LTM4646 datasheet(12/30 Pages) AD

12 / 30 page

LTM4638

Rev. C

For more information www.analog.com

APPLICATIONS INFORMATION

cold or hot temperature or high output voltage case, ad-

ditional ceramic capacitor or tantalum-polymer capacitor

is required due to variation of actual capacitance over bias

voltage and temperature. Table 7 shows a matrix of dif-

ferent output voltages and output capacitors to minimize

the voltage droop and overshoot during a 4A load-step

transient. Additional output filtering may be required by

the system designer if further reduction of output ripple or

dynamictransientspikesisrequired.TheLinearTechnology

LTpowerCAD™ design tool is available to download online

for output ripple, stability and transient response analysis

for further optimization.

Discontinuous Current Mode (DCM)

In applications where low output ripple and high efficiency

at intermediate current are desired, discontinuous current

mode (DCM) should be used by connecting the MODE/

CLKIN pin to GND. At light loads the internal current com-

paratormayremaintrippedforseveralcyclesandforcethe

top MOSFET to stay off for several cycles, thus skipping

cycles. The inductor current does not reverse in this mode.

Forced Continuous Current Mode (CCM)

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/CLKIN pin to INTVCC. In this mode, induc-

tor current is allowed to reverse during low output loads,

the COMP voltage is in control of the current comparator

thresholdthroughout,andthetopMOSFETalwaysturnson

witheachoscillatorpulse.Duringstart-up,forcedcontinuous

mode is disabled and inductor current is prevented from

reversinguntiltheLTM4638’soutputvoltageisinregulation.

Operating Frequency

The operating frequency of the LTM4638 is optimized

to achieve the compact package size and the minimum

out-put ripple voltage while still keeping high efficiency.

The default operating frequency is 600kHz. In most ap-

plications,noadditionalfrequencyadjustmentisrequired.

If an operating frequency other than 600kHz is required by

the application, the operating frequency can be increased

by adding a resistor, RFSET, between the FREQ pin and

SGND, as shown in Figure 24. The operating frequency

can be calculated as:

f Hz

( )=

1.67e11

274k ||RFSET Ω

( )

The programmable operating frequency range is from

400kHz to 3MHz.

Frequency Synchronization and Clock In

The power module has a phase-locked loop comprised

of an internal voltage controlled oscillator and a phase

detector.ThisallowstheinternaltopMOSFETturn-ontobe

locked to the rising edge of the external clock. The external

clock frequency range must be within ±30% around the

resistor set operating frequency. A pulse detection circuit

is used to detect a clock on the CLKIN pin to turn on the

phase-locked loop. The pulse width of the clock has to

be at least 100ns. The clock high level must be above 1V

and clock low level below 0.3V. During the start-up of

the regulator, the phase-locked loop function is disabled.

Multiphase Operation

For output loads that demand more than 15A of current,

multiple LTM4638s can be paralleled to run out of phase

to provide more output current without increasing input

and output voltage ripples.

The CLKOUT signal can be connected to the MODE/CLKIN

pin of the following LTM4638 stage to line up both the

frequency and the phase of the entire system. Tying the

PHMODE pin to INTVCC, GND or FLOAT generates a phase

difference (between CLKIN and CLKOUT) of 180°, 120°, or

90° respectively, which corresponds to 2-phase, 3-phase

or 4-phase operation. A total of 6 phases can be cascaded

to run simultaneously out of phase with respect to each

other by programming the PHMODE pin of each LTM4638

to different levels. Figure 3 shows a 4-phase design and

a 6-phase design example for clock phasing.

Table 2. PHMODE Pin Status and Corresponding Phase

Relationship (Relative to CLKIN)

PHMODE

INTVCC

GND

FLOAT

CLKOUT

180°

120°

90°

LTM4646 Datasheet(PDF) 12 Page - Analog Devices

LTM4646 Datasheet(HTML) 12 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

Datasheet Download

Link URL