LTM4657 datasheet(9/24 Pages) AD

9 / 24 page

LTM4705

Rev. 0

For more information www.analog.com

Adding a resistor RFB from FB pin to GND programs the

output voltage (Equation 3).

RFB =

0.6V

VOUT – 0.6V

• 60.4k

(3)

Table 1. VFB Resistor Table vs Various Output Voltages

VOUT (V)

0.6

1.0

1.2

1.5

1.8

2.5

3.3

5.0

RFB (k)

OPEN 90.9

60.4

40.2

30.1

19.1

13.3

8.25

For parallel operation of N-channels LTM4705, Equation 4

can be used to solve for RFB:

RFB =

0.6V

VOUT – 0.6V

•

60.4k

(4)

Input Decoupling Capacitors

The LTM4705 module should be connected to a low

AC-impedance DC source. For each regulator channel,

one piece 10µF input ceramic capacitor is required for

RMS ripple current decoupling. A bulk input capacitor is

only needed when the input source impedance is com-

promised by long inductive leads, traces or not enough

source capacitance. The bulk capacitor can be an electro-

lytic aluminum capacitor and/or polymer capacitor.

Without considering the inductor current ripple, for each

output, the RMS current of the input capacitor can be

estimated using Equation 5.

ICIN(RMS) =

IOUT(MAX)

η%

• D • 1– D

(

)

(5)

where is the estimated efficiency of the power module.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth design,

only a single piece of 47µF low ESR output ceramic capac-

itor is required for each LTM4705 output to achieve low

output voltage ripple and very good transient response.

Additional output filtering may be required by the system

designer, if further reduction of output ripples or dynamic

transient spikes is required. Table 8 shows a matrix of dif-

ferent output voltages and output capacitors to minimize

the voltage droop and overshoot during a 2.5A (50%) load

step transient. Multiphase operation will reduce effec-

tive output ripple as a function of the number of phases.

Application Note 77 discusses this noise reduction versus

output ripple current cancellation, but the output capaci-

tance will be more a function of stability and transient

response. The Analog Devices LTpowerCAD® Design Tool

is available to download online for output ripple, stability

and transient response analysis and calculating the output

ripple reduction as the number of phases implemented

increases by N times.

Burst Mode Operation

In applications where high efficiency at intermediate cur-

rent is preferred over lower output voltage ripple, Burst

Mode operation could be used by connecting SYNC/

MODE pin to INTVCC to improve light load efficiency. In

Burst Mode operation, a current reversal comparator

(IREV) detects the negative inductor current and shuts off

the bottom power MOSFET, resulting in discontinuous

operation and increased efficiency. Both power MOSFETs

will remain off and the output capacitor will supply the

load current until the COMP voltage rises above the zero

current level to initiate another cycle.

Forced Continuous Current Mode (CCM) Operation

In applications where fixed frequency operation is more

critical than low current efficiency, and where the low-

est output ripple is desired, forced continuous opera-

tion should be used. Forced continuous operation can

be enabled by tying the SYNC/MODE pin to GND. In this

mode, inductor current is allowed to reverse during low

output loads, the COMP voltage is in control of the cur-

rent comparator threshold throughout. During start-up,

forced continuous mode is disabled and inductor current

is prevented from reversing until the LTM4705’s output

voltage is in regulation.

Operating Frequency

The operating frequency of the LTM4705 is optimized

to achieve the compact package size and the minimum

output ripple voltage while still keeping high efficiency.

The default operating frequency is internally set to 1MHz.

In most applications, no additional frequency adjusting

is required.

APPLICATIONS INFORMATION

LTM4657 Datasheet(PDF) 9 Page - Analog Devices

LTM4657 Datasheet(HTML) 9 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL