參數(shù)資料
型號(hào)： LT3757MPMSE#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO10
封裝： 3 X 3 MM, LEAD FREE, PLASTIC, MSOP-10
文件頁(yè)數(shù)： 5/36頁(yè)
文件大小： 436K
代理商： LT3757MPMSE#TRPBF
LT3757
13
3757fc
applicaTions inForMaTion
FBX Frequency Foldback
When VOUT is very low during start-up or a short-circuit
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
decayrateisverylowduringswitchofftime.Theminimum
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
frequency foldback function prevents IL from exceeding
the 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.
Theoptimumvaluesdependontheconvertertopology,the
componentvaluesandtheoperatingconditions(including
the input voltage, load current, etc.). To compensate the
feedback loop of the LT3757, a series resistor-capacitor
network is usually connected from the VC pin to GND.
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
VC voltage ripple induced from the output voltage ripple
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 (RSENSE)
between GND and the MOSFET source. Figure 4 shows a
typical waveform of the sense voltage (VSENSE) across the
sense resistor. It is important to use Kelvin traces between
the SENSE pin and RSENSE, and to place the IC GND as
close as possible to the GND terminal of the RSENSE for
proper operation.
Figure 4. The Sense Voltage During a Switching Cycle
3757 F04
VSENSE(PEAK)
VSENSE = χ VSENSE(MAX)
VSENSE
t
DTS
VSENSE(MAX)
TS
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