LT3694/LT3694-1
9
36941fb
OPERATION
Unless specifically noted, this data sheet refers to both the
LT3694 and the LT3694-1 generically as the LT3694.
The LT3694 is a constant-frequency, current mode, buck
regulator with an internal power switch plus two low
dropout linear regulator controllers. The three regulators
share common circuitry including input source, voltage
reference, undervoltage lockout, and enable, but are oth-
erwise independent. Operation can be best understood by
referring to the Block Diagram (Figure 1).
If the EN/UVLO pin is below 0.35V (min), the LT3694 is
shut down and draws <2礎 from the input source tied to
V
IN1
. If the EN/UVLO pin is driven above 0.5V (typ), the
internal bias circuits turn on, including the internal regulator,
reference and master oscillator. The switching regulator
will only begin to operate when the EN/UVLO pin reaches
>1.20V (typ). The EN/UVLO pin can be driven from a logic
gate or can be used as an undervoltage lockout by using
a resistor divider to V
IN
.
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 bipolar 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
C1
, the current comparator resets the RS flip-flop,
turning off the switch. The current in the inductor flows
through the external, Schottky, catch diode, and begins to
decrease. The cycle begins again at the next pulse from the
oscillator. In this way, the voltage on the V
C1
pin controls
the current through the inductor to the output. The internal
error amplifier regulates the output voltage by continually
adjusting the V
C1
pin voltage. The threshold for switching
on the V
C1
pin is 0.75V and an active clamp of 2V limits
the output current.
Overcurrent protection is provided by the DA comparator.
The DA comparator senses the catch diode current and
will delay the switch-on cycle if the diode current is too
high at the beginning of a cycle.
The TRK/SS pins override the 0.75V reference for the FB
pins when the TRK/SS pins are below 0.75V. This allows
either coincident or ratiometric supply tracking on start-up
as well as a soft-start capability.
The switch driver operates either from V
IN
or from the BST
pin. An external capacitor is used to generate a voltage at
the BST pin that is higher than the input supply. This al-
lows the driver to saturate the internal bipolar NPN power
switch for efficient operation.
The BIAS pin allows the internal circuitry to draw its cur-
rent from a voltage supply lower than V
IN
, reducing power
dissipation and increasing efficiency. If the voltage on the
BIAS pin falls below 2.7V, then its quiescent current will
flow from V
IN
.
The LDO regulator uses an external NPN pass transistor to
form a linear regulator. The loop is internally compensated
to be stable with a minimum load capacitance of 2.2 礔. The
LDO also has a foldback current limiter available to protect
the external transistor under overload conditions
The overvoltage detection shuts down the LT3694 if the
input voltage goes above 38V. This will prevent the switch
from turning on under high voltage conditions and allows
the LT3694 to survive transient input voltages up to 70V.
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