Datasheet

LT3758/LT3758A
14
3758afd
ApplicAtions inForMAtion
FBX Frequency Foldback
When V
OUT
is very low during start-up or a GND 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 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
per
formance. The LT3758/LT3758A use current mode
control to regulate the output which simplifies loop com-
pensation. The LT3758A improves the no-load to heavy
load transient response, when compared to the LT3758.
New internal circuits ensure that the transient from not
switching to switching at high current can be made in a
few cycles. The optimum values depend on the converter
topology, the component values and the operating condi-
tions (including the input voltage, load current, etc.). To
compensate the feedback loop of the LT3758/LT3758A,
a series resistor-capacitor network is usually connected
from the V
C
pin to GND. Figure 1 shows the typical V
C
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 parallel with the RC compensation
network to attenuate the V
C
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 compensa-
tion 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
SENSE
)
between GND and the MOSFET source. Figure 4 shows a
typical waveform of the sense voltage (V
SENSE
) across the
sense resistor. It is important to use Kelvin traces between
the SENSE pin and R
SENSE
, and to place the IC GND as
close as possible to the GND terminal of the R
SENSE
for
proper operation.
Figure 4. The Sense Voltage During a Switching Cycle
3758 F04
V
SENSE(PEAK)
∆V
SENSE = χ •
V
SENSE(MAX)
V
SENSE
t
DT
S
V
SENSE(MAX)
T
S
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