LTC3633A-3_15 Datasheet, PDF (14/28 Pages) Linear Technology – Dual Channel 3A, 20V Monolithic Synchronous Step-Down Regulator

LTC3633A-2/LTC3633A-3
APPLICATIONS INFORMATION
Several capacitors may also be paralleled to meet size or
height requirements in the design. For low input voltage
applications, sufficient bulk input capacitance is needed
to minimize transient effects during output load changes.
Even though the LTC3633A-2 design includes an over-
voltage protection circuit, care must always be taken to
ensure input voltage transients do not pose an overvoltage
hazard to the part.
The selection of C_OUT is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
and load step transients as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response. The output ripple, ΔV_OUT, is
approximated by:
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 PV_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 PV_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.
∆V_OUT
<
∆I_L

ESR +

8
•
f
1
• C_OUT



When using low-ESR ceramic capacitors, it is more useful
to choose the output capacitor value to fulfill a charge stor-
age requirement. During a load step, the output capacitor
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 • ∆I_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
INTV_CC Regulator Bypass Capacitor
An internal low dropout (LDO) regulator draws power
from the SV_IN input and 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 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 LTC3633A-2. This sup-
ply is intended only to supply additional DC load currents
as desired and not intended to regulate large transient or
AC behavior, as this may impact LTC3633A-2 operation.
As long as the INTV_CC rail is powered by SV_IN, the regula-
tor control circuitry will operate, regardless of the PV_IN
voltages. Thus, the SV_IN input can be powered from a
different supply voltage than either PV_IN1 or PV_IN2. This
characteristic makes the LTC3633A-2/LTC3633A-3 very
flexible and easy to use in systems with multiple power
sources.
Operating from Multiple Power Sources
Channel 1 and channel 2 may be operated from separate
input power sources. In cases where one power source is
disconnected, the other regulator can continue to operate
provided that SV_IN remain powered. This can be done with
a simple diode-OR circuit, as shown in Figure 2.
3633a23fb
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