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LT3797

3797f

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APPLICATIONS INFORMATION

Switch Current Sense Resistors Selection

The LT3797 measures each channel’s power N-channel

MOSFET current by using a sense resistor (see RSW_SENin

Figure 1) between GND and the MOSFET source. Figure 7

showsatypicalwaveformofthesensevoltage(VSW_SENSE)

across the sense resistor in CCM. The placement of the

senseresistorRSW_SENshouldbeclosetothesourceofthe

MOSFET and GND. The SENSEP and SENSEN sense node

traces should run parallel to each other to a Kelvin con-

nection on the positive and negative terminals of RSW_SEN.

Due to the current limit function of the power switch cur-

rent control, RSW_SENshouldbeselectedtoguaranteethat

the peak current sense voltage VSW_SENSE(PEAK) during

steady-state normal operation is lower than the SENSE

current limit threshold (100mV minimum). It is recom-

mended to give a 20% margin and set VSW_SENSE(PEAK)

to be 80mV. Then, the switch current sense resistor value

can be calculated as:

RSW _SEN =

80mV

ISW(PEAK)

where ISW(PEAK) is the peak switch current. ISW(PEAK) of

the boost, buck mode and buck-boost mode LED driver is:

ISW(PEAK) = IL(PEAK)

ISW(PEAK) of the SEPIC LED driver is:

ISW(PEAK) = IL1(PEAK) + IL2(PEAK)

Sense Voltage Ripple Verification

After the inductor ripple current and the switch current

sense resistor value have been selected according to the

previous sections, the sense voltage ripple

∆VSW_SENSE

(refer to Figure 7) of the boost, buck, and buck-boost LED

drivers can be determined using the following equation:

∆VSW_SENSE = ∆IL • RSW_SEN

∆VSW_SENSE of the SEPIC LED driver can be determined

using the following equation:

∆VSW_SENSE = 2 • ∆IL • RSW_SEN

The LT3797 has internal slope compensation to stabilize

the control loop against sub-harmonic oscillation. When

the LT3797 operates at a duty cycle greater than 0.66

in CCM, the sense voltage ripple,

∆VSW_SENSE (refer to

Figure 7), needs to be limited to ensure the internal slope

compensation is sufficient to stabilize the control loop.

Figure 8 shows the maximum

∆VSW_SENSE over the duty

cycle.Itisrecommendedtocheckandensure

∆VSW_SENSE

isbelowthiscurveatthehighestdutycycle.If

∆VSW_SENSE

is above the maximum

∆VSW_SENSE curve at the highest

dutycycle,the

∆ILneedstobereducedandtheparameters

in the previous two sections need to be recalculated until

the optimized values are obtained.

Figure 7. The Sense Voltage Across the Sense Resistor in CCM

∆VSW_SENSE

VSW_SENSE(PEAK)

VSW_SENSE

D/f

3797 F07

1/f

Figure 8. The Maximum Sense Voltage Ripple

vs Duty Cycle for CCM

DUTY CYCLE

0.5

100

110

3797 F08

0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95

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