LT3757 datasheet(14/38 Pages) AD

14 / 38 page

LT3757/LT3757A

Rev. G

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APPLICATIONS INFORMATION

Due to the current limit function of the SENSE pin,

RSENSE should be selected to guarantee that the peak

current sense voltage VSENSE(PEAK) during steady state

normal operation is lower than the SENSE current limit

threshold (see the Electrical Characteristics table). Given

a 20% margin, VSENSE(PEAK) is set to be 80mV. Then,

the maximum switch ripple current percentage can be

calculated using the following equation:

χ =

ΔVSENSE

80mV

− 0.5 • ΔVSENSE

c is used in subsequent design examples to calculate

inductor value. ∆VSENSE is the ripple voltage across

RSENSE.

The LT3757 switching controller incorporates 100ns tim-

ing interval to blank the ringing on the current sense signal

immediately after M1 is turned on. This ringing is caused

by the parasitic inductance and capacitance of the PCB

trace, the sense resistor, the diode, and the MOSFET. The

100ns timing interval is adequate for most of the LT3757

applications. In the applications that have very large and

long ringing on the current sense signal, a small RC filter

can be added to filter out the excess ringing. Figure 5

shows the RC filter on SENSE pin. It is usually suffi-

cient to choose 22Ω for RFLT and 2.2nF to 10nF for CFLT.

Keep RFLT’s resistance low. Remember that there is 65µA

(typical) flowing out of the SENSE pin. Adding RFLT will

affect the SENSE current limit threshold:

VSENSE_ILIM = 108mV – 65µA • RFLT

APPLICATION CIRCUITS

The LT3757 can be configured as different topologies. The

first topology to be analyzed will be the boost converter,

followed by the flyback, SEPIC and inverting converters.

Boost Converter: Switch Duty Cycle and Frequency

The LT3757 can be configured as a boost converter for

the applications where the converter output voltage is

higher than the input voltage. Remember that boost con-

verters are not short-circuit protected. Under a shorted

output condition, the inductor current is limited only by

the input supply capability. For applications requiring a

step-up converter that is short-circuit protected, please

refer to the Applications Information section covering

SEPIC converters.

The conversion ratio as a function of duty cycle is

VOUT

VIN

1– D

in continuous conduction mode (CCM).

For a boost converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT) and the input voltage (VIN). The maximum

duty cycle (DMAX) occurs when the converter has the

minimum input voltage:

DMAX =

VOUT – VIN(MIN)

VOUT

Discontinuous conduction mode (DCM) provides higher

conversion ratios at a given frequency at the cost of

reduced efficiencies and higher switching currents.

Figure 5. The RC Filter on SENSE Pin

CFLT

3757 F05

LT3757

RFLT

RSENSE

SENSE

GATE

GND

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