LT8331 datasheet(10/24 Pages) AD

10 / 24 page

LT8334

Rev. 0

For more information www.analog.com

INTVCC REGULATOR

A low dropout (LDO) linear regulator, supplied from VIN,

produces a 3.2V supply at the INTVCC pin. A minimum 1µF

low ESR ceramic capacitor must be used to bypass the

INTVCC pin to ground to supply the high transient currents

required by the internal power MOSFET gate driver.

No additional components or loading is allowed on this

pin. The INTVCC rising threshold (to allow soft-start and

switching) is typically 2.65V. The INTVCC falling threshold

(to stop switching and reset soft-start) is typically 2.5V.

To improve efficiency across all loads, the major-

ity of INTVCC current can be drawn from the BIAS pin

(4.4V ≤ BIAS ≤ VIN) instead of the VIN pin. For SEPIC

applications with VIN often greater than VOUT, the BIAS

pin can be directly connected to VOUT. If the BIAS pin is

connected to a supply other than VOUT, be sure to bypass

the pin with a local ceramic capacitor.

PROGRAMMING SWITCHING FREQUENCY

The LT8334 uses a constant frequency PWM architec-

ture that can be programmed to switch from 300kHz to

2MHz by using a resistor tied from the RT pin to ground.

Table 1 shows the necessary RT value for a desired

switching frequency.

The RT resistor required for a desired switching frequency

can be calculated with Equation 2.

RT =

51.2

fOSC

– 5.6

(2)

where RT is in kΩ and fOSC is the desired switching fre-

quency in MHz.

Table 1. SW Frequency vs RT Value

fOSC (MHz)

RT (kΩ)

0.3

165

0.45

107

0.75

63.4

45.3

1.5

28.7

SYNCHRONIZATION AND MODE SELECTION

To select low ripple Burst Mode operation, for high effi-

ciency at light loads, tie the SYNC/MODE pin below 0.14V

(this can be ground or a logic low output).

To synchronize the LT8334 oscillator to an external fre-

quency connect a square wave (with 20% to 80% duty

cycle) to the SYNC pin. The square wave amplitude should

have valleys that are below 0.4V and peaks above 1.7V

(up to 6V). The LT8334 will not enter Burst Mode opera-

tion at low output loads while synchronized to an external

clock, but instead will pulse skip to maintain regulation.

The LT8334 may be synchronized over a 300kHz to 2MHz

range. The RT resistor should be chosen to set the LT8334

switching frequency equal to or below the lowest synchro-

nization input. For example, if the synchronization sig-

nal will be 500kHz and higher, the RT should be selected

for 500kHz.

For some applications, it is desirable for the LT8334 to

operate in pulse-skipping mode, offering two major dif-

ferences from Burst Mode operation. Firstly, the clock

stays always awake and all switching cycles are aligned to

the clock. Secondly, the full switching frequency is main-

tained at lower output load than in Burst Mode operation.

These two differences come at the expense of increased

quiescent current. To enable pulse-skipping mode, float

the SYNC pin.

To improve EMI/EMC, the LT8334 can provide spread

spectrum frequency modulation (SSFM). This feature var-

ies the clock with a triangle frequency modulation of 20%.

For example, if the LT8334’s frequency was programmed

to switch at 2MHz, spread spectrum mode will modulate

the oscillator between 2MHz and 2.4MHz. The 20% mod-

ulation will occur at a frequency: fOSC/256 where fOSC is

the switching frequency programmed using the RT pin.

The LT8334 can also be configured to operate in

pulse-skipping/SSFM mode by tying the SYNC/MODE pin

above 1.7V. The LT8334 can also be configured for Burst

Mode operation at light loads (for improved efficiency)

and SSFM at heavy loads (for low EMI) by tying a 100k

from the SYNC/MODE pin to GND.

APPLICATIONS INFORMATION

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