LTC1530CS8-1.9(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC1530CS8-1.9
(Rev.:RevA)

High Power Synchronous Switching Regulator Controller

Linear Technology 

LTC1530

APPLICATIO S I FOR ATIO

either Q1 or Q2 with the power dissipation split up accord-

ing to the duty cycle:

DC(Q1) = VOUT

VIN

( ) DC(Q2) = 1− VOUT = VIN − VOUT

VIN

VIN

The RDS(ON) 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 or allowable thermal dissipation. A high efficiency

buck converter designed for the Pentium II with 5V input

and a 2.8V, 11.2A output might allow no more than 4%

efficiency loss at full load for each MOSFET. Assuming

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

value of:

(2.8)(11.2A/0.9)(0.04) = 1.39W per FET

and a required RDS(ON) of:

( ) 5V 1.39W

RDS(ON)Q1 = 2.8V11.2A2 = 0.020Ω

( ) 5V 1.39W

( ) RDS(ON)Q2 =

= 0.025Ω

5V − 2.8V 11.2A2

12

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

MOSFETs, the power dissipation numbers are only 1.39W

per device or less — large TO-220 packages and heat

sinks are not necessarily required in high efficiency appli-

cations. Siliconix Si4410DY or International Rectifier

IRF7413 (both in SO-8) or Siliconix SUD50N03 or Motorola

MTD20N03HDL (both in DPAK) are small footprint sur-

face mount devices with RDS(ON) values below 0.03Ω at 5V

of VGS that work well in LTC1530 circuits. With higher

output voltages, the RDS(ON) of Q1 may need to be signifi-

cantly lower than that for Q2. These conditions can often

be met by paralleling two MOSFETs for Q1 and using a

single device for Q2. Using a higher PMAX value in the

RDS(ON) calculations generally decreases the MOSFET

cost and the circuit efficiency and increases the MOSFET

heat sink requirements.

In most LTC1530 applications, RDS(ON) is used as the

current sensing element. MOSFET RDS(ON) has a positive

temperature coefficient. Therefore, the LTC1530 IMAX sink

current is designed with a positive 3300ppm/°C tempera-

ture coefficient. The positive tempco of IMAX provides first

order correction for current limit vs temperature. There-

fore, current limit does not have to be set to an increased

level at room temperature to guarantee a desired output

current at elevated temperatures.

Table 1 highlights a variety of power MOSFETs that are

suitable for use in LTC1530 applications.

Inductor Selection

The inductor is often the largest component in an LTC1530

design and must be chosen carefully. Choose the inductor

value and type based on output slew rate requirements

and expected peak current. The required output slew rate

primarily controls the inductor value. The maximum rate

of rise of inductor current is set by the inductor’s value, the

input-to-output voltage differential and the LTC1530’s

maximum duty cycle. In a typical 5V input, 2.8V output

application, the maximum rise time will be:



DCMAX 

VIN

− VOUT

L 

=

1.85

L

A

µs

1530fa

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