LTC3313EVPBF datasheet(16/28 Pages) AD

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LTC3313

Rev. 0

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APPLICATIONS INFORMATION

The phasing of a slave phase relative to the master phase

is programmed with a resistor divider on the RT pin. Use

of 1% resistors is recommended. See Table 5 and Figure 2

for more information.

Table 5. LTC3313 Programming Slave Phase Angle

SYNC

PHASE

ANGLE

VALUE

EXAMPLE

0°

0Ω

None

0Ω

None

90°

3 • R

301k

100k

120°

7 • R

5 • R

243k

174k

180°

None

0Ω

None

0Ω

240°

5 • R

7 • R

174k

243k

270°

3 • R

100k

301k

When configured for master/slave operation, the slave

phases operate in forced continuous mode.

Figure 2.

LTC3313

VIN

3313 F02

AGND

Phase Programming

Mode of Operation

For most configurations, the LTC3313 operates in forced

continuous mode. While in forced continuous mode, reg-

ulation at low currents is achieved by allowing negative

inductor current. Switching cycles are not skipped.

The LTC3313 operates in pulse-skipping mode when both

the RT and MODE/SYNC pins are connected to VIN. In this

mode, the switching frequency is set with the nominal

2MHz internal clock. While in pulse-skipping mode nega-

tive current is disallowed and regulation at low currents

is achieved by skipping switching cycles.

Table 6. LTC3313 Single-Phase Configuration

RT PIN

CONNECTION

MODE/SYNC PIN

CONNECTION

MODE OF

OPERATION

SWITCHING

FREQUENCY

VIN

Clock Input

Forced

Continuous

External Clock

VIN

AGND

Forced

Continuous

2MHz Default

VIN

Pulse-

Skipping

2MHz Default

Resistor to AGND

Clock Output

Forced

Continuous

RT-Programmed

Synchronization

To synchronize the LTC3313 oscillator to an external fre-

quency, configure the MODE/SYNC pin as an input by con-

necting the RT pin to VIN. Drive the MODE/SYNC pin with a

square wave in the frequency range of 500kHz to 2.25MHz

with an amplitude swinging above 1.2V and below 0.4V,

and a pulse width greater than 40ns.

The LTC3313 phase-locked loop (PLL) will synchronize the

internal oscillator to the clock applied to the MODE/SYNC

pin. At start up, before the LTC3313 recognizes the external

clock applied to MODE/SYNC, the LTC3313 will switch at its

default frequency of 2MHz. Once the externally applied clock

is recognized, the switching frequency will gradually transi-

tion from the default frequency to the applied frequency. If the

external clock is removed, the LTC3313 will slowly transition

back to the default frequency.

The LTC3313 operates in forced continuous mode during

synchronization. An internal 200k resistor from the MODE/

SYNC pin to AGND allows the MODE/SYNC pin to be left

floating.

Transient Response and Loop Compensation

When determining the compensation components, CFF,

RC, and CC, control loop stability and transient response

are the two main considerations.

The LTC3313 has been designed to operate at a high band-

width for fast transient response capability. Operating at

a high loop bandwidth reduces the output capacitance

required to meet transient response requirements.

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