LTM4660 datasheet(14/28 Pages) AD

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LTM4660

Rev. 0

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APPLICATIONS INFORMATION

TIME

VOUT1

VOUT2

VOUT1

VOUT2

4660 F03

(a) Coincident Tracking

(b) Ratio-Metric Tracking

Figure 3. Two Different Methods of Output Voltage Tracking

60.4k (INTERNAL)

VOUT1

EXTERNAL

SUPPLY

TO TRACK/SS

PIN OF LTM4660

TO VFB PIN

OF LTM4660

VOUT1

EXTERNAL

SUPPLY

VOUT2

FROM LTM4660

(a) Coincident Tracking Setup

(b) Ratio-Metric Tracking Setup

4660 F04

VOUT2

FROM LTM4660

TO TRACK/SS

PIN OF LTM4660

TO VFB2

PIN OF

LTM4660

Figure 4. Setup for Coincident and Ratio-Metric Tracking

Burst Mode Operation, Pulse-Skipping Mode, or

Forced Continuous Mode

The LTM4660 can be enabled to enter high efficiency

Burst Mode operation, constant-frequency pulse-skipping

mode, or forced continuous mode. To select forced con-

tinuous operation, tie the MODE/PLLIN pin to SGND. To

select pulse-skipping mode of operation, tie the MODE/

PLLIN pin to INTVCC. To select Burst Mode operation,

float the MODE/PLLIN pin. When the controller is enabled

for Burst Mode operation, if the average inductor current

is higher than the load current, the error amplifier will

decrease the voltage on the COMPa pin. When the COMPa

voltage drops below 0.5V, the internal sleep signal goes

high (enabling sleep mode) and switching is turned off.

In sleep mode, the load current is supplied by the output

capacitor. As the output voltage decreases, the voltage

on COMPa pin begins to rise. When the output voltage

drops enough, the sleep signal goes low, and the control-

ler resumes normal operation by turning on the power

switches on the next cycle of the internal oscillator. When

a controller is enabled for Burst Mode operation, the con-

troller operates in discontinuous operation. In forced con-

tinuous operation, the peak inductor current is determined

by the voltage on the COMPa pin. Switching frequency

is constant as set by RFREQ resistor and inductor cur-

rent remains continuous through the switching period.

In this mode, the efficiency at light loads is lower than in

Burst Mode operation. However, continuous mode has

the advantages of faster response to load transients and

less interference with audio circuitry. When the MODE/

PLLIN pin is connected to INTVCC, the LTM4660 oper-

ates in PWM pulse-skipping mode at light loads. At very

light loads, switching is off for few numbers of cycles

(i.e., skipping pulses) and the part operates in discon-

tinuous mode. This mode, like forced continuous opera-

tion, exhibits low output ripple as well as low audio noise

and reduced RF interference as compared to Burst Mode

operation. At light loads, pulse-skipping mode provides

higher efficiency than forced continuous mode, but not

nearly as high as Burst Mode operation. Regardless of the

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