LTC3374 Datasheet, PDF (17/24 Pages) Linear Technology – 8-Channel Parallelable 1A Buck DC/DCs

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.
V_OUT
BUCK
SWITCHING
REGULATOR
FB
+
R2
C_FF
C_OUT
(OPTIONAL)
R1
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Figure 2. Feedback Components
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.
PCB Considerations
When laying out the printed circuit board, the following
list should be followed to ensure proper operation of the
LTC3374:
1. The exposed pad of the package (Pin 39) should connect
directly to a large ground plane to minimize thermal and
electrical impedance.
For more information www.linear.com/LTC3374
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