LT3757HMSE#TRPBF Linear Technology, LT3757HMSE#TRPBF Datasheet - Page 13

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

Specifications of LT3757HMSE#TRPBF

• 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
FBX Frequency Foldback
When V
fault on the output, the switching regulator must operate
at low duty cycles to maintain the power switch current
within the current limit range, since the inductor current
decay rate is very low during switch off time. The minimum
on-time limitation may prevent the switcher from attaining
a sufficiently low duty cycle at the programmed switch-
ing frequency. So, the switch current will keep increasing
through each switch cycle, exceeding the programmed
current limit. To prevent the switch peak currents from
exceeding the programmed value, the LT3757 contains
a frequency foldback function to reduce the switching
frequency when the FBX voltage is low (see the Normal-
ized Switching Frequency vs FBX graph in the Typical
Performance Characteristics section).
The typical frequency foldback waveforms are shown in
the Typical Performance Characteristics section. The fre-
quency foldback function prevents I
programmed limits because of the minimum on-time.
During frequency foldback, external clock synchroniza-
tion is disabled to prevent interference with frequency
reducing operation.
Thermal Lockout
If LT3757 die temperature reaches 165°C (typical), the
part will go into thermal lockout. The power switch will
be turned off. A soft-start operation will be triggered. The
part will be enabled again when the die temperature has
dropped by 5°C (nominal).
Loop Compensation
Loop compensation determines the stability and transient
performance. The LT3757 uses current mode control to
regulate the output which simplifies loop compensation.
The optimum values depend on the converter topology, the
OUT
is very low during start-up or a short-circuit
L
from exceeding the
network is usually connected from the VC pin to GND.
component values and the operating conditions (including
the input voltage, load current, etc.). To compensate the
feedback loop of the LT3757, a series resistor-capacitor
Figure 1 shows the typical VC compensation network. For
most applications, the capacitor should be in the range of
470pF to 22nF , and the resistor should be in the range of
5k to 50k. A small capacitor is often connected in paral-
lel with the RC compensation network to attenuate the
V
through the internal error amplifier. The parallel capacitor
usually ranges in value from 10pF to 100pF . A practical
approach to design the compensation network is to start
with one of the circuits in this data sheet that is similar
to your application, and tune the compensation network
to optimize the performance. Stability should then be
checked across all operating conditions, including load
current, input voltage and temperature.
SENSE Pin Programming
For control and protection, the LT3757 measures the
power MOSFET current by using a sense resistor (R
between GND and the MOSFET source. Figure 4 shows a
typical waveform of the sense voltage (V
sense resistor. It is important to use Kelvin traces between
the SENSE pin and R
close as possible to the GND terminal of the R
proper operation.
C
V
voltage ripple induced from the output voltage ripple
SENSE
Figure 4. The Sense Voltage During a Switching Cycle
V
SENSE(MAX)
DT
S
SENSE
V
SENSE(PEAK)
T
V
S
, and to place the IC GND as
SENSE =
V
SENSE(MAX)
SENSE
LT3757
) across the
SENSE

SENSE
3757 F04
3757fb
for
t
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