LTM4603 Просмотр технического описания (PDF) - Linear Technology

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LTM4603

6A DC/DC µModule with PLL, Output Tracking and Margining

Linear Technology 

LTM4603/LTM4603-1

APPLICATIO S I FOR ATIO

or choose a capacitor rated at a higher temperature than

required. Always contact the capacitor manufacturer for

derating requirements.

In Figure 18, the 10µF ceramic capacitors are together

used as a high frequency input decoupling capacitor. In a

typical 6A output application, two very low ESR, X5R or

X7R, 10µF ceramic capacitors are recommended. These

decoupling capacitors should be placed directly adjacent

to the module input pins in the PCB layout to minimize

the trace inductance and high frequency AC noise. Each

10µF ceramic is typically good for 2A to 3A of RMS ripple

current. Refer to your ceramics capacitor catalog for the

RMS current ratings.

Multiphase operation with multiple LTM4603 devices in

parallel will lower the effective input RMS ripple current due

to the interleaving operation of the regulators. Application

Note 77 provides a detailed explanation. Refer to Figure 2

for the input capacitor ripple current requirement as a func-

tion of the number of phases. The figure provides a ratio

of RMS ripple current to DC load current as a function of

duty cycle and the number of paralleled phases. Pick the

corresponding duty cycle and the number of phases to

arrive at the correct ripple current value. For example, the

2-phase parallel LTM4603 design provides 10A at 2.5V

output from a 12V input. The duty cycle is DC = 2.5V/12V

= 0.21. The 2-phase curve has a ratio of ~0.25 for a duty

cycle of 0.21. This 0.25 ratio of RMS ripple current to a

DC load current of 10A equals ~2.5A of input RMS ripple

current for the external input capacitors.

0.6

0.5

0.4

1-PHASE

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)

4603 F02

Figure 2. Normalized Input RMS Ripple Current

vs Duty Factor for One to Six Modules (Phases)

Output Capacitors

The LTM4603 is designed for low output voltage ripple.

The bulk output capacitors defined as COUT are chosen

with low enough effective series resistance (ESR) to meet

the output voltage ripple and transient requirements. COUT

can be a low ESR tantalum capacitor, a low ESR polymer

capacitor or a ceramic capacitor. The typical capacitance is

200µF if all ceramic output capacitors are used. Additional

output filtering may be required by the system designer,

if further reduction of output ripple or dynamic transient

spike is required. Table 2 shows a matrix of different output

voltages and output capacitors to minimize the voltage

droop and overshoot during a 2.5A/µs transient. The table

optimizes total equivalent ESR and total bulk capacitance

to maximize transient performance.

Multiphase operation with multiple LTM4603 devices in

parallel will lower the effective output ripple current due

to the interleaving operation of the regulators. For ex-

ample, each LTM4603’s inductor current of a 12V to 2.5V

multiphase design can be read from the “Inductor Ripple

versus Duty Cycle” (Figure 3). The large ripple current at

low duty cycle and high output voltage can be reduced

by adding an external resistor from fSET to ground which

increases the frequency. If we choose the duty cycle of

DC = 2.5V/12V = 0.21, the inductor ripple current for 2.5V

output at 21% duty cycle is ~2.5A in Figure 3.

5

2.5V OUTPUT

4

5V OUTPUT

1.8V OUTPUT

1.5V OUTPUT

3

1.2V OUTPUT

3.3V OUTPUT WITH

82.5k ADDED FROM

2

VOUT TO fSET

5V OUTPUT WITH

1

150k ADDED FROM

fSET TO GND

0

0

20

40

60

80

DUTY CYCLE (VOUT/VIN)

4603 F03

Figure 3. Inductor Ripple Current vs Duty Cycle

4603f

11

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