LTC3633 Datasheet, PDF (14/28 Pages) Linear Technology – Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator

LTC3633
APPLICATIONS INFORMATION
must instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation and the
output capacitor size. Typically, 3 to 4 cycles are required
to respond to a load step, but only in the first cycle does
the output drop linearly. The output droop, V_DROOP, is
usually about 3 times the linear drop of the first cycle.
Thus, a good place to start is with the output capacitor
size of approximately:
C_OUT
≈
3 • ΔI_OUT
f • 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 values, lower cost ceramic capacitors are available
in small case sizes. Their high ripple current, high voltage
rating and low ESR make them ideal for switching regulator
applications. 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. 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 and X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
INTV_CC Regulator Bypass Capacitor
An internal low dropout (LDO) regulator produces the
3.3V supply that powers the internal bias circuitry and
drives the gate of the internal MOSFET switches. The
INTV_CC pin connects to the output of this regulator and
must have a minimum of 1μF ceramic decoupling capaci-
tance 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 LTC3633. This supply is intended only to supply ad-
ditional DC load currents as desired and not intended to
regulate large transient or AC behavior, as this may impact
LTC3633 operation.
Boost Capacitor
The LTC3633 uses a “bootstrap” circuit to create a voltage
rail above the applied input voltage V_IN. Specifically, a boost
capacitor, C_BOOST, is charged to a voltage approximately
equal to 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
+ 3.3V. For most applications, a 0.1μF ceramic capacitor
closely connected between the BOOST and SW pins will
provide adequate performance.
Low Power 2.5V Linear Regulator
The V2P5 pin can be used as a low power 2.5V regulated
rail. This pin is the output of a 10mA linear regulator
powered from the INTV_CC pin. Note that the power from
V2P5 eventually comes from V_IN1 since the INTV_CC power
is supplied from V_IN1. When using this output, this pin
must be bypassed with a 1μF ceramic capacitor. If this
output is not being used, it is recommended to short this
output to INTV_CC to disable the regulator.
3633f
14