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LT3684

3684f

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 LT3684 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

Figure 3. Transient Load Response of the LT3684 Front Page

Application as the Load Current is Stepped from 500mA to

1500mA. VOUT = 3.3V

Figure 2. Model for Loop Response

APPLICATIONS INFORMATION

1.265V

GND

LT3684

3684 F02

OUTPUT

ESR

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

gm = 3.5mho

gm =

330

µmho

POLYMER

TANTALUM

CERAMIC

CPL

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.

BOOST and BIAS Pin Considerations

Capacitor C3 and the internal boost Schottky diode (see

the Block Diagram) are used to generate a boost volt-

age that is higher than the input voltage. In most cases

a 0.22µF capacitor will work well. Figure 2 shows three

ways to arrange the boost circuit. The BOOST pin must be

more than 2.3V above the SW pin for best efﬁciency. For

outputs of 3V and above, the standard circuit (Figure 4a)

is best. For outputs between 2.8V and 3V, use a 1µF boost

capacitor. A 2.5V output presents a special case because it

is marginally adequate to support the boosted drive stage

while using the internal boost diode. For reliable BOOST pin

operation with 2.5V outputs use a good external Schottky

diode (such as the ON Semi MBR0540), and a 1µF boost

capacitor (see Figure 4b). For lower output voltages the

boost diode can be tied to the input (Figure 4c), or to

another supply greater than 2.8V. The circuit in Figure 4a

is more efﬁcient because the BOOST pin current and BIAS

pin quiescent current comes from a lower voltage source.

You must also be sure that the maximum voltage ratings

of the BOOST and BIAS pins are not exceeded.

The minimum operating voltage of an LT3684 application

is limited by the minimum input voltage (3.6V) and by the

maximum duty cycle as outlined in a previous section. For

3684 F03

1A/DIV

VOUT

100mV/DIV

µs/DIV

VOUT = 12V, FRONT PAGE APPLICATION

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