LTC3370 datasheet(20/28 Pages) AD

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LTC3376

Rev 0

For more information www.analog.com

OPERATION

APPLICATIONS INFORMATION

pin powers all of the MOSFET gate drivers, must have its

own10µFbypasscap,andmustbeconnectedontheboard

to INTVCC.Goodbypassingisnecessarytosupplythehigh

transient currents required by the power MOSFET drivers.

To improve efficiency the internal LDO can also draw cur-

rent from the EXTVCC pin if the EXTVCC pin is 3V or higher.

VCC has to be present even if EXTVCC is used. Typically

the EXTVCC pin can be tied to an output of one of the

LTC3376 bucks, or it can be tied to an external supply of

3V or above. If EXTVCC is connected to a supply other than

a buck output, be sure to bypass it with a local ceramic

capacitor. If the EXTVCC pin is below 2.8V, the internal LDO

will consume current from VCC. Applications with high

input voltage and high switching frequency in which the

LDO pulls current from VCC will increase die temperature

because of the higher power dissipation in the LDO. Do not

load the INTVCC pin with external circuitry exceeding 2mA.

Buck Switching Regulator Output Voltage and

Feedback Network

The output voltage of each buck switching regulator is

programmed by a resistor divider across the switching

regulator’s output connecting to its feedback pin and is

given by VOUT = VFB+(1 + R2/R1) as shown in Figure 1

where VFB+ = 400mV. Typical values for R1 range from

20k to 200k. 1% or better resistors are recommended

to maintain output voltage accuracy. 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 capacitor values between 2pF

and 22pF may improve transient response if the resistor

divider has a large VOUT/VFB+ ratio.

The LTC3376 includes low offset, high input impedance

differential sense for applications that require remote

sensing. Connect FB+ to the center tap of the feedback

divider across the output load, and FB– to the load ground.

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 operating frequency for all of the LTC3376 buck regu-

lators can be determined by an external resistor that is

connected from the RT pin to ground. The operating fre-

quency is calculated using the following equation:

fOSC = 2MHz

402kΩ

⎛

⎝

⎜

⎞

⎠

⎟

(4)

While the LTC3376 is designed to function with operat-

ing frequencies between 1MHz and 3MHz, it has internal

safety clamps that prevent the oscillator from running

faster than 4MHz (typical) or slower than 500kHz (typi-

cal). Tying the RT pin to INTVCC sets the oscillator to the

default internal operating frequency of 2MHz (typical).

Although the maximum programmable switching fre-

quency is 3MHz for the LTC3376, the minimum on-time

of the LTC3376 imposes a minimum operating duty

cycle. The typical minimum on-time is 53ns. The highest

Figure 1. Feedback Components

BUCK

SWITCHING

REGULATOR

FB+

CFF

COUT

OPTIONAL

3376 F01

FB–

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