LTC3602 datasheet(10/20 Pages) LINER

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LTC3602

3602fb

APPLICATIONS INFORMATION

characteristics. The choice of which style inductor to use

mainly depends on the price vs size requirements and any

radiated ﬁeld/EMI requirements. New designs for surface

mount inductors are available from Coiltronics, Coilcraft,

Toko and Sumida.

CIN and COUT Selection

The input capacitance, CIN, is needed to ﬁlter the trapezoi-

dal current at the source of the top MOSFET. To prevent

large ripple voltage, a low ESR input capacitor sized for

the maximum RMS current should be used. RMS current

is given by:

RMS

OUT MAX

OUT

() ••

– 1

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is com-

monly used for design because even signiﬁcant deviations

do not offer much relief. Note that ripple current ratings

from capacitor manufacturers are often based on only

2000 hours of life which makes it advisable to further

derate the capacitor, or choose a capacitor rated at a

higher temperature than required. Several capacitors may

also be paralleled to meet size or height requirements in

the design.

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 as described in a later section.

The output ripple,

ΔVOUT, is determined by:

ΔV

OUT ≤ΔIL •ESR +

8fC

OUT

⎛

⎝⎜

⎞

⎠⎟

The output ripple is highest at maximum input voltage

since

ΔIL increases with input voltage. Multiple capacitors

placed in parallel may be needed to meet the ESR and

RMS current handling requirements. Dry tantalum, special

polymer, aluminum electrolytic and ceramic capacitors are

all available in surface mount packages. Special polymer

capacitors offer very low ESR but have lower capacitance

density than other types. Tantalum capacitors have the

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 but can have a high

voltage coefﬁcient and audible piezoelectric effects. The

high Q of ceramic capacitors with trace inductance can

also lead to signiﬁcant ringing.

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them ideal

for switching regulator applications. However, 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 input,

VIN. 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.

Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following equation:

OUT = 0.6V • 1+

⎛

⎝⎜

⎞

⎠⎟

The resistive divider allows the VFB pin to sense a fraction

of the output voltage as shown in Figure 1.

LTC3602

3602 F01

SGND

VFB

VOUT

Figure 1. Setting the Output Voltage

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