingthecompensationnetworkisabit

complicated and the best values depend on the application

and in particular the type of output capacitor. A practical

approach is to start with one of the circuits in this data

sheet that is similar to your application and tune the com-

pensation network to optimize the performance. Stability

should then be checked across all operating conditions,

including load current, input voltage and temperature. The

LT1375datasheetcontainsamorethoroughdiscussionof

loop compensation and describes how to test the stability

using a transient load.

Figure11showsanequivalentcircuitfortheLT3505control

loop. The error amp is a transconductance ampliﬁer with

ﬁnite output impedance. The power section, consisting of

the modulator, power switch and inductor, is modeled as

a transconductance ampliﬁer generating an output cur-

rent proportional to the voltage at the VC node. Note that

the output capacitor integrates this current and that the

capacitor on the VC node (CC) integrates the error ampli-

ﬁer output current, resulting in two poles in the loop. RC

provides a zero. With the recommended output capacitor,

theloopcrossoveroccursabovetheRCCCzero.Thissimple

model works well as long as the value of the inductor is

not too high and the loop crossover frequency is much

lower than the switching frequency. With a larger ceramic

capacitor (very low ESR), crossover may be lower and a

phaseleadcapacitor(CPL)acrossthefeedbackdividermay

improve the phase margin and transient response. Large

electrolytic capacitors may have an ESR large enough to

create an additional zero and the phase lead may not be

necessary.

If the output capacitor is different than the recommended

capacitor, stability should be checked across all operat-

ing conditions, including load current, input voltage and

temperature.

PCB Layout

ForproperoperationandminimumEMI,caremustbetaken

during printed circuit board layout. Figure 12 shows the

recommended component placement with trace, ground

plane and via locations. Note that large, switched currents

ﬂow in the LT3505’s VIN and SW pins, the catch diode (D1)

and the input capacitor (C2). The loop formed by these

components should be as small as possible and tied to

SYSTEM

GROUND

VOUT

3505 F12

SHUTDOWN

VIN

: VIAS TO LOCAL GROUND PLANE

: OUTLINE OF LOCAL GROUND PLANE

SIGNAL

GROUND

POWER

GROUND

BOOST

Figure 12. A Good PCB Layout Ensures Proper, Low EMI Operation

780mV

LT3505

3505 F11

OUT

ESR

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

gm =

200

µA/V

gm =

1.1A/V

CPL

0.8V

Figure 11. Model for Loop Response

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