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LT3754
APPLICATIONS INFORMATION
V SUPPLY OFF = 1.476 ? 1 +
duringstart-upoftheLT3754thesoft-startramphasnot
timed out before PWM is asserted high. Without this ‘wait
for PWM high’ feature, systems which apply PWM after V IN
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
start-up of LED displays for applications using very high
PWM Dimming ratios.
Shutdown and Programming Undervoltage Lockout
The LT3754 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 /UVLO
V SUPPLY
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 LT3754 SHDN /UVLO pin can be
made as follows :
? R1 ?
?
? R2 ?
V SUPPLY ON = V SUPPLY 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 LT3754.
Programming Switching Frequency
The switching frequency of the LT3754 boost converter
can be programmed between 100kHz and 1MHz using a
single resistor (R T ) connected from the RT pin to ground
(Figure 4). Connect the R T 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 R T values
required for a range of frequencies.
1000
900
800
R1
13 SHDN /UVLO
–
700
600
OFF ON
R2
600k
1.476V
+
500
400
300
200
100
0
100
200
300
400
500
600
RT (k )
Figure 3. Programming Undervoltage
Lockout (UVLO) with Hysteresis
3754 F03
3754 F04
Figure 4. Switching Frequency vs RT
3754fc
15
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