LTC3732CUHF View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC3732CUHF

3-Phase, 5-Bit VID, 600kHz, Synchronous Buck Switching Regulator Controller

Linear Technology 

LTC3732

APPLICATIO S I FOR ATIO

A 2A to 8A Schottky is generally a good compromise for

both regions of operation due to the relatively small

average current. Larger diodes result in additional transi-

tion loss due to their larger junction capacitance.

CIN and COUT Selection

In continuous mode, the source current of each top

N-channel MOSFET is a square wave of duty cycle VOUT/VIN.

A low ESR input capacitor sized for the maximum RMS

current must be used. The details of a close form equation

can be found in Application Note 77. Figure 6 shows the

input capacitor ripple current for different phase configu-

rations with the output voltage fixed and input voltage

varied. The input ripple current is normalized against the

DC output current. The graph can be used in place of

tedious calculations. The minimum input ripple current

can be achieved when the product of phase number and

output voltage, N(VOUT), is approximately equal to the

input voltage VIN or:

VOUT = k where k = 1, 2, ..., N – 1

VIN N

So the phase number can be chosen to minimize the input

capacitor size for the given input and output voltages.

In the graph of Figure 4, the local maximum input RMS

capacitor currents are reached when:

VOUT = 2k – 1 where k = 1, 2, ..., N

VIN

N

These worst-case conditions are commonly used for de-

sign because even significant deviations do not offer much

relief. Note that capacitor manufacturer’s ripple current

ratings are often based on only 2000 hours of life. This

makes it advisable to further derate the capacitor or to

choose a capacitor rated at a higher temperature than re-

quired. Several capacitors may also be paralleled to meet

size or height requirements in the design. Always consult

the capacitor manufacturer if there is any question.

The Figure 6 graph shows that the peak RMS input current

is reduced linearly, inversely proportional to the number N

of stages used. It is important to note that the efficiency

loss is proportional to the input RMS current squared and

therefore a 3-stage implementation results in 90% less

power loss when compared to a single phase design.

Battery/input protection fuse resistance (if used), PC

board trace and connector resistance losses are also

reduced by the reduction of the input ripple current in a

PolyPhase system. The required amount of input capaci-

tance is further reduced by the factor, N, due to the

effective increase in the frequency of the current pulses.

0.6

0.5

1-PHASE

0.4

2-PHASE

3-PHASE

0.3

4-PHASE

6-PHASE

0.2

0.1

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

DUTY FACTOR (VOUT/VIN)

3732 F06

Figure 6. Normalized Input RMS Ripple Current

vs Duty Factor for One to Six Output Stages

Ceramic capacitors are becoming very popular for small

designs but several cautions should be observed. “X7R”,

“X5R” and “Y5V” are examples of a few of the ceramic

materials used as the dielectric layer, and these different

dielectrics have very different effect on the capacitance

value due to the voltage and temperature conditions

applied. Physically, if the capacitance value changes due

to applied voltage change, there is a concommitant piezo

effect which results in radiating sound! A load that draws

varying current at an audible rate may cause an attendant

varying input voltage on a ceramic capacitor, resulting in

an audible signal. A secondary issue relates to the energy

flowing back into a ceramic capacitor whose capacitance

value is being reduced by the increasing charge. The

voltage can increase at a considerably higher rate than the

constant current being supplied because the capacitance

value is decreasing as the voltage is increasing! Ceramic

capacitors, when properly selected and used however, can

provide the lowest overall loss due to their extremely low

ESR.

3732f

15

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