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LTC1430

High Power Step-Down Switching Regulator Controller

Linear Technology 

LTC1430

APPLICATIO S I FOR ATIO

Once the threshold voltage has been selected, RON should

be chosen based on input and output voltage, allowable

power dissipation and maximum required output current.

In a typical LTC1430 buck converter circuit operating in

continuous mode, the average inductor current is equal to

the output load current. This current is always flowing

through either M1 or M2 with the power dissipation split

up according to the duty cycle:

DC (M1) = VOUT

VIN

DC (M2) = 1− VOUT

VIN

= (VIN − VOUT )

VIN

The RON required for a given conduction loss can now be

calculated by rearranging the relation P = I2R:

RON

(M1)

=

PMAX (M1)

DC (M1)• IMAX2

=

VIN • PMAX(M1)

VOUT

•

I

2

MAX

RON

(M2)

=

PMAX (M2)

DC (M2)• IMAX2

( ) =

VIN • PMAX(M2)

VIN − VOUT

•

I

2

MAX

PMAX should be calculated based primarily on required

efficiency. A typical high efficiency circuit designed for 5V

in, 3.3V 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 (3.3V • 10A/0.9) • 0.03 = 1.1W per FET and

a required RON of:

RON (M1) =

5V • 1.1W

3.3V • 10A2

= 0.017Ω

RON (M2) =

(5V

5V • 1.1W

− 3.3V) • 10A2

= 0.032Ω

Note that the required RON for M2 is roughly twice that of

M1 in this example. This application might specify a single

0.03Ω device for M2 and parallel two more of the same

devices to form M1. Note also that while the required RON

values suggest large MOSFETs, the dissipation numbers

are only 1.1W per device or less—large TO-220 packages

and heat sinks are not necessarily required in high effi-

ciency applications. Siliconix Si4410DY (in SO-8) and

Motorola MTD20N03HL (in DPAK) are two small, surface

mount devices with RON values of 0.03Ω or below with 5V

of gate drive; both work well in LTC1430 circuits with up

to 10A output current. 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 LTC1430

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 LTC1430. In a typical 5V to

3.3V application, the maximum rise time will be:

90%• (VIN − VOUT ) AMPS = 1.53A I

L SECOND µs L

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

have a 0.76A/µs rise time in this application, resulting in a

6.5µs delay in responding to a 5A load current step. During

this 6.5µs, 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 5V to 3.xV LTC1430 circuits.

Different combinations of input and output voltages and

expected loads may require different values.

Once the required value is known, the inductor core type

can be chosen based on peak current and efficiency

requirements. Peak current in the inductor will be equal to

the maximum output load current added to half the peak-

to- peak inductor ripple current. Ripple current is set by the

8

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