8

LT3471

3471f

ƒ=

Z

PL

RC

1

23

π ••

By choosing the appropriate values for the resistor and

capacitor, the zero frequency can be designed to improve

the phase margin of the overall converter. The typical

target value for the zero frequency is between 35kHz to

55kHz. Figure 3 shows the transient response of the step-

up converter from Figure 2 without the phase lead capaci-

margin is not ideal as evidenced by 2-3 “bumps” in both

the output voltage and inductor current. A 33pF capacitor

Figure 4 as a more damped response and less overshoot.

Figure 3. Transient Response of Figure 2’s Step-Up

Converter without Phase Lead Capacitor

Figure 4. Transient Response of Figure 2’s Step-Up

Converter with 33pF Phase Lead Capacitor

50µs/DIV

3471 F04

LOAD CURRENT

100mA/DIV

AC COUPLED

200mV/DIV

AC COUPLED

50µs/DIV

3471 F03

0.5A/DIV

AC COUPLED

LOAD CURRENT

100mA/DIV

AC COUPLED

200mV/DIV

AC COUPLED

0.5A/DIV

AC COUPLED

Pin 3 of the LT3471 is a bandgap voltage reference that has

been divided down to 1.00V and buffered for external use.

This pin must be bypassed with at least 0.01µF and no

more than 1µF. This will ensure stability as well as reduce

the noise on this pin. The buffer has a built-in current limit

of at least 1mA (typically 1.4mA). This not only means that

you can use this pin as an external reference for supple-

mental circuitry, but it also means that it is possible to

provide a soft-start feature if this pin is used as one of the

feedback pins for the error amplifier. Normally the soft-

start time will be dominated by the RC time constant

discussed in the soft-start and shutdown section. How-

ever, because of the finite current limit of the buffer for the

ramp up, and this action provides an alternate means for

soft-starting the circuit. If the largest recommended by-

pass capacitor is used, 1µF, the worst-case (longest) soft-

11 00

10

10

µ

=

FV

mA

ms

•.

.

.

Choose the RC network such that the soft-start time is

longer than this time, or choose a smaller bypass capaci-

the RC network dominates the soft-starting of the LT3471.

used for one of the feedback pins for the error amplifier.

This is especially useful in LED driver applications, where

the current through the LEDs is set using the voltage

reference across a sense resistor in the LED chain. Using

a smaller or divided down reference leads to less wasted

power in the sense resistor. See the Typical Applications

section for an example of LED driving applications.

DIODE SELECTION

A Schottky diode is recommended for use with the LT3471.

For high efficiency, a diode with good thermal character-

istics at high currents should be used such as the On