LTC3312SAAVPBF datasheet(16/28 Pages) AD

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LTC3312SA

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Output Voltage and Feedback Network

The output voltage of the buck switching regulators is

programmed by a resistor divider between the output

and the FB pin. Choose the resistor values according to

Equation 1.

RA =RB

VOUT

500mV

– 1

⎛

⎝

⎜

⎞

⎠

⎟

(1)

as shown in Figure 1:

COUT

CFF

VOUT

3312SA F01

BUCK

SWITCHING

REGULATOR

(OPTIONAL)

Figure 1. Feedback Components

Typical values for RB range from 40k to 400k. 0.1% resis-

tors are recommended to maintain output voltage accu-

racy. The buck regulator transient response may improve

with an optional phase lead capacitor CFF that helps cancel

the pole created by the feedback resistors and the input

capacitance of the FB pin. Experimentation with capaci-

tor values between 2pF and 40pF may improve transient

response. The values used in the typical application cir-

cuits are a good starting point.

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.

The minimum on-time of the buck regulators imposes

a minimum operating duty-cycle. The highest switching

frequency (fSW(MAX)) for a given application can be cal-

culated with Equation 2.

fSW MAX

(

) =

VOUT

tON MIN

( ) •PVIN(MAX)

(2)

where PVIN(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 switch-

ing frequency might be necessary to accommodate a high

VIN/VOUT ratio.

The LTC3312SA is capable of a maximum duty-cycle of

98%. Therefore, the VIN-to-VOUT dropout is limited by

0.98 times the input supply, the RDS(ON) of the top switch,

the inductor DCR, and the load current.

Setting the Switching Frequency

The LTC3312SA uses a constant frequency peak current

mode control architecture. There are three methods to set

the switching frequency.

The first method, connecting the RT pin to VIN, sets the

switching frequency to the internal default with a nominal

value of 2MHz.

The second method is with a resistor (RT) tied from the RT

pin to ground. The frequency can be programmed from

1MHz to 3MHz. Table 1 and Equation 3 show the neces-

sary RT value for a desired switching frequency:

RT =

73.4

fSW

−1.9

(3)

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

quency in MHz, ranging from 1MHz to 3MHz.

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