LT3685E Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LT3685E

36V, 2A, 2.4MHz Step-Down Switching Regulator

Linear Technology 

LT3685

APPLICATIONS INFORMATION

forms a high quality (under damped) tank circuit. If the

LT3685 circuit is plugged into a live supply, the input volt-

age can ring to twice its nominal value, possibly exceeding

the LT3685’s rating. This situation is easily avoided (see

the Hot Plugging Safely section).

Frequency Compensation

The LT3685 uses current mode control to regulate the

output. This simplifies loop compensation. In particular, the

LT3685 does not require the ESR of the output capacitor

for stability, so you are free to use ceramic capacitors to

achieve low output ripple and small circuit size. Frequency

compensation is provided by the components tied to the

VC pin, as shown in Figure 2. Generally a capacitor (CC)

and a resistor (RC) in series to ground are used. In addi-

tion, there may be lower value capacitor in parallel. This

capacitor (CF) is not part of the loop 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. 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 LT3685 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.

LT3685

CURRENT MODE

POWER STAGE

gm = 3.5mho

SW

ERROR

AMPLIFIER

FB

gm =

420μmho

0.8V

3M

VC

GND

CF RC

CC

R1

CPL

OUTPUT

ESR

+

C1

POLYMER

OR

TANTALUM

R2

C1

CERAMIC

3685 F02

Figure 2. Model for Loop Response

VOUT

100mV/DIV

IL

0.5A/DIV

VIN = 12V; FRONT PAGE APPLICATION

10μs/DIV

3685 F03

Figure 3. Transient Load Response of the LT3685 Front Page

Application as the Load Current is Stepped from 500mA to

1500mA. VOUT = 3.3V

3685fb

13

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