LTM4657 datasheet(11/28 Pages) AD

11 / 28 page

LTM4657

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

capacitor or tantalum-polymer capacitor is required due

to variation of actual capacitance over bias voltage and

temperature. Table 7 shows a matrix of different output

voltages and output capacitors to minimize the voltage

droop and overshoot during a 2A load-step transient.

Additional output filtering may be required by the sys-

tem designer if further reduction of output ripple or

dynamic transient spikes is required. The Analog Devices

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-

parator may remain tripped for several cycles and force the

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 low-

est output ripple is desired, forced continuous opera-

tion should be used. Forced continuous operation can

be enabled by tying the MODE/CLKIN pin to INTVCC. In

this mode, inductor current is allowed to reverse during

low output loads, the COMP voltage is in control of the

current comparator threshold throughout, and the top

MOSFET always turns on with each oscillator pulse.

During start-up, forced continuous mode is disabled and

inductor current is prevented from reversing until the

LTM4657’s output voltage is in regulation.

Operating Frequency

The operating frequency of the LTM4657 is optimized

to achieve the compact package size and the minimum

output ripple voltage while still keeping high efficiency.

The default operating frequency is 500kHz. In most appli-

cations, no additional frequency adjustment is required.

If an operating frequency other than 500kHz is required by

the application, the operating frequency can be increased

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

GND, as shown in Figure 24. The operating frequency

can be calculated as:

f Hz

( )= 1.67•10

332k||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 detec-

tor. This allows the internal top MOSFET turn-on to be

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

nal 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 8A of current,

multiple LTM4657s 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 LTM4657 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 LTM4657

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°

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