LTC3646-1_15 Datasheet, PDF (13/28 Pages) Linear Technology – 40V, 1A Synchronous Step-Down Converter

LTC3646/LTC3646-1
APPLICATIONS INFORMATION
Thus, a good place to start is with the output capacitor
size of approximately:
C_OUT
≈
3 • ∆I_OUT
f_O • V_DROOP
Though this equation provides a good approximation, more
capacitance may be required depending on the duty cycle
and load step requirements. The actual V_DROOP should be
verified by applying a load step to the output.
Using Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
available in small case sizes. Their high voltage rating
and low ESR make them ideal for switching regulator ap-
plications. However, due to the self-resonant and high-Q
characteristics of some types of ceramic capacitors, care
must be taken when these capacitors are used at the input
and output. When a ceramic capacitor is used at the input,
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
V_IN input. At best, this ringing can couple to the output and
be mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V_IN large enough to damage the part. For
a more detailed discussion, refer to Application Note 88.
When choosing the input and output ceramic capacitors
choose the X5R or X7R dielectric formulations. These
dielectrics provide the best temperature and voltage
characteristics for a given value and size.
INTV_CC Regulator and EXTV_CC
An internal low dropout (LDO) regulator produces a 5V
supply voltage used to power much of the internal LTC3646
circuitry including the power MOSFET gate drivers. The
INTV_CC pin connects to the output of this regulator and
should have 4.7μF of decoupling capacitance to ground.
The decoupling capacitor should have low impedance
electrical connections to the INTV_CC and PGND pins to
provide the transient currents required by the LTC3646.
The user may draw a maximum load current of 5mA from
this pin but must take into account the increased power
dissipation and die temperature that results. Furthermore,
this supply is intended only to supply additional DC load
currents as desired; it is not intended to regulate large
transient or AC behavior as this may impact LTC3646
operation.
Alternatively, if a suitable supply is available or can be
generated, the power required to operate the low voltage
circuitry of the LTC3646 can be supplied through the
EXTV_CC pin. When the voltage on the EXTV_CC pin is be-
low 4.5V, the chip power is supplied by the internal LDO.
As shown in the Block Diagram, when EXTV_CC is above
4.5V, the internal LDO is shut off, and an internal switch
is closed between the EXTV_CC and INTV_CC pins. Connect
EXTV_CC to SGND if an external supply meeting these con-
straints is not available.If the voltage on the EXTV_CC pin is
efficiently generated, this will result in the highest overall
system efficiency and the least amount of heat generated
by the LTC3646. This effectively decreased the no-load
quiescent current by a factor of V_OUT/V_IN. This topic is
further discussed in the Thermal Considerations section.
Boost Capacitor and Diode
The boost capacitor, C_BOOST, is used to create a voltage rail
above the applied input voltage V_IN. Specifically, the boost
capacitor is charged to a voltage equal to approximately
INTV_CC each time the bottom power MOSFET is turned
on. The charge on this capacitor is then used to supply
the required transient current during the remainder of the
switching cycle. When the top MOSFET is turned on, the
BOOST pin voltage will be equal to approximately V_IN +
INTV_CC. For most applications a 0.1μF ceramic capacitor
will provide adequate performance.
An internal switch is used to charge the boost capacitor
when the synchronous MOSFET is turned on. An external
Schottky diode can be connected between BOOST and
INTV_CC in parallel with this switch in order to improve
the capacitor refresh. For best performance and sufficient
design margin an external diode must be used in circuits
where V_OUT is programmed to be above 12V or the IC
operates at a die temperature above 85°C. Forward cur-
rents through this diode are small, on the order of 10mA
to 20mA, but the diode chosen must have low reverse
leakage current at the expected voltage and temperature.
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