Datasheet

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
capacitor C
PL
. Although adequate for many applications,
phase margin is not ideal as evidenced by 2-3 “bumps”
in both the output voltage and inductor current. A 33pF
capacitor for C
PL
results in ideal phase margin, which
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
V
REG
VOLTAGE REFERENCE
Pin 3 of the LT3471 is a bandgap voltage reference that has
been divided down to 1.00V and buffered for external use.
T h i s p i n m u s t b e b y p a s s e d w i t h a t l e a s t 0 . 0 1μ F a n d n o m o r e
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 amplifi er. Normally the soft-start time
will be dominated by the RC time constant discussed in
the soft-start and shutdown section. However, because of
the fi nite current limit of the buffer for the V
REG
pin, it will
take some time to charge up the bypass capacitor. During
this time, the voltage at the V
REG
pin will ramp up, and
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
that would be provided from the V
REF
pin is:
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
for the V
REF
pin (but always larger than 0.01μF ) so that the
RC network dominates the soft-starting of the LT3471. The
voltage at the V
REF
pin can also be divided down and used
for one of the feedback pins for the error amplifi er. 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
I
L1
0.5A/DIV
AC/COUPLED
LOAD CURRENT
100mA/DIV
AC/COUPLED
V
OUT
200mV/DIV
AC COUPLED
V
IN
= 3.3V
V
IN
> V
SHDN/SS
50μs/DIV
I
L1
0.5A/DIV
AC/COUPLED
LOAD CURRENT
100mA/DIV
AC/COUPLED
V
OUT
200mV/DIV
AC COUPLED
V
IN
= 3.3V
V
IN
> V
SHDN/SS