LT3478/LT3478-1
APPLICATIO S I FOR ATIO
Shutdown and Programming Undervoltage Lockout
The LT3478/LT3478-1 have an accurate 1.4V shutdown
threshold at the ⎯S⎯H⎯D⎯N 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 2). ⎯S⎯H⎯D⎯N pin current
hysteresis allows programming of hysteresis voltage for
this UVLO threshold. Just before part turn on, 10µA flows
into the ⎯S⎯H⎯D⎯N pin. After part turn on, 0µA flows from the
S⎯ H⎯ D⎯ N⎯ pin. Calculation of the on/off thresholds for a system
input supply using the LT3478/LT3478-1 ⎯S⎯H⎯D⎯N pin can
be made as follows:
VSUPPLY OFF = 1.4 [1 + R1/R2)]
VSUPPLY ON = VSUPPLY OFF + (10µA • R1)
An open drain transistor can be added to the resistor
divider network at the ⎯S⎯H⎯D⎯N pin to independently control
the turn off of the LT3478/LT3478-1.
VSUPPLY
R1
11 SHDN
–
Programming Switching Frequency
The switching frequency is programmed using an external
resistor (RT) connected between the RT pin and ground. The
internal free-running oscillator is programmable between
200kHz and 2.25MHz. Table 4 shows the typical RT values
required for a range of switching frequencies.
Selecting the optimum switching frequency depends
on several factors. Inductor size is reduced with higher
frequency but efficiency drops due to higher switching
losses. In addition, some applications require very high duty
cycles to drive a large number of LEDs from a low supply.
Low switching frequency allows a greater operational duty
cycle and hence a greater number of LEDs to be driven.
In each case the switching frequency can be tailored to
provide the optimum solution. When programming the
switching frequency the total power losses within the IC
should be considered. See “Thermal Calculations” in the
Applications Information section.
10000
TA = 25°C
R2
OFF ON
1.4V +
1000
10µA
3478 F02
Figure 2. Programming Undervoltage Lockout (UVLO)
with Hysteresis
With the ⎯S⎯H⎯D⎯N pin connected directly to the VIN pin, an
internal undervoltage lockout threshold exists for the VIN
pin (2.8V max). This prevents the converter from operat-
ing in an erratic mode when supply voltage is too low.
The LT3478/LT3478-1 provide a soft-start function when
recovering from such faults as ⎯S⎯H⎯D⎯N <1.4V and/or VIN
<2.8V. See details in the Applications Information section
“Soft-Start”.
100
1
10
100
RT (kΩ)
1000
3478 F03
Figure 3. Switching Frequency vs RT Resistor Value
Table 4. Switching Frequencies vs RT Values
SWITCHING FREQUENCY (MHz)
2.25
1
0.2
RT (kΩ)
9.09
31.6
200
34781f
11