6
LT3439
sn3439 3439fs
vary from 2.8V to 17.5V with very little change in device
performance. When the part is in shutdown mode, the
internal regulator is turned off, drawing less than 20µA of
current from V
IN
.
Overcurrent Protection
A linearly controlled current limit circuit is provided to
protect the circuit from excessive currents and to facilitate
start-up into a highly capacitive load. Upon reaching cur-
rent limit, the switching cycle is not terminated, instead the
base drive to the output transistor is regulated to maintain
the maximum current over the entire switch cycle. Very high
power dissipation in the switches occurs during this mode
of operation. If the current limit is enabled for a long enough
period of time, over temperature protection shutdown will
be enabled to protect the device.
OPERATIO
U
Overtemperature Protection
When the IC has exceeded the maximum temperature the
part will trigger the overtemperature protection circuit
where both output drivers are turned off.
Undervoltage Lockout Protection
When V
IN
is below 2.55V the part will go into undervoltage
lockout mode where both output drivers are turned off.
No Load Operation
The operation of the supply is stable all the way down to
zero load and a preload is not required.
APPLICATIO S I FOR ATIO
WUUU
Reducing EMI from switching power supplies has tradi-
tionally invoked fear in designers. Many switchers are
designed solely on efficiency and, as such, produce wave-
forms filled with high frequency harmonics that propagate
through the rest of the supply.
The LT3439 provides control of two of the primary vari-
ables for controlling EMI while switching inductive loads:
switch voltage slew rate and switch current slew rate. The
use of this part will reduce noise and EMI over conven-
tional switch mode controllers. Because these variables
are under control, a supply built with this part will exhibit
far less tendency to create EMI and less chance of running
into problems during production.
It is beyond the scope of this data sheet to get into EMI
fundamentals. AN70, “A Monolithic Switching Regulator
with 100µV Output Noise” contains much information
concerning noise in switching regulators and should be
consulted.
Oscillator Frequency
The internal oscillator generates the switching frequency
that determines the fundamental positioning of the
harmonics. Using quality external components is impor-
tant to ensure oscillator frequency stability. A current
defined by external resistor R
T
charges and discharges
the capacitor C
T
creating a saw tooth waveform where the
outputs’ states change at the peak. The frequency of each
output is one half of the frequency of the oscillator.
By having both components external, the user has greater
flexibility in setting the frequency and the frequency is less
susceptible to any temperature variations in the device.
The external capacitance C
T
is chosen by:
C
T
(nF) = 1183/[f
OSC
(kHz) • R
T
(kΩ)]
where f
OSC
is the desired oscillator frequency.
For R
T
equal to 16.9k, this simplifies to:
C
T
(nF) = 70/f
OSC
(kHz)
e.g., C
T
= 1nF for f
OSC
= 70kHz
Nominally, R
T
should be set to 16.9k.
Low tolerance and low temperature coefficient compo-
nents are recommended.
