3681 datasheet(13/24 Pages) LINER

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LT3681

3681f

Loop compensation determines the stability and transient

performance. Designing the compensation network is a

bit complicated and the best values depend on the ap-

plication 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 compensation network to optimize the performance.

Stability should then be checked across all operating

conditions, including load current, input voltage and

temperature. The LT1375 data sheet contains a more

thorough discussion of loop compensation and describes

how to test the stability using a transient load. Figure 2

shows an equivalent circuit for the LT3681 control loop.

The error ampliﬁer 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 pin. Note that

the output capacitor integrates this current, and that the

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

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

most cases a zero is required and comes from either the

output capacitor ESR or from a resistor RC in series with

CC. 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.

A phase lead capacitor (CPL) across the feedback divider

may improve the transient response. Figure 3 shows the

transient response when the load current is stepped from

500mA to 1500mA and back to 500mA.

Burst Mode Operation

To enhance efﬁciency at light loads, the LT3681 auto-

matically switches to Burst Mode operation which keeps

the output capacitor charged to the proper voltage while

minimizing the input quiescent current. During Burst Mode

operation, the LT3681 delivers single cycle bursts of current

to the output capacitor followed by sleep periods where

the output power is delivered to the load by the output

capacitor. In addition, VIN and BIAS quiescent currents are

reduced to typically 20µA and 50µA respectively during

the sleep time. As the load current decreases towards a

no load condition, the percentage of time that the LT3681

operates in sleep mode increases and the average input

current is greatly reduced resulting in higher efﬁciency.

See Figure 4.

1.265V

GND

3Meg

LT3681

3681 F02

OUTPUT

ESR

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

gm = 3.5mho

gm =

330

µmho

POLYMER

TANTALUM

CERAMIC

CPL

Figure 2. Model for Loop Response

Figure 3. Transient Load Response of the LT3681 3.3V Application

as the Load Current is Stepped from 500mA to 1500mA.

3681 F03

1A/DIV

VOUT

100mV/DIV

ms/DIV

VIN = 12V

APPLICATIONS INFORMATION

Figure 4. Burst Mode Operation

3681 F04

0.5A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

ms/DIV

VIN = 12V; VOUT = 3.3V

ILOAD = 10mA

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