LTC3601IMSE datasheet(12/24 Pages) LINER

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LTC3601

3601f

APPLICATIONS INFORMATION

highest capacitance density, but it is important to only

use types that have been surge tested for use in switching

power supplies. Aluminum electrolytic capacitors have

signiﬁcantly higher ESR but can be used in cost-sensitive

applications provided that consideration is given to ripple

current ratings and long-term reliability. Ceramic capacitors

have excellent low ESR characteristics and small footprints.

Their relatively low value of bulk capacitance may require

multiple capacitors in parallel.

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

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

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 Bypass Capacitor

An internal low dropout (LDO) regulator produces a

3.3V supply voltage used to power much of the internal

LTC3601 circuitry including the power MOSFET gate

drivers. The INTVCC pin connects to the output of this

regulator and must have a minimum of 1μF 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 LTC3601. The user may connect a maximum load

current of 5mA to 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 and not intended

to regulate large transient or AC behavior this may impact

LTC3601 operation.

Boost Capacitor

The boost capacitor, CBOOST, is used to create a voltage rail

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 +

3.3V. For most applications a 0.1μF ceramic capacitor will

provide adequate performance.

Output Voltage Programming

The LTC3601 will adjust the output voltage such that VFB

equals the reference voltage of 0.6V according to:

VOUT = 0.6V 1+

The desired output voltage is set by appropriate selection of

resistors R1 and R2 as shown in Figure 2. Choosing large

values for R1 and R2 will result in improved efﬁciency but

may lead to undesirable noise coupling or phase margin

reduction due to stray capacitances at the FB node. Care

should be taken to route the FB line away from any noise

source, such as the SW line.

To improve the frequency response of the main control

loop a feedforward capacitor, CF, may be used as shown

in Figure 2.

3601 F02

VOUT

SGND

LTC3601

Figure 2. Optional Feedforward Capacitor

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