LTC3416 datasheet(10/20 Pages) LINER

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LTC3617

3617fa

applications inForMation

The basic LTC3617 application circuit is shown in Figure 1.

Operating Frequency

Selection of the operating frequency is a trade-off between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing internal gate charge losses but requires larger

inductance values and/or capacitance to maintain low

output voltage ripple.

The operating frequency of the LTC3617 is determined

by an external resistor that is connected between the RT

pin and ground. The value of the resistor sets the ramp

current that is used to charge and discharge an internal

timing capacitor within the oscillator and can be calculated

by using the following equation:

RT =

3.82 • 1011Hz

fOSC Hz

( )

Ω – 16kΩ

Although frequencies as high as 4MHz are possible, the

minimum on-time of the LTC3617 imposes a minimum

limit on the operating duty cycle. The minimum on-time

is typically 80ns; therefore, the minimum duty cycle is

equal to 80ns • fOSC(Hz)•100%.

RUN

VDDQIN

PGOOD

ITH

SGND

PGND

VIN

2.5V

VTTR

SVIN

LTC3617

PVIN

CIN

22µF

×4

680pF

0.33µH

3617 F01

SYNC

VFB

COUT

100µF

VOUT

1.25V

±6A

0.1µF

VTTR

1.25V

±10mA

10k

365k

RPG

100k

CC1

10pF

(OPT)

Figure 1. 1.25V, ±6A at 1MHz from 2.5V

Tying the RT pin to SVIN sets the default internal operating

frequency to 2.25MHz ±20%.

Frequency Synchronization

The LTC3617’s internal oscillator can be synchronized to

an external frequency by applying a square wave clock

signal to the SYNC pin. During synchronization, the top

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

frequency source. The synchronization frequency range

is 300kHz to 4MHz.

The frequency set by the resistor on the RT pin should be

the same as the external clock frequency to ensure the

internal oscillator properly adjusts when the clock signal

is applied or removed.

LTC3617

SVIN

VIN

SYNC

SGND

LTC3617

SVIN

fOSC

2.25MHz

fOSC

1/TP

fOSC ∝1/RT

LTC3617

SVIN

VIN

SYNC

SGND

VIN

SYNC

SGND

1.2V

0.3V

3617 F02

0.4V

Figure 2. Setting Switching Frequency

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