LTM4609EVPBF datasheet(10/24 Pages) LINER

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LTM4609

4609fa

locked loop comprised of an internal voltage controlled

oscillator and a phase detector. This allows turning on the

internal top MOSFET for locking to the rising edge of the

external clock. A pulse detection circuit is used to detect

a clock on the PLLIN pin to turn on the phase-lock loop.

The input pulse width of the clock has to be at least 400ns,

and 2V in amplitude. The synchronized frequency ranges

from 200kHz to 400kHz, corresponding to a DC voltage

input from 0V to 2.4V at PLLFLTR. During the start-up of

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

APPLICATIONS INFORMATION

Figure 2. Frequency vs PLLFLTR Pin Voltage

Low Current Operation

To improve the efﬁciency at low current operation, LTM4609

provides three modes for both buck and boost operations

by accepting a logic input on the FCB pin. Table 2 shows

the different operation modes.

Table 2. Different Operating Modes

FCB PIN

BUCK

BOOST

0V to 0.75V

Force Continuous Mode

0.85V to 5V

Skip-Cycle Mode

Burst Mode Operation

>5.3V

DCM with Constant Freq

When the FCB pin voltage is lower than 0.8V, the controller

behaves as a continuous, PWM current mode synchronous

switching regulator. When the FCB pin voltage is below

VINTVCC – 1V, but greater than 0.8V, the controller enters

Burst Mode operation in boost operation or enters skip-

cycle mode in buck operation. During boost operation,

Burst Mode operation is activated if the load current is

lower than the preset minimum output current level. The

MOSFETs will turn on for several cycles, followed by a

variable “sleep” interval depending upon the load current.

During buck operation, skip-cycle mode sets a minimum

positive inductor current level. In this mode, some cycles

will be skipped when the output load current drops below

1% of the maximum designed load in order to maintain

the output voltage.

When the FCB pin voltage is tied to the INTVCC pin, the

controller enters constant frequency discontinuous current

mode (DCM). For boost operation, if the output voltage is

high enough, the controller can enter the continuous current

buck mode for one cycle to discharge inductor current. In

the following cycle, the controller will resume DCM boost

operation. for buck operation, constant frequency discon-

tinuous current mode is turned on if the preset minimum

negative inductor current level is reached. At very light

loads, this constant frequency operation is not as efﬁcient

as Burst Mode operation or skip-cycle, but does provide

low noise, constant frequency operation.

Input Capacitors

In boost mode, since the input current is continuous, only

minimum input capacitors are required. However, the input

current is discontinuous in buck mode. So the selection

of input capacitor CIN is driven by the need of ﬁltering the

input square wave current.

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

estimated as:

VOUT

VIN

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

ICIN(RMS) =

IOUT(MAX)

•D •(1−D)

In the above equation,

η is the estimated efﬁciency of the

power module. CIN can be a switcher-rated electrolytic

aluminum capacitor, OS-CON capacitor or high volume

ceramic capacitors. Note the capacitor ripple current rat-

PLLFLTR PIN VOLTAGE (V)

0.5

2.0

450

400

350

300

250

200

150

100

4609 F02

1.0

1.5

2.5

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