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LT3686

3686fa

ApplicAtions inForMAtion

Figure 16. Model for Loop Response

Frequency Compensation

The LT3686 uses current mode control to regulate the

output. This simplifies loop compensation. In particular,

the LT3686 does not require the ESR of the output capaci-

tor for stability allowing the use of ceramic capacitors to

achieve low output ripple and small circuit size. Figure 16

shows an equivalent circuit for the LT3686 control 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 current

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

outputcapacitorintegratesthiscurrent,andthatthecapaci-

torontheVCnode(CC)integratestheerroramplifieroutput

current, resulting in two poles in the loop. RC provides a

zero. With the recommended output capacitor, the loop

crossover occurs above the RCCC zero. This simple 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 phase lead

capacitor (CPL) across the feedback divider may improve

thephasemarginandtransientresponse.Largeelectrolytic

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 operating

conditions, including load current, input voltage and tem-

perature. The LT1375 data sheet contains a more thorough

discussion of loop compensation and describes how to

test the stability using a transient load.

800mV

CPL

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

GND

LT3686

160k

100pF

gm =

200µA/V

gm =

2A/V

ESR

3686 F16

OUT

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