LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 15

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
FBX Frequency Foldback
When V
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
suffi ciently low duty cycle at the programmed switching
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 LT3758 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).
During frequency foldback, external clock synchroniza-
tion is disabled to prevent interference with frequency
reducing operation.
Thermal Lockout
If LT3758 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 LT3758 uses current mode control to
regulate the output which simplifi es loop compensation.
The optimum values depend on the converter topology, the
component values and the operating conditions (including
OUT
is very low during start-up or a GND fault on
V
SENSE
Figure 4. The Sense Voltage During a Switching Cycle
V
SENSE(MAX)
DT
S
V
SENSE(PEAK)
T
V
S
SENSE =
the input voltage, load current, etc.). To compensate the
feedback loop of the LT3758, 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 amplifi er. 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 LT3758 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.
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
SENSE(PEAK)
V
SENSE(MAX)
SENSE(PEAK)
is set to be 80mV. Then, the maximum
during steady state normal operation
3758 F04
SENSE
t
, and to place the IC GND as
SENSE
LT3758
) across the
SENSE
15
SENSE
SENSE
3758f
for
)