LTC3703 datasheet(27/32 Pages) LINER

27 / 32 page

LTC3703

3703f

watching the output. If this isn’t convenient, a current step

generator is needed. This generator needs to be able to

turn on and off in nanoseconds to simulate a typical

switching logic load, so stray inductance and long clip

leads between the LTC3703 and the transient generator

must be minimized.

Figure 19 shows an example of a simple transient genera-

tor. Be sure to use a noninductive resistor as the load

element—many power resistors use an inductive spiral

pattern and are not suitable for use here. A simple solution

is to take ten 1/4W film resistors and wire them in parallel

to get the desired value. This gives a noninductive resistive

load which can dissipate 2.5W continuously or 50W if

pulsed with a 5% duty cycle, enough for most LTC3703

circuits. Solder the MOSFET and the resistor(s) as close to

the output of the LTC3703 circuit as possible and set up

the signal generator to pulse at a 100Hz rate with a 5% duty

cycle. This pulses the LTC3703 with 500

µstransients10ms

apart, adequate for viewing the entire transient recovery

time for both positive and negative transitions while keep-

ing the load resistor cool.

With 10

µH inductor, ripple current will vary from 3.2A to

4A (32% to 40%) over the input supply range.

Next, verify that the minimum on-time is not violated. The

minimum on-time occurs at maximum VIN:

kHz

ON MIN

OUT

IN MIN

()

()()

(

)

72 250

667

which is above the LTC3703’s 200ns minimum on-time.

Next, choose the top and bottom MOSFET switch. Since

the drain of each MOSFET will see the full supply voltage

72V(max) plus any ringing, choose a 100V MOSFET to

provide a margin of safety. Si7456DP has a 100V BVDSS,

RDS(ON) = 25mΩ(max), δ = 0.009/°C, CMILLER = (19nC –

10nC)/50V = 180pF, VGS(MILLER) = 4.7V, θJA = 20°C/W.

The power dissipation can be estimated at maximum input

voltage, assuming a junction temperature of 100

°C (30°C

above an ambient of 70

°C):

MAIN =+

[]

















1 0 009 100 25 0 025

2 180

10 4 7

250

070

0 94

164

()

(

–

) ( .

)

()

( )(

)•

–.

()

And double check the assumed TJ in the MOSFET:

TJ = 70°C + (1.64W)(20°C/W) = 103°C

Since the synchronous MOSFET will be conducting over

twice as long each period (almost 100% of the period in

short circuit) as the top MOSFET, use two Si7456DP

MOSFETs on the bottom:

SYNC =

−









[]









72 12

1 0 009 100 25

0 025

174

()

(

–

) •

TJ = 70°C + (1.74W)(20°C/W) = 105°C

Next, set the current limit resistor. Since IMAX = 10A, the

limit should be set such that the minimum current limit is

>10A. Minimum current limit occurs at maximum RDS(ON).

APPLICATIO S I FOR ATIO

Figure 19. Transient Load Generator

LTC3703

VOUT

IRFZ44 OR

EQUIVALENT

RLOAD

Ω

0V TO 10V

100Hz, 5%

DUTY CYCLE

LOCATE CLOSE TO THE OUTPUT

3703 F19

PULSE

GENERATOR

Design Example

As a design example, take a supply with the following

specifications: VIN = 36V to 72V (48V nominal), VOUT =

12V

±5%, IOUT(MAX) = 10A, f=250kHz. First, calculate RSET

to give the 250kHz operating frequency:

RSET = 7100/(250-25) = 31.6k

Next, choose the inductor value for about 40% ripple

current at maximum VIN:

kHz









=µ

250

0 4 10

(

)( . )(

)

–

LTC3703 датащи(PDF) 27 Page - Linear Technology

LTC3703 датащи(HTML) 27 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

датащи скачать

ссылки URL