LT3758 datasheet(16/36 Pages) LINER

16 / 36 page

LT3758

3758f

APPLICATIONS INFORMATION

switch ripple current percentage can be calculated using

the following equation:

χ=

−

SENSE

0 5

.•

χisusedinsubsequentdesignexamplestocalculateinduc-

tor value. ΔVSENSE is the ripple voltage across RSENSE.

The LT3758 switching controller incorporates 100ns timing

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 LT3758

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 the SENSE pin. It is usually suf-

ﬁcient 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 = 110mV – 65μA • RFLT

APPLICATION CIRCUITS

The LT3758 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 LT3758 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

OUT

−

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:

MAX

OUT

IN MIN

OUT

−

()

Discontinuous conduction mode (DCM) provides higher

conversion ratios at a given frequency at the cost of reduced

efﬁciencies and higher switching currents.

Boost Converter: Inductor and Sense Resistor Selection

For the boost topology, the maximum average inductor

current is:

L MAX

O MAX

MAX

()

() •

−

Then, the ripple current can be calculated by:

ΔII

L MAX

O MAX

MAX

−

χχ

••

•

()

Figure 5. The RC Filter on the SENSE Pin

CFLT

3758 F05

LT3758

RFLT

RSENSE

SENSE

GATE

GND

LT3758 Datasheet(PDF) 16 Page - Linear Technology

LT3758 Datasheet(HTML) 16 Page - Linear Technology

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 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL