LTC3829 Datasheet, PDF(30/40 Page) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

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LTC3829 Datasheet, PDF (30/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

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LTC3829

Applications Information

4. EXTVCC connected to an output-derived boost network.

For 3.3V and other low voltage regulators, efficiency

gains can still be realized by connecting EXTVCC to an

output-derived voltage that has been boosted to greater

than 4.7V.

For applications where the main input power is 5V, tie

the VIN and INTVCC pins together and tie the combined

pins to the 5V input with a 1Î© or 2.2Î© resistor as shown

in Figure 13 to minimize the voltage drop caused by the

gate charge current. This will override the INTVCC linear

regulator and will prevent INTVCC from dropping too low

due to the dropout voltage. Make sure the INTVCC voltage

is at or exceeds the RDS(ON) test voltage for the MOSFET

which is typically 4.5V for logic-level devices

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors, CB, connected to the BOOST

pins supply the gate drive voltages for the topside MOSFETs.

Capacitor CB in the Functional Diagram is charged though

external diode DB from INTVCC when the SW pin is low.

When one of the topside MOSFETs is to be turned on, the

driver places the CB voltage across the gate source of the

desired MOSFET. This enhances the MOSFET and turns on

the topside switch. The switch node voltage, SW, rises to

VIN and the BOOST pin follows. With the topside MOSFET

on, the boost voltage is above the input supply:

VBOOST = VIN + VINTVCC

The value of the boost capacitor, CB, needs to be 100 times

that of the total input capacitance of the topside MOSFET(s).

The reverse breakdown of the external Schottky diode

must be greater than VIN(MAX). When adjusting the gate

drive level, the final arbiter is the total input current for

the regulator. If a change is made and the input current

decreases, then the efficiency has improved. If there is

no change in input current, then there is no change in

efficiency.

Setting Output Voltage

The LTC3829 output voltage is set by an external feed-

back resistive divider carefully placed across the output,

as shown in Figure 14. The regulated output voltage is

determined by:

VOUT

0.6V

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To improve the frequency response, a feedforward ca-

pacitor, CFFâ, may be used. Great care should be taken to

route the VFB line away from noise sources, such as the

inductor or the SW line.

If diffamp is used, then the resistor, RB, should connect

to the output of the diffamp, DIFFOUT.

LTC3829

VIN

INTVCC

RVIN

1â¦

+ CINTVCC

4.7ÂµF

CIN

3829 F13

Figure 13. Setup for a 5V Input

VOUT

LTC3829

VFB

CFF

3829 F14

Figure 14. Setting Output Voltage

3829f

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