LT3669 Ver la hoja de datos (PDF) - Linear Technology
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LT3669 Datasheet PDF : 40 Pages
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LT3669/LT3669-2
APPLICATIONS INFORMATION
BST and BD Pin Considerations
Capacitor CBST and the internal boost Schottky diode
(see the Block Diagram) are used to generate a boost
voltage that is higher than the input voltage. In most
cases a 0.1µF (LT3669) and 0.22µF (LT3669-2) capaci-
tor will work well. Figure 19 shows two ways to arrange
the boost circuit. The BST pin must be more than 1.9V
above the SW pin for best efficiency. For outputs of 2.2V
and above, the standard circuit (Figure 19a) is best. For
outputs between 2.2V and 2.5V, use a 0.22µF (LT3669)
and 0.47μF (LT3669-2) boost capacitor. For output volt-
ages below 2.2V the boost diode can be tied to the input
through pin DIO to preserve reverse-polarity protection
(Figure 19b), or to another external supply greater than
2.2V. However the circuit in Figure 19a is more efficient
because the BST pin current comes from a lower voltage
source. Be sure that the maximum voltage ratings of the
BST and BD pins are not exceeded.
The minimum operating voltage of an LT3669 applica-
tion is limited by the minimum input voltage and by the
maximum duty cycle as outlined previously. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly, or
the LT3669 is turned on with its EN/UVLO pin (when the
output is already in regulation), then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load generally goes to zero
once the circuit has started. In many cases the discharged
output capacitor will present a load to the switcher, which
will allow it to start. For a given programmed output volt-
age VOUT, the minimum input voltage that guarantees a
proper start-up regardless of load current is VOUT + 2V.
Synchronization
Synchronizing the LT3669 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.5V
and peaks that are above 1.5V (up to 6V).
The LT3669 may be synchronized over a 300kHz to
2.2MHz range. Choose the RT resistor to set the LT3669
switching frequency 20% below the lowest synchroniza-
tion input. For example, if the synchronization signal will
be 360kHz, choose RT for 300kHz. To assure a reliable
DIO BD BST
VOUT
VL+
L+ LT3669/
LT3669-2
CBST
GND SW
(a) For VOUT ≥ 2.2V
DIO BD BST
VL+
L+ LT3669/
LT3669-2
CBST
GND SW
VOUT
36692 F19
(b) For VOUT < 2.2V; VL+ < 25V
Figure 19. Two Circuits for Generating the Boost Voltage
For more information www.linear.com/LT3669
3669fa
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