LTM4664 datasheet(63/138 Pages) AD

63 / 138 page

LTM4664A

Rev. 0

For more information www.analog.com

DUAL 25A/30A PSM APPLICATIONS INFORMATION

VIN TO VOUT STEP-DOWN RATIOS

There are restrictions in the maximum VIN and VOUT step-

down ratio that can be achieved for a given input voltage.

Each output of the LTM4664A PSM is capable of 95% duty

cycle at 500kHz, but the VIN to VOUT minimum dropout is

still a function of its load current and will limit output current

capability related to high duty cycle on the topside switch.

Minimum on-time tON(MIN) is another consideration in oper-

ating at a specified duty cycle while operating at a certain

frequency due to the fact that tON(MIN) < D/fSW, where D is

duty cycle and fSW is the switching frequency. tON(MIN) is

specified in the electrical parameters as 60ns. See Note 6

in the Electrical Characteristics section for output current

guideline. Since the LTM4664A front end 4:1 divider feeds

the two 25A/30A PSM channels with a VOUT2 range of 7.5V

to 14.5V, there should be no minimum on time issue.

INPUT CAPACITORS

The LTM4664A PSM channels should be connected to

a low AC impedance DC source. For the regulator input,

four 22µF input ceramic capacitors are used to handle

the RMS ripple current. A 47µF to 100µF surface mount

aluminum electrolytic bulk capacitor can be used for more

input bulk capacitance. This bulk input capacitor is only

needed if the input source impedance is compromised by

long inductive leads, traces or not enough source capaci-

tance. If low impedance power planes are used, then this

bulk capacitor is not needed.

For a buck converter, the switching duty-cycle can be

estimated as:

Dn =

VOUTn

VINn

Without considering the inductor current ripple, for each

output, the RMS current of the input capacitor can be

estimated as:

ICINn(RMS) =

IOUTn(MAX)

η%

• Dn • 1−Dn

(

)

In the above equation, η% is the estimated efficiency of the

power module. The bulk capacitor can be a switcher-rated

electrolytic aluminum capacitor, or a polymer capacitor.

OUTPUT CAPACITORS

The LTM4664A PSM channel outputs are designed for low

output voltage ripple noise and good transient response.

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 ceramic capacitor. The typical output capac-

itance range for each output is from 400µF to 1000µF.

Additional output filtering may be required by the system

designer, if further reduction of output ripple or dynamic

transient spikes is required. Table 12 shows a matrix of dif-

ferent output voltages and output capacitors to minimize

the voltage droop and overshoot during a 12.5A to 25A

step, 12A/µs transient each channel. Table 12 optimizes

total equivalent ESR and total bulk capacitance to optimize

the transient performance. Stability criteria are consid-

ered in the Table 12 matrix, and the LTpowerCAD Design

Tool will be provided for stability analysis. Multiphase

operation reduces effective output ripple as a function

of the number of phases. Application Note 77 discusses

this noise reduction versus output ripple current cancel-

lation, but the output capacitance should be considered

carefully as a function of stability and transient response.

The LTpowerCAD Design Tool can calculate the output

ripple reduction as the number of implemented phases

increases by N times. A small value 10Ω resistor can be

placed in series from VOUTn to the VOSNS0+ pin to allow for

a bode plot analyzer to inject a signal into the control loop

and validate the regulator stability. The LTM4664A PSM

stability compensation can be adjusted using two external

capacitors, and the MFR_PWM_COMP commands.

LIGHT LOAD CURRENT OPERATION

The LTM4664A PSM channels have two modes of oper-

ation including high efficiency, discontinuous conduction

mode or forced continuous conduction mode. The mode of

operation is configured by bit 0 of the MFR_PWM_MODEn

command (discontinuous conduction is always the start-up

mode, forced continuous is the default running mode).

If a channel is enabled for discontinuous mode operation,

the inductor current is not allowed to reverse. The reverse

LTM4664 Datasheet(PDF) 63 Page - Analog Devices

LTM4664 Datasheet(HTML) 63 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 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL