LTM4609 datasheet(13/38 Pages) LINER

13 / 38 page

LTM4636-1

46361fa

For more information www.linear.com/LTM4636-1

APPLICATIONS INFORMATION

Without considering the inductor ripple current, for each

output the RMS current of the input capacitor can be

estimated as:

ICIN(RMS)

IOUT(MAX)

• D •(1– D)

where % is the estimated efficiency of the power mod-

ule. The bulk capacitor can be a switcher-rated aluminum

electrolytic capacitor or a Polymer capacitor.

Output Capacitors

The LTM4636-1 is designed for low output voltage ripple

noise. The bulk output capacitors defined as COUT are

chosen with low enough effective series resistance (ESR)

to meet the output voltage ripple and transient require-

ments. COUT can be a low ESR tantalum capacitor, low

ESR Polymer capacitor or ceramic capacitors. The typi-

cal output capacitance range 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 5 shows a matrix of dif-

ferent output voltages and output capacitors to minimize

the voltage droop and overshoot during a 15A/µs tran-

sient. The table optimizes total equivalent ESR and total

bulk capacitance to optimize the transient performance.

Stability criteria are considered in the Table 5 matrix, and

LTpowerCAD is available for stability analysis. Multiphase

operation will reduce 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.

LTpowerCAD can be used to calculate the output ripple

reductionasthenumberofimplementedphasesincreases

by N times. External loop compensation can be used for

transient response optimization.

Burst Mode Operation

The LTM4636-1 is capable of Burst Mode operation in

which the power MOSFETs operate intermittently based

on load demand, thus saving quiescent current. For ap-

plications where maximizing the efficiency at very light

loads is a high priority, Burst Mode operation should be

applied. To enable Burst Mode operation, simply float

the MODE_PLLIN pin. During Burst Mode operation, the

peak current of the inductor is set to approximately 30%

of the maximum peak current value in normal operation

even though the voltage at the COMPA pin indicates a

lower value. The voltage at the COMPA pin drops when

the inductor’s average current is greater than the load

requirement. As the COMPA voltage drops below 0.5V,

the burst comparator trips, causing the internal sleep line

to go high and turn off both power MOSFETs.

In sleep mode, the internal circuitry is partially turned

off, reducing the quiescent current. The load current is

now being supplied from the output capacitors. When the

output voltage drops, causing COMPA to rise, the internal

sleep line goes low, and the LTM4636-1 resumes normal

operation. The next oscillator cycle will turn on the top

power MOSFET and the switching cycle repeats.

Pulse-Skipping Mode Operation

In applications where low output ripple and high effi-

ciencyatintermediatecurrentsaredesired,pulse-skipping

mode should be used. Pulse-skipping operation allows

the LTM4636-1 to skip cycles at low output loads, thus

increasing efficiency by reducing switching loss. Tying

the MODE_PLLIN pin to INTVCC enables pulse-skipping

operation. With pulse-skipping mode at light load, the

internalcurrentcomparatormayremaintrippedforseveral

cycles, thus skipping operation cycles. This mode has

lower ripple than Burst Mode operation and maintains a

higher frequency operation than Burst Mode operation.

Forced Continuous Operation

In applications where fixed frequency operation is more

critical than low current efficiency, and where the lowest

outputrippleisdesired,forcedcontinuousoperationshould

be used. Forced continuous operation can be enabled

by tying the MODE_PLLIN pin to ground. In this mode,

inductor current is allowed to reverse during low output

loads, the COMPA voltage is in control of the current

comparator threshold throughout, and the top MOSFET

alwaysturnsonwitheachoscillatorpulse.Duringstart-up,

forced continuous mode is disabled and inductor current

is prevented from reversing until the LTM4636-1’s output

voltage is in regulation.

LTM4609 Datasheet(PDF) 13 Page - Linear Technology

LTM4609 Datasheet(HTML) 13 Page - Linear Technology

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

Datasheet Download

Link URL