LT8620 datasheet(13/28 Pages) LINER

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LTC3646/LTC3646-1

36461fc

For more information www.linear.com/LTC3646

APPLICATIONS INFORMATION

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 • ∆IOUT

fO • VDROOP

Thoughthisequationprovidesagoodapproximation,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 value, lower cost ceramic capacitors are now

available in small case sizes. Their high voltage rating

and low ESR make them ideal for switching regulator ap-

plications. 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

and output. 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 or X7R dielectric formulations. These

dielectrics provide the best temperature and voltage

characteristics for a given value and size.

INTVCC Regulator and EXTVCC

An internal low dropout (LDO) regulator produces a 5V

supplyvoltageusedtopowermuchoftheinternalLTC3646

circuitry including the power MOSFET gate drivers. The

INTVCC pin connects to the output of this regulator and

should have 4.7μF of 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 LTC3646.

The user may draw a maximum load current of 5mA from

this pin but must take into account the increased power

dissipation and die temperature that results. Furthermore,

this supply is intended only to supply additional DC load

currents as desired; it is not intended to regulate large

transient or AC behavior as this may impact LTC3646

operation.

Alternatively, if a suitable supply is available or can be

generated, the power required to operate the low voltage

circuitry of the LTC3646 can be supplied through the

EXTVCC pin. When the voltage on the EXTVCC pin is be-

low 4.5V, the chip power is supplied by the internal LDO.

As shown in the Block Diagram, when EXTVCC is above

4.5V, the internal LDO is shut off, and an internal switch

is closed between the EXTVCC and INTVCC pins. Connect

EXTVCC to SGND if an external supply meeting these con-

straints is not available.If the voltage on the EXTVCC pin is

efficiently generated, this will result in the highest overall

system efficiency and the least amount of heat generated

by the LTC3646. This effectively decreased the no-load

quiescent current by a factor of VOUT/VIN. This topic is

further discussed in the Thermal Considerations section.

Boost Capacitor and Diode

Theboostcapacitor,CBOOST,isusedtocreateavoltagerail

above the applied input voltage VIN. Specifically, the boost

capacitor is charged to a voltage equal to approximately

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 +

INTVCC. For most applications a 0.1μF ceramic capacitor

will provide adequate performance.

An internal switch is used to charge the boost capacitor

when the synchronous MOSFET is turned on. An external

Schottky diode can be connected between BOOST and

INTVCC in parallel with this switch in order to improve

the capacitor refresh. For best performance and sufficient

design margin an external diode must be used in circuits

where VOUT is programmed to be above 12V or the IC

operates at a die temperature above 85°C. Forward cur-

rents through this diode are small, on the order of 10mA

to 20mA, but the diode chosen must have low reverse

leakage current at the expected voltage and temperature.

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