LTC3374
17
3374fc
For more information www.linear.com/LTC3374
APPLICATIONS INFORMATION
Buck Switching Regulator Output Voltage
and Feedback Network
The output voltage of the buck switching regulators is
programmed by a resistor divider connected from the
switching regulator’s output to its feedback pin and is
given by V
OUT
= V
FB
(1 + R2/R1) as shown in Figure 2.
Typical values for R1 range from 40k to 1M. The buck
regulator transient response may improve with optional
capacitor C
FF
that helps cancel the pole created by the
feedback resistors and the input capacitance of the FB
pin. Experimentation with capacitor values between 2pF
and 22pF may improve transient response.
Figure 2. Feedback Components
BUCK
SWITCHING
REGULATOR
V
OUT
C
(OPTIONAL)
C
FF
R2
R1
FB
Buck Regulators
All eight buck regulators are designed to be used with
inductors ranging from 1µH to 3.3µH depending on the
lowest switching frequency that the buck regulator must
operate at. To operate at 1MHz a 3.3µH inductor should
be used, while to operate at 3MHz a 1µH inductor may be
used. Table 1 shows some recommended inductors for
the buck regulators.
The input supply needs to be decoupled
with a 10µF
capacitor while the output needs to be decoupled with a
22µF capacitor. Refer to the Capacitor Selection section
for details on selecting a proper capacitor.
Combined Buck Regulators
A single 2A buck regulator is available by combining two
adjacent 1A buck regulators together. Likewise a 3A or 4A
buck regulator is available by combining any three or four
adjacent buck regulators respectively. Tables 2, 3, and 4
show recommended inductors for these configurations.
The input supply needs to be decoupled with a 22µF capaci-
tor while the output needs to be decoupled with a 47µF
capacitor for a 2A combined buck regulator. Likewise for
3A and 4A configurations the input and output capacitance
must be scaled up to account for the increased load. Refer
to the Capacitor Selection section for details on selecting
a
proper capacitor.
In many cases, any extra unused buck converters may be
used to increase the efficiency of the active regulators.
In general the efficiency will improve for any regulators
running close to their rated load currents. If there are
unused regulators, the user should look at their specific
applications and current requirements to decide whether
to add extra stages.
Input and Output Decoupling Capacitor Selection
The
LTC3374 has individual input supply pins for each
buck switching regulator and a separate V
CC
pin that
supplies power to all top level control and logic. Each of
these pins must be decoupled with low ESR capacitors
to GND. These capacitors must be placed as close to
the pins as possible. Ceramic dielectric capacitors are a
good compromise between high dielectric constant and
stability versus temperature
and DC bias. Note that the
capacitance of a capacitor deteriorates at higher DC bias.
It is important to consult manufacturer data sheets and
obtain the true capacitance of a capacitor at the DC bias
voltage it will be operated at. For this reason, avoid the
use of Y5V dielectric capacitors. The X5R/X7R dielectric
capacitors offer good overall performance.
The input supply voltage
Pins 2/5, 5/8, 8/11, 11/14, 21/24,
24/27, 27/30, 30/33, and 35/38 (QFN/TSSOP packages)
all need to be decoupled with at least 10µF capacitors.
