LTC3602 datasheet(14/28 Pages) LINER

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LTC3633A-2/LTC3633A-3

3633a23f

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 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 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 PVIN

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 PVIN 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 SVIN 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-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 INTVCC rail is powered by SVIN, the regula-

tor control circuitry will operate, regardless of the PVIN

voltages. Thus, the SVIN input can be powered from a

different supply voltage than either PVIN1 or PVIN2. 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 SVIN remain powered. This can be done with

a simple diode-OR circuit, as shown in Figure 2.

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