LTC3370 datasheet(16/24 Pages) LINER

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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 VOUT = VFB(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, CFF, 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.

the C1, C2, and C3 pins (see Table 1). Tables 3, 4, and 5

show recommended inductors for the combined power

stage configurations.

Theinputsupplyshouldbedecoupledwitha22µFcapacitor

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 VCC 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.Ceramicdielectriccapacitorsareagoodcompro-

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

powerstagesarecombinedthesuppliesshouldbeshorted

with as short of a trace as possible, and the decoupling

capacitor should be scaled accordingly.

BUCK

SWITCHING

REGULATOR

VOUT

3370 F01

CFF

OPTIONAL

COUT

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 VIN

and VOUT voltages when the appropriate inductance is

used for the desired switching frequency.

Theinputsupplyshouldbedecoupledwitha10µFcapacitor

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

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