LTC3313 datasheet(13/28 Pages) AD

13 / 28 page

LTC3313

Rev. 0

For more information www.analog.com

Refer to the Block Diagram for reference.

FB Resistor Network

The output voltage on the LTC3313 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

3313 F01

BUCK

SWITCHING

REGULATOR

(OPTIONAL)

VOUT

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 LTC3313

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

tON MIN

( ) •VIN MAX

(

)

(2)

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

output voltage, and tON(MIN) is the minimum top switch on-

time. This equation shows that a slower switching fre-

quency is necessary to accommodate a high VIN/VOUT ratio.

The LTC3313 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 LTC3313 uses a constant frequency PWM architecture.

There are three methods to set the switching frequency. 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 necessary RT value for

a desired switching frequency.

The RT resistor required for a desired switching frequency

is calculated using the following formula:

RT =

580

fSW

− 16

(3)

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

quency in MHz.

Table 1. Switching Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.5

1210

549

274

2.2

243

178

130

100

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