LT3471

9

3471fb

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

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

in both the output voltage and inductor current. A 33pF

is revealed in 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

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 supplemental

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. However, because of

take some time to charge up the bypass capacitor. During

this action provides an alternate means for soft-starting

the circuit. If the largest recommended bypass capacitor

is used, 1μF, the worst-case (longest) soft-start function

1μF • 1.00V

1.0mA

=1.0ms

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

longer than this time, or choose a smaller bypass capacitor

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

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.

APPLICATIONS INFORMATION

50μs/DIV

0.5A/DIV

AC/COUPLED

LOAD CURRENT

100mA/DIV

AC/COUPLED

200mV/DIV

AC COUPLED

50μs/DIV

0.5A/DIV

AC/COUPLED

LOAD CURRENT

100mA/DIV

AC/COUPLED

200mV/DIV

AC COUPLED