LT3762 datasheet(21/34 Pages) LINER

21 / 34 page

LT3762

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

includes a spread spectrum frequency feature. If there

is a capacitor (CSSFM) at the SSFM pin, a triangle wave

sweeping between 1V and 2V is generated. This signal is

then fed into the internal oscillator to modulate the switch-

ing frequency between 70% of the base frequency and

the base frequency, which is set by the RT resistor. The

modulation frequency is set by 12µA / (2 • 1V • CSSFM).

Figure 10 shows the noise spectrum comparison between

aconventionalboostswitchingconverter(withtheLT3762

SSFM pin tied to GND) and a spread spectrum modula-

tion enabled boost switching converter with 6.8nF at the

SSFM pin. The results of EMI measurements are sensitive

to the SSFM frequency selected with the capacitor. 1kHz

is a good starting point to optimize peak measurements,

but some fine tuning of this selection may be necessary

to get the best overall EMI results in a particular system.

Consult factory application for more detailed information

about EMI reduction.

Duty Cycle Considerations

Switching duty cycle is a key variable in defining converter

operation, therefore, its limits must be considered when

programming the switching frequency for a particular ap-

plication. The fixed BG minimum on-time and minimum

off-time(seeFigure11)andtheswitchingfrequencydefine

the minimum and maximum duty cycle of the switch, re-

spectively.Thefollowingequationsexpresstheminimum/

maximum duty cycle:

Min Duty Cycle = min on-time • switching frequency

Max Duty Cycle = 1 – min off-time • switching frequency

When calculating the operating limits, the typical values

for on/off-time in the data sheet should be increased by

at least 100ns to allow margin for PWM comparator and

logic delays, TG & BG rise/fall times and SW node rise/

fall times.

(10b) Conducted Peak EMI Comparison

Figure 11. Typical Minimum On-Time and

Off-Time vs Temperature

SPREAD SPECTRUM

SPREAD SPECTRUM DISABLED

FREQUENCY (kHz)

500

900

1300

1700

2100

2500

3762 F10a

SPREAD SPECTRUM

SPREAD SPECTRUM DISABLED

FREQUENCY (kHz)

500

900

1300

1700

2100

2500

3762 F10b

Figure 10.

OFF TIME

ON TIME

TEMPERATURE (°C)

–50 –25

75 100 125 150

100

150

200

250

300

3762 F11

(10a) Conducted Average EMI Comparison

Synchronous vs Non-Synchronous Operation

The TG synchronous driver is designed to improve the

efficiency of the power converter by replacing the con-

ventional power diode with a power NMOS device. The

synchronous driver feature is enabled when the following

criteria are met:

1. The switch current sense resistor RSENSE is placed in

series with the inductor so that SNSP and SNSN can

monitor the power switch currents in both phases of

the switching cycle.

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