Datasheet

LT3478/LT3478-1
11
34781f
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 defi ne 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 fl ows
into the
⎯
S
⎯
H
⎯
D
⎯
N pin. After part turn on, 0µA fl ows 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:
V
SUPPLY
OFF = 1.4 [1 + R1/R2)]
V
SUPPLY
ON = V
SUPPLY
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.
Programming Switching Frequency
The switching frequency is programmed using an external
resistor (R
T
) connected between the R
T
pin and ground. The
internal free-running oscillator is programmable between
200kHz and 2.25MHz. Table 4 shows the typical R
T
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 effi ciency 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.
APPLICATIO S I FOR ATIO
WUU
U
With the
⎯
S
⎯
H
⎯
D
⎯
N pin connected directly to the V
IN
pin, an
internal undervoltage lockout threshold exists for the V
IN
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 V
IN
<2.8V. See details in the Applications Information section
“Soft-Start”.
Figure 2. Programming Undervoltage Lockout (UVLO)
with Hysteresis
Table 4. Switching Frequencies vs R
T
Values
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
2.25 9.09
1 31.6
0.2 200
Figure 3. Switching Frequency vs R
T
Resistor Value
R
T
(kΩ)
1
100
SWITCHING FREQUENCY (kHz)
1000
10000
10 100 1000
3478 F03
T
A
= 25°C
SHDN
1.4V
10µA
V
SUPPLY
R1
11
R2
3478 F02
ONOFF
+
–