lt3471edd-trpbf Linear Technology Corporation, lt3471edd-trpbf Datasheet - Page 8

• Manufacturer Part Number: lt3471edd-trpbf
• Description: Dual 1.3a, 1.2mhz Boost/inverter In 3mm ? 3mm Dfn
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LT3471
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-
tor C
margin is not ideal as evidenced by 2-3 “bumps” in both
the output voltage and inductor current. A 33pF capacitor
for C
Figure 4 as a more damped response and less overshoot.
LOAD CURRENT
LOAD CURRENT
8
Figure 4. Transient Response of Figure 2’s Step-Up
Converter with 33pF Phase Lead Capacitor
AC COUPLED
AC COUPLED
AC COUPLED
AC COUPLED
AC COUPLED
AC COUPLED
ƒ =
200mV/DIV
100mA/DIV
200mV/DIV
100mA/DIV
Z
Figure 3. Transient Response of Figure 2’s Step-Up
Converter without Phase Lead Capacitor
PL
PL
0.5A/DIV
0.5A/DIV
. Although adequate for many applications, phase
V
V
results in ideal phase margin, which is revealed in
OUT
OUT
I
I
2
L1
L1
π •
R
1
3
•
C
U
PL
INFORMATION
U
50µs/DIV
50µs/DIV
W
3471 F04
3471 F03
U
V
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
V
capacitor. During this time, the voltage at the V
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-
start function that would be provided from the V
Choose the RC network such that the soft-start time is
longer than this time, or choose a smaller bypass capaci-
tor for the V
the RC network dominates the soft-starting of the LT3471.
The voltage at the V
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
REG
REG
1
µ
VOLTAGE REFERENCE
pin, it will take some time to charge up the bypass
1 0
F
.
• .
1 00
mA
REF
V
pin (but always larger than 0.01µF) so that
=
1 0
.
REF
ms
pin can also be divided down and
REG
REF
pin will
pin is:
3471fa