LTC3370_15 Datasheet, PDF (17/24 Pages) Linear Technology – 4-Channel 8A Configurable Buck DC/DCs

LTC3370
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 1.
Typical values for R1 range from 40kΩ to 1MΩ. 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
+
C_FF
C_OUT
R2
3370 F01
OPTIONAL
R1
Figure 1. Feedback Components
Buck Regulators
All four buck regulators are designed to be used with
inductors ranging from 1µH to 3.3µH depending on the
lowest switching frequency at which the buck regulator
must operate. When operating at 1MHz a 3.3µH inductor
should be used, while at 3MHz a 1µH inductor may be
used, or a higher value inductor may be used if reduced
current ripple is desired. Table 2 shows some recom-
mended inductors for the buck regulators. The bucks are
compensated to operate across the range of possible V_IN
and V_OUT voltages when the appropriate inductance is
used for the desired switching frequency.
The input supply should be decoupled with a 10µF capacitor
while the output should be decoupled with a 22µF capaci-
tor. Refer to the Capacitor Selection section for details on
selecting a proper capacitor.
Combined Buck Power Stages
The LTC3370 has eight power stages that can handle aver-
age load currents of 1A each. These power stages may be
combined in any one of eight possible combinations, via
the C1, C2, and C3 pins (see Table 1). Tables 3, 4, and 5
show recommended inductors for the combined power
stage configurations.
The input supply should be decoupled with a 22µF capacitor
while the output should be decoupled with a 47µF capaci-
tor 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 some cases it may be beneficial to use more power
stages than needed to achieve increased efficiency of the
active regulators. In general the efficiency will improve by
adding stages for any regulator running close to what the
rated load current would be without the additional stage.
For example, if the application requires a 1A regulator that
supplies close to 1A at a high duty cycle, a 3A regulator
that only peaks at 3A but averages a lower current, and
a 2A regulator that runs at 1.5A at a high duty cycle, bet-
ter efficiency may be achieved by using the 3A, 3A, 2A
configuration.
Input and Output Decoupling Capacitor Selection
The LTC3370 has individual input supply pins for each
buck power stage 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 compro-
mise 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 that 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 1, 4, 5, 8, 17, 20, 21, 24 and
29 all need to be decoupled with at least 10µF capacitors. If
power stages are combined the supplies should be shorted
with as short of a trace as possible, and the decoupling
capacitor should be scaled accordingly.
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3370f
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