LTC3602 datasheet(21/36 Pages) LINER

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LTC3613

3613fa

APPLICATIONS INFORMATION

TIME

Coincident Tracking

EXTERNAL

SUPPLY

EXTERNAL

SUPPLY

VOUT

TIME

3613 F06

Ratiometric Tracking

Figure 6. Two Different Modes of Output Tracking

Figure 7. Setup for Coincident and Ratiometric Tracking

RFB2

EXT. V

RFB1

Coincident Tracking Setup

TRACK/SS

RFB2

VOUT

TO VOSNS

RFB1

TO VOSNS

–

EXT. V

R1+ R2

≥

TRACK/SS

RFB2

VOUT

TO VOSNS

RFB1

TO VOSNS

–

3613 F07

Ratiometric Tracking Setup

0.6V

EXT. V

Phase and Frequency Synchronization

For applications that require better control of EMI and

switching noise or have special synchronization needs,

the LTC3613 can phase and frequency synchronize the

turn-on of the switching cycle to an external clock signal

applied to the MODE/PLLIN pin. The applied clock signal

needs to be within ±30% of the RT pin programmed free-

running frequency to assure proper frequency and phase

lock. The clock signal levels should generally comply to VIH

> 2V and VIL < 0.5V. The MODE/PLLIN pin has an internal

600k pull-down resistor to ensure pulse-skipping mode

if the pin is left floating.

The LTC3613 uses the voltages on SVIN and VOUT pins as

well as the RT programmed frequency to determine the

steady-state on-time as follows:

tON ≈

VOUT

VIN • f

An internal PLL system adjusts this on-time dynamically

in order to maintain phase and frequency lock with the

external clock. The LTC3613 will maintain phase and fre-

quency lock under steady-state conditions for VIN, VOUT

and load current.

As shown in the previous equation, the on-time is a

function of the switching regulator’s output. This output

is measured by the VOUT pin and is used to calculate the

required on-time. Therefore, simply connecting VOUT to

the regulator’s local output point is preferable for most

applications. However, there could be applications where

the internally calculated on-time differs significantly from

the real on-time required by the application. For example,

if there are differences between the local output point and

the remotely regulated output point due to line losses, then

the internally calculated on-time will be inaccurate. Lower

efficiencies in the switching regulator can also cause the

real on-time to be significantly different from the internally

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