LTC3602 datasheet(14/28 Pages) LINER

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LTC3633A/LTC3633A-1

3633a1f

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 design includes an overvoltage

protection circuit, care must always be taken to ensure

input voltage transients do not pose an overvoltage hazard

to the part.

The selection of COUT 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, ΔVOUT, is

approximated by:

ΔVOUT < ΔIL ESR +

8• f • COUT

⎛

⎝

⎜

⎞

⎠

⎟

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, 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:

COUT ≈

3• ΔIOUT

f• VDROOP

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 regula-

tor applications. However, due to the self-resonant and

high-Q characteristics of some types of ceramic capaci-

tors, 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 draws power

from the VIN1 input and 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 capacitance 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 LTC3633A. This supply

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 operation.

Boost Capacitor

The LTC3633A 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.

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