LT3760EFE View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT3760EFE' PDF : 32 Pages View PDF
LT3760
Applications Information
the soft start of VC pin to begin. This feature ensures that
during startup of the LT3760 the soft start ramp has not
timed out before PWM is asserted high. Without this ‘wait
for PWM high’ feature, systems which apply PWM after VIN
and SHDN/UVLO are valid, can potentially turn on without
soft start and experience high inductor currents during
wake up of the converter’s output voltage. It is important
to note that when PWM subsequently goes low, the soft
start ramp is not held at its present voltage but continues
to ramp upwards. If the soft start ramp voltage was held
every time PWM goes low, this would cause very slow
startup of LED displays for applications using very high
PWM Dimming ratios.
Shutdown and Programming Undervoltage Lockout
The LT3760 has an accurate 1.476V shutdown threshold
at the SHDN/UVLO pin. This threshold can be used in
conjunction with a resistor divider from the system input
supply to define an accurate undervoltage lockout (UVLO)
threshold for the system (Figure 3). An internal hysteresis
current at the SHDN/UVLO pin allows programming of
hysteresis voltage for this UVLO threshold. Just before
part turn on, an internal 2.4µA flows from the SHDN/
OFF ON
VSUPPLY
R1
15 SHDN/UVLO
–
600k
R2
1.476V +
3760 F03
Figure 3. Programming Undervoltage
Lockout (UVLO) with Hysteresis
UVLO pin. After part turn on, 0µA flows from the SHDN/
UVLO pin. Calculation of the turn on/off thresholds for a
system input supply using the LT3760 SHDN/UVLO pin
can be made as follows :
VSUPPLY
OFF
=
1.476ï£ï£¬1+
R1
R2 
VSUPPLY ON = VSUPPLY OFF + (2.4µA • R1)
An open drain transistor can be added to the resistor
divider network at the SHDN/UVLO pin to independently
control the turn off of the LT3760.
Programming Switching Frequency
The switching frequency of the LT3760 boost converter
can be programmed between 100kHz and 1MHz using a
single resistor (RT) connected from the RT pin to ground
(Figure 4). Connect the RT resistor as close as possible to
the RT pin to minimize noise pick up and stray capacitance
(see “Circuit Layout Considerations†in the Applications
Information section). Table 5 shows the typical RT values
required for a range of frequencies.
1000
900
800
700
600
500
400
300
200
100
0
100 200 300 400 500 600
RT (kΩ)
3760 F04
Figure 4. Switching Frequency vs RT
3760fc
15
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