LT8331 datasheet(14/24 Pages) AD

14 / 24 page

LT8337/LT8337-1

Rev. 0

For more information www.analog.com

Spread Spectrum Frequency Modulation

The LT8337/LT8337-1 features spread spectrum fre-

quency modulation to further reduce EMI emissions. The

user can select spread spectrum frequency modulation

with Burst Mode operation by connecting the SYNC/MODE

pin to ground through a 50k resistor, or spread spectrum

frequency modulation with pulse-skipping operation by

connecting the SYNC/MODE pin to INTVCC. When spec-

trum frequency modulation is selected, a stepped trian-

gular frequency modulation is used to vary the internal

oscillator frequency between the value programmed by

the RT resistor to approximately 20% higher than that

value. The modulation frequency is approximately 0.45%

of the switching frequency. For example, when the IC is

programmed to 2MHz, and spread spectrum frequency

modulation is selected, the oscillator frequency var-

ies from 2MHz to 2.4MHz at a 9kHz rate (see Oscillator

Frequency with Spread Spectrum Modulation curve in the

Typical Performance Characteristics section). When oper-

ating at light load, the spread spectrum frequency modu-

lation is more effective in pulse-skipping mode than in

Burst Mode operation, due to the fact that pulse-skipping

operation maintains the programmed switching frequency

down to a much lower load current as compared to Burst

Mode operation.

VIN to VOUT PassThru Mode Operation

In the boost pre-regulator applications for automotive

stop-start and cold crank, VIN is normally above the regu-

lated VOUT voltage. In this condition, LT8337/LT8337-1

enters PassThru operation. LT8337/LT8337-1 is designed

to have an accurate, well controlled PassThru operation

with low quiescent current consumption. If VIN transiently

falls below the VOUT regulation setpoint, the boost con-

verter commences switching to maintain the output volt-

age in regulation.

As shown in Block Diagram, VIN is compared with VOUT

using the comparator A3 with 0.6V hysteresis. When

VIN rises above VOUT (causing A3’s output high), and at

the same time VOUT is higher than its regulation voltage

programmed by the FB resistor network, the IC boost

converter enters PassThru operation, where the synchro-

nous power switch M2 is kept on continuously and the

power switch M1 is kept off continuously. The voltage

across the boost capacitor (CBST), VBST_SW, is constantly

monitored. When VBST_SW drops below 3.2V, an inter-

nal charge pump is turned on to charge VBST_SW up to

3.6V, and then turned off. In PassThru mode the VOUT is

essentially shorted to VIN by the inductor and M2, and

VIN pin quiescent current is limited to 15µA (LT8337)

or 30µA (LT8337-1) regardless of the SYNC/MODE pin’s

configuration. VOUT pin draws 30µA (typ). A typical

waveforms drawing is shown in the Typical Performance

Characteristics section.

Several conditions cause the IC to exit from the PassThru

mode operation. First, when VOUT drops below its regula-

tion voltage programmed by the FB resistor network, the

IC exits from PassThru mode operation and normal boost

switching operation resumes to maintain the regulated

VOUT voltage. Second, when VOUT is still higher than its

regulation voltage but VIN drops below VOUT by the com-

parator A3’s hysteresis of 0.6V (typ) or more to cause

A3’s output low, M2 is turned off to prevent the reverse

current from VOUT to VIN from ramping up. IC is back to

the PassThru mode when A3’s output is high again. Third,

when VOUT is still higher than its regulation voltage but

M2’s reverse current (flowing from its drain to source)

rises above 1.5A (typ), M2 is turned off to prevent the

reverse current from VOUT to VIN from ramping up. The

IC re-enters the PassThru mode when A3’s output is high

again. Waveforms for typical reverse current protection

are shown in the Typical Performance Characteristics

section.

To ensure the PassThru mode operation works properly,

the IC’s VIN pin must be connect to the input of the power

stage (the input terminal of inductor as shown in Block

Diagram).

APPLICATIONS INFORMATION

LT8331 датащи(PDF) 14 Page - Analog Devices

LT8331 датащи(HTML) 14 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

датащи скачать

ссылки URL