LT3757HMSE#PBF Linear Technology, LT3757HMSE#PBF Datasheet - Page 14

• Manufacturer Part Number: LT3757HMSE#PBF
• Description: IC DC-DC CTRLR ADJ 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Inverting, Flyback, Sepicr

Specifications of LT3757HMSE#PBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 100kHz ~ 1MHz
• Voltage - Input: 2.9 ~ 40 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP Exposed Pad, 10-HMSOP, 10-eMSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

applicaTions inForMaTion
LT3757
Due to the current limit function of the SENSE pin, R
should be selected to guarantee that the peak current sense
voltage V
is lower than the SENSE current limit threshold (see the
Electrical Characteristics table). Given a 20% margin,
V
switch ripple current percentage can be calculated using
the following equation:
c is used in subsequent design examples to calculate induc-
tor value. ∆V
The LT3757 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 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 sufficient
to choose 22Ω for R
Keep R
(typical) flowing out of the SENSE pin. Adding R
affect the SENSE current limit threshold:
V

SENSE(PEAK)
c =
SENSE_ILIM
80
FLT
SENSE(PEAK)
mV
’s resistance low. Remember that there is 65µA
Figure 5. The RC Filter on SENSE Pin
SENSE
is set to be 80mV. Then, the maximum
−
∆
= 108mV – 65µA • R
V
0 5 . •
LT3757
SENSE
GND
is the ripple voltage across R
during steady state normal operation
∆
SENSE
V
GATE
FLT
SENSE
and 2.2nF to 10nF for C
C
R
FLT
FLT
FLT
M1
R
3757 F05
SENSE
SENSE
FLT
SENSE
will
FLT
.
.
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
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 (V
duty cycle (D
minimum input voltage:
Discontinuous conduction mode (DCM) provides higher
conversion ratios at a given frequency at the cost of reduced
efficiencies and higher switching currents.
D
V
V
MAX
OUT
IN
=
=
OUT
1
V
−
OUT
1
) and the input voltage (V
MAX
D
V
−
) occurs when the converter has the
OUT
V
IN MIN
(
)
IN
). The maximum
3757fb