LTC3589 datasheet(26/36 Pages) LINER

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LTC3375

3375fa

For more information www.linear.com/3375

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 VOUT = VFB(1 + R2/R1) as shown in Figure 5.

Typical values for R1 range from 40k to 1M. 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.

Figure 5. Feedback Components

BUCK

SWITCHING

REGULATOR

VOUT

COUT

(OPTIONAL)

CFF

3375 F05

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 3 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 Capacitor Selection for details on

selecting a proper capacitor.

EachbuckregulatorcanbeprogrammedviaI2C.Toprogram

buck regulator 1 use sub-address 01h, buck regulator 2

sub-address 02h, buck regulator 3 sub-address 03h, buck

regulator4subaddress04h,buckregulator5sub-address

05h, buck regulator 6 sub-address 06h, buck regulator 7

sub-address 07h, and buck regulator 8 sub-address 08h.

The bit format is explained in Table 7.

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 4, 5, and 6

show recommended inductors for these configurations.

Theinputsupplyneedstobedecoupledwitha22µFcapaci-

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 Capacitor Selection in the Applications Information sec-

tion 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.

Table 3. Recommended Inductors for 1A Buck Regulators

PART NUMBER

L (µH)

MAX IDC (A)

MAX DCR (MΩ)

SIZE IN mm (L

× W × H) MANUFACTURER

IHLP1212ABER1R0M-11

1.0

× 3.6 × 1.2

Vishay

1239AS-H-1R0N

2.5

× 2.0 × 1.2

Toko

XFL4020-222ME

2.2

3.5

23.5

× 4 × 2.1

CoilCraft

1277AS-H-2R2N

2.2

2.6

3.2

× 2.5 × 1.2

Toko

IHLP1212BZER2R2M-11

2.2

× 3.6 × 1.2

Vishay

XFL4020-332ME

3.3

2.8

38.3

× 4 × 2.1

CoilCraft

IHLP1212BZER3R3M-11

3.3

2.7

× 3.6 × 1.2

Vishay

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