LTC3315B datasheet(12/28 Pages) AD

12 / 28 page

LTC3311S

Rev. A

For more information www.analog.com

Refer to the Block Diagram for reference.

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the resistor

values according to:

RA =RB

VOUT

500mV

– 1

⎛

⎝

⎜

⎞

⎠

⎟

(1)

as shown in Figure 1:

Figure 1.

COUT

CFF

VOUT

3311S F01

BUCK

SWITCHING

REGULATOR

(OPTIONAL)

Feedback Resistor Network

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy. When optimizing the control loop for high band-

width and optimal transient response add a phase-lead

capacitor connected from VOUT to FB.

Operating Frequency Selection and Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency, component size, transient response and input

voltage range.

The advantage of high frequency operation is that smaller

inductor and capacitor values may be used. Higher

switching frequencies allow for higher control loop

bandwidth and, therefore, faster transient response. The

disadvantages of higher switching frequencies are lower

efficiency, because of increased switching losses, and a

smaller input voltage range, because of minimum switch

on-time limitations.

Although the maximum programmable switching fre-

quency is 5MHz, the minimum on-time of the LTC3311S

imposes a minimum operating duty cycle. The highest

APPLICATIONS INFORMATION

switching frequency (fSW(MAX)) for a given application

can be calculated as follows:

fSW MAX

(

) =

VOUT + VSW BOT

(

)

tON MIN

(

) VIN MAX

(

) – VSW TOP

(

) + VSW BOT

(

)

(

)

(2)

where VIN(MAX) is the maximum input voltage, VOUT is

the output voltage, VSW(TOP) and VSW(BOT) are the inter-

nal switch drops and tON(MIN) is the minimum top switch

on-time. This equation shows that a slower switching

frequency is necessary to accommodate a high VIN/VOUT

ratio.

The LTC3311S is capable of a maximum duty cycle of

100%, therefore, the VIN-to-VOUT dropout is limited by

the RDS(ON) of the top switch, the inductor DCR and the

load current.

Setting the Switching Frequency

The LTC3311S uses a constant frequency PWM archi-

tecture. There are three methods to set the switching fre-

quency. The first method is with a resistor (RT) tied from

the RT pin to ground. The frequency can be programmed

to switch from 500kHz to 5MHz. Table 1 shows the neces-

sary RT value for a desired switching frequency.

The RT resistor required for a desired switching frequency

is calculated using the following formula:

RT = 568 • fSW(–1.08)

(3)

where RT is in kΩ and fSW is the desired switching fre-

quency in MHz.

Table 1. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.5

1210

549

274

2.2

243

178

130

100

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