LTC1649CS View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC1649CS

3.3V Input High Power Step-Down Switching Regulator Controller

Linear Technology 

LTC1649

APPLICATIONS INFORMATION

EXTERNAL COMPONENT SELECTION

Power MOSFETs

Two N-channel power MOSFETs are required for most

LTC1649 circuits. These should be selected primarily by

on-resistance considerations; thermal dissipation is often

a secondary concern in high efficiency designs. The

LTC1649 is designed to be used with 5V logic-level MOS-

FETs; “standard” threshold MOSFETs with RDS(ON) speci-

fied at 10V only will not provide satisfactory performance.

MOSFET RDS(ON) should be chosen based on input and

output voltage, allowable power dissipation and maxi-

mum required output current. In a typical LTC1649 buck

converter circuit operating in continuous mode, the aver-

age inductor current is equal to the output load current.

This current is always flowing through either Q1 or Q2 with

the power dissipation split up according to the duty cycle:

DC (Q1) =

VOUT

VIN

DC

(Q2)

=

1

–

VOUT

VIN

=

(VIN

– VOUT)

VIN

The RON required for a given conduction loss can now be

calculated by rearranging the relation P = I2R:

RDS(ON)

(Q1)

=

PMAX(Q1)

DC(Q1)(IMAX2)

=

VIN(PMAX)(Q1)

VOUT(IMAX2)

RDS(ON)

(Q2)

=

PMAX(Q2)

DC(Q2)(IMAX2)

=

VIN(PMAX)(Q2)

(VIN – VOUT)(IMAX2)

PMAX should be calculated based primarily on required

efficiency. A typical high efficiency circuit designed for

3.3V in, 2.5V at 10A out might require no more than 3%

efficiency loss at full load for each MOSFET. Assuming

roughly 90% efficiency at this current level, this gives a

PMAX value of (2.5V)(10A/0.9)(0.03) = 833mW per FET

and a required RDSON of:

RDS(ON)

(Q1)

=

(3.3V)(833mW)

(2.5V)(10A2)

=

0.011Ω

RDS(ON)

(Q2)

=

(3.3V)(833mW)

(3.3V – 2.5V)(10A2)

=

0.034Ω

Note that while the required RDS(ON) values suggest large

MOSFETs, the dissipation numbers are less than a watt per

device— large TO-220 packages and heat sinks are not

necessarily required in high efficiency applications. Siliconix

Si4410DY and International Rectifier IRF7801 are two

small, surface mount devices with RON values of 0.03Ω or

below with 5V of gate drive; both work well in LTC1649

circuits. A higher PMAX value will generally decrease

MOSFET cost and circuit efficiency and increase MOSFET

heat sink requirements.

Inductor

The inductor is often the largest component in an LTC1649

design and should be chosen carefully. Inductor value and

type should be chosen based on output slew rate require-

ments and expected peak current. Inductor value is prima-

rily controlled by the required current slew rate. The

maximum rate of rise of the current in the inductor is set

by its value, the input-to-output voltage differential and the

maximum duty cycle of the LTC1649. In a typical 3.3V to

2.5V application, the maximum rise time will be:

93%

(VIN

– VOUT)

L

AMPS

SECOND

=

0.744A

µs

I

L

where L is the inductor value in µH. A 2µH inductor would

have a 0.37A/µs rise time in this application, resulting in a

14µs delay in responding to a 5A load current step. During

this 14µs, the difference between the inductor current and

the output current must be made up by the output capaci-

tor, causing a temporary droop at the output. To minimize

this effect, the inductor value should usually be in the 1µH

to 5µH range for most typical 3.3V to 2.xV LTC1649

circuits. Different combinations of input and output volt-

8

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