LTC1629CG datasheet(22/28 Pages) LINER

22 / 28 page

LTC1629/LTC1629-PG

Although the LTC1629 has a maximum input voltage of

36V, most applications will be limited to 30V by the

MOSFET BVDSS.

Since the output voltage is below 2.4V the output resistive

divider will need to be sized to not only set the output

voltage but also to absorb the sense pin input current for

both channels.

MIN

OUT

22 4

() =

−









−









Choosing 1% resistors: R1=13.2k and R2=16.5k yields an

output voltage of 1.80V and satisfies the above condition.

The power dissipation on the topside MOSFET can be

easily estimated. Using a Siliconix Si4420DY for example;

RDS(ON) = 0.013Ω, CRSS = 300pF. At maximum input

voltage with Tj (estimated) = 110°C at an elevated ambient

temperature:

kHz

MAIN =

() +() °− °

()

[]

() ( )( )

()=

0 005 110

0 013

1 7 5 5

300

310

0 61

Ω

The worst-case power disipated by the synchronous

MOSFET under normal operating conditions at elevated

ambient temperature and estimated 50

°C junction tem-

perature rise is:

SYNC =

−

() ( )

Ω

()

1 48 0 013

129

A short-circuit to ground will result in a folded back current

of:

SC =

Ω

()













0 005

200

5 5

528

APPLICATIO S I FOR ATIO

Figure 8. Automotive Application Protection

VIN

1629 F08

12V

50A IPK RATING

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

LTC1629

Design Example (Using Two Phases)

As a design example, assume VIN=5V(nominal),VIN = 5.5V

(max), VOUT = 1.8V, IMAX = 20A, TA = 70°C and f = 300kHz.

The inductance value is chosen first based on a 30% ripple

current assumption. The highest value of ripple current

occurs at the maximum input voltage. Tie the FREQSET pin

to the INTVCC pin for 300kHz operation. The minimum

inductance for 30% ripple current is:

kHz

OUT

≥

∆

()

−









≥ ()( )( ) −









≥µ

300

135

A 2

µH inductor will produce 20% ripple current. The peak

inductor current will be the maximum DC value plus one

half the ripple current, or 11.5A. The minimum on-time

occurs at maximum VIN:

kHz

ON MIN

OUT

() == ()(

)

=µ

5 5

300

The RSENSE resistors value can be calculated by using the

maximum current sense voltage specification with some

accomodation for tolerances:

SENSE =≈

Ω

11 5

0 005

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