LTC3622 datasheet(13/24 Pages) LINER

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LTC3622/

LTC3622-2/LTC3622-23/5

3622fc

For more information www.linear.com/LTC3622

APPLICATIONS INFORMATION

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.

When choosing the input and output ceramic capacitors,

choose the X5R and X7R dielectric formulations. These

dielectrics have the best temperature and voltage char-

acteristics of all the ceramics for a given value and size.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement.Duringaloadstep,theoutputcapacitormust

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough

to support the load. Typically, five cycles are required to

respond to a load step, but only in the first cycle does the

output voltage drop linearly. The output droop, VDROOP, is

usually about three times the linear drop of the first cycle.

Thus, a good place to start with the output capacitor value

is approximately:

COUT = 3

ΔIOUT

ƒO • VDROOP

More capacitance may be required depending on the duty

cycle and load step requirements. In most applications,

the input capacitor is merely required to supply high

frequency bypassing, since the impedance to the supply

is very low. A 10µF ceramic capacitor is usually enough

for these conditions. Place this input capacitor as close

to the VIN1 and VIN2 pins as possible.

Output Power Good

When the LTC3622’s output voltages are within the ±7.5%

windowoftheregulationpoint,theoutputvoltagesaregood

andthePGOODpinsarepulledhighwithexternalresistors.

Otherwise, internal open-drain pull-down devices (275Ω)

willpullthePGOODpinslow.TopreventunwantedPGOOD

glitches during transients or dynamic VOUT changes, the

LTC3622’s PGOOD falling edge includes a blanking delay

of approximately 32 switching cycles.

Frequency Synchronization Capability

The LTC3622 has the capability to synchronize to a ±50%

range of the internal programmed frequency. It takes

several cycles of external clock to engage the sync mode,

and roughly 2μs for the part to detect the absence of the

external clock signal. Once engaged in sync, the LTC3622

immediately runs at the external clock frequency.

Inductor Selection

Given the desired input and output voltages, the inductor

valueandoperatingfrequencydeterminetheripplecurrent:

ΔIL=

VOUT

ƒ •L

1– VOUT

VIN(MAX)













Lower ripple current reduces power losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efficiency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a trade-off between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 50% of IOUT(MAX). To guarantee that ripple

current does not exceed a specified maximum, the induc-

tance should be chosen according to:

L =

VOUT

ƒ • ΔIL(MAX)

1– VOUT

VIN(MAX)













Once the value for L is known, the type of inductor must

be selected. Actual core loss is independent of core size

for a fixed inductor value, but is very dependent on the

inductance selected. As the inductance or frequency in-

creases, core loss decreases. Unfortunately, increased

inductance requires more turns of wire and therefore

copper losses increase.

Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard,” which means that

inductancecollapsesabruptlywhenthepeakdesigncurrent

is exceeded. This results in an abrupt increase in inductor

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