LTM4659EVPBF datasheet(10/26 Pages) AD

10 / 26 page

LTM4659

Rev. 0

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APPLICATIONS INFORMATION

The frequency can be programmed to switch from 1MHz

to 3MHz. Table 2 shows the necessary RFREQ value for a

desired switching frequency.

Table 2. SW Frequency vs RFREQ Value

fSW

(MHz)

RFREQ

(kΩ)

549

274

2.2

243

178

Synchronizing the Oscillator to an External Clock

The LTM4659 switching frequency can also be adjusted

by synchronizing the internal PLL circuit to an external

clock to the MODE/SYNC pin. The synchronization fre-

quency range is 1MHz to 2.6MHz. The LTM4659 operates

in FCM when synchronized to an external clock.

Connect the FREQ pin to VIN configures the MODE/SYNC

pinasaclockinput.Duringsynchronization,thetoppower

switch turn-on is locked to the rising edge of the external

frequencysource.Theslopecompensationisautomatically

adapted to the external clock frequency.

At start-up, before the LTM4659 recognizes the external

clock applied to MODE/SYNC, the LTM4659 will switch at

its default frequency of 2MHz. Once the externally applied

clock is recognized, the switching frequency will gradu-

ally transition from the default frequency to the applied

frequency. If the external clock is removed, the LTM4659

will slowly transition back to the default frequency.

The synchronizing clock amplitude should be greater than

1.2V and less than 0.4V with a pulse width greater than

40ns. An internal 200k resistor on the MODE/SYNC pin

to AGND allows the MODE/SYNC to be floating. Note that

a low switching frequency will increase the inductor peak

current and the output voltage ripple.

transient response analysis and calculation of the output

ripple reduction as the number of phases implemented

increases by N times.

Modes of Operation

The MODE/SYNC pin either synchronizes the external

switching frequency, or a clock output, to set the PWM

mode. The PWM modes of operation are either pulse-

skipping (PSM) or forced continuous mode (FCM).

See Table 1.

Table 1. LTM4659 Single-Phase Configuration

FREQ PIN

CONNECTION

MODE/SYNC

PIN CONNECTION

MODE OF

OPERATION

SWITCHING

FREQUENCY

VIN

Clock Input

FCM

External

Clock

VIN

AGND

FCM

2MHz Default

VIN

PSM

2MHz Default

Resistor

to AGND

Clock Output

FCM

RFREQ

Programmed

The LTM4659 operates in FCM for low noise or PSM for

high efficiency at light load. The LTM4659 operates in

PSM when both FREQ and MODE/SYNC pins are con-

nected to VIN. In PSM, switching cycles are skipped at

light load to regulate the output voltage. The LTM4659

defaults to FCM in regulation and during synchroniza-

tion. During FCM, the top switch turns on every cycle

and light load regulation is achieved by allowing negative

inductor current.

Setting the Operating Frequency

The operating frequency defaults to 2MHz when the FREQ

pin is connected to VIN. If any frequency higher than the

default frequency is required, the frequency can be pro-

grammed by tying a resistor from the FREQ pin to AGND

using Equation 4.

RFREQ = 568 • f(SW)(–1.08)

(4)

where RFREQ is in kΩ and fSW is the desired switching

frequency in MHz.

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