LT3508
OPERATION
The LT3508 is a dual constant frequency, current mode
regulator with internal power switches. Operation can be
best understood by referring to the Block Diagram. If the
SHDN
pin is tied to ground, the LT3508 is shut down and
draws minimal current from the input source tied to the
V
IN
pins. If the
SHDN
pin exceeds 1V, the internal bias
circuits turn on, including the internal regulator, reference
and oscillator. The switching regulators will only begin to
operate when the
SHDN
pin exceeds 2.63V.
The switcher is a current mode regulator. Instead of directly
modulating the duty cycle of the power switch, the feedback
loop controls the peak current in the switch during each
cycle. Compared to voltage mode control, current mode
control improves loop dynamics and provides cycle-by-
cycle current limit. A pulse from the oscillator sets the
RS flip-flop and turns on the internal NPN power switch.
Current in the switch and the external inductor begins to
increase. When this current exceeds a level determined
by the voltage at V
C
, current comparator C1 resets the
flip-flop, turning off the switch. The current in the inductor
flows through the external Schottky diode and begins to
decrease. The cycle begins again at the next pulse from the
oscillator. In this way, the voltage on the V
C
pin controls
the current through the inductor to the output. The internal
error amplifier regulates the output current by continually
adjusting the V
C
pin voltage. The threshold for switching
on the V
C
pin is 0.8V, and an active clamp of 1.75V limits
the output current.
The switching frequency is set either by the resistance to
GND at the R
T
/SYNC pin or the frequency of the logic-level
signal driving the R
T
/SYNC pin. A detection circuit monitors
for the presence of a SYNC signal on the pin and switches
between the two modes. Unique circuitry generates the
appropriate slope compensation ramps and generates the
180° out-of-phase clocks for the two channels.
The switching regulator performs frequency foldback
during overload conditions. An amplifier senses when
V
FB
is less than 0.625V and begins decreasing the oscil-
lator frequency down from full frequency to 12% of the
nominal frequency when V
FB
= 0V. The FB pin is less than
0.8V during start-up, short-circuit and overload conditions.
Frequency foldback helps limit switch current under these
conditions.
The switch driver operates either from V
IN
or from the
BOOST pin. An external capacitor and Schottky diode
are used to generate a voltage at the BOOST pin that is
higher than the input supply. This allows the driver to
saturate the internal bipolar NPN power switch for ef-
ficient operation.
The TRACK/SS pin serves as an alternative input to the
error amplifier. The amplifier will use the lowest voltage
of either the reference of 0.8V or the voltage on the
TRACK/SS pin as the positive input of error amplifier.
Since the TRACK/SS pin is driven by a constant current
source, a single capacitor on the pin will generate a linear
ramp on the output voltage. Tying the TRACK/SS pin to a
resistor divider from the output of one of the switching
regulators allows one output to track another.
The PG output is an open-collector transistor that is off
when the output is in regulation, allowing an external
resistor to pull the PG pin high. Power good is valid when
the LT3508 is enabled (SHDN is high) and V
IN1
is greater
than ~2.4V.
3508fb
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LINER [ LINEAR TECHNOLOGY ]
