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LTC3633

3633f

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, VDROOP, 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:

OUT ≈

3• ΔIOUT

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 VDROOP 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

VIN 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 VIN 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.

INTVCC 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

INTVCC 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 INTVCC 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 VIN.Speciﬁcally,aboost

capacitor, CBOOST, is charged to a voltage approximately

equal to INTVCC 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 VIN

+ 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 INTVCC pin. Note that the power from

V2P5 eventually comes from VIN1 since the INTVCC power

is supplied from VIN1. 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 INTVCC to disable the regulator.

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