LT3756 datasheet(17/31 Pages) AD

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LT8355-1

Rev. A

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APPLICATIONS INFORMATION

Figure 5. Maximum Duty as a Function of Switching Frequency

CHANNEL 1

CHANNEL 2

CHANNEL 1 SWITCHING FREQUENCY (Hz)

100k

8355-1 F05

Table 2. Recommended PWM Voltage for Selected PWM Duties

PWM DUTY

PWM PIN VOLTAGE

0.560V

0.880V

10%

1.022V

14.75%

1.095V

20%

1.160V

25%

1.205V

33.3%

1.260V

50%

1.345V

76.5%

1.430V

100%

>1.6V

Spread Spectrum Frequency Modulation

The internal spread spectrum frequency modulation is

always enabled in the LT8355-1. The modulating wave-

form is a triangle wave with steps at the positive and neg-

ative peaks. The modulation frequency is around 2kHz,

and the switching frequency range is from 100% to 125%

of the programmed value. If spread spectrum frequency

modulation does not provide sufficient EMI attenuation

even with good PCB design, consider slowing the switch

turn-on speed by placing a 5Ω to 10Ω resistor between

the GATE pin and the external NMOS switch. Please refer

to the EVAL-LT8355-1-AZ evaluation circuit for a low EMI

reference design and layout.

Maximum Duty Ratio

Since LT8355-1 uses a switch to connect an inductor from

VIN to ground, having a duty ratio of 100% would result in

zero current flowing to the load. To prevent this situation,

the part enforces a minimum off time. During this time,

irrespective of load or demand, the switch turns off and

allows the inductor current to flow into the load. The duty

ratio can, therefore, never reach 100%. The maximum duty

ratio varies with frequency. Due to the very wide range of

input and output voltages offered by LT8355-1, a signifi-

cant minimum off time of around 170ns exists to allow the

switch node more time to slew up to the output. However,

as switching frequency increases, this minimum off time

folds back to 60ns to avoid limiting switch duty ratio

unreasonably. For this reason, lower switching frequency

(<400kHz) is recommended for high voltage (>60V) output

applications. See Figure 5 for the typical maximum duty

ratio as a function of switching frequency. Consider adding

margin to account for variations in component values and

LT8355-1 switching frequencies variations.

Maximum Switch Current

An important system parameter is the current limit. This

prevents damage to system components by limiting the

maximum instantaneous current conducting through the

power switch. In a well-designed system, there will be

margin between the maximum switch current to drive

the LED load and the switch current limit. The LT8355-1

offers a current limit that has sufficient slope compensa-

tion so that reaching the current limit during line and load

transients does not result in subharmonic oscillations.

The switch current sense resistor programs the switch

current limit as 0.1/RSENSE. Normally, the loop limits

switch current based on the value of the sense resistor.

The loop will not typically command a voltage VSENSEP

– VSENSEN of more than 100mV. A good rule of thumb

for setting maximum LED current for boost and buck-

boost power stages appears in Equation 6. This equation

assumes that the inductor selection used limits current

ripple to around 20% of average current. For more infor-

mation, see the Inductor Selection section.

ILED,MAX(20%ripple)=

0.9 •(100mV)

RSENSE

•

VIN

VISP

(6)

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