SC339 datasheet(9/15 Pages) SEMTECH

9 / 15 page

www.semtech.com

SC339

POWER MANAGEMENT

This is a very severe example, since the output

voltage is so low, therefore the allowable window

is ver y small. See Solution 2 for an alternate

circuit. For higher output voltages the components

required will be less stringent.

The input capacitance needs to be large enough to stop

the input supply from collapsing below -5% (i.e., the

design minimum) during output load steps. If the input

to the pass MOSFET is not local to the supply bulk

capacitance then additional bulk capacitance may be

required.

MOSFET selection: since the input voltage to the SC339

is 5V±5%, the minimum available gate drive is:

GS = (4.4 ― 1.1025) = 3.3V

So a MOSFET rated for V

GS = 2.7V will be required, with an

DS(ON)(MAX) (over-temperature) given by:

(VIN(MIN) ― VOUT )

(

1.14 ― 1.05)

DSON(MAX) = ——————

= —————— =

36m

OUT(MAX)

2.5

MOSFET SELECTION

SO-8 Footprint

Vgs

Rds-on

Imax

FDS6682

1.7V

14A

IRF7456

1.5V

6.5m

16A

1206 Footprint

Vgs

Rds-on

Imax

Si5406DC

0.6V

20m

9.5A

NTHS5404

0.6V

25m

7.2A

SOT-23 Footprint

Vgs

Rds-on

Imax

IRLMS2002

30m

5.2A

FDN337N

(when Input

is > 1.2V

for 1.05V

output)

0.7V

65m

2.2A

Solution 2: Using Passive Droop

Passive droop allows us to use almost the full output

tolerance window for transients, therefore making the

output capacitor selection simpler and less

expensive. The trade-offs are the cost of the droop

resistor versus the reduction in output capacitor cost,

and the reduction in headroom which impacts MOSFET

selection. The top of the feedback chain connects to the

input side of R

DROOP, and the output is set for 1.075V. Thus

at no load, V

OUT will be 1.075V (or 1.05V + 2.4%) and at IOUT

= 2.5A, V

OUT will be 1.025V (or 1.05V - 2.4%).

If 1% set resistors are used, the total DC error will be

±2.25% = 24mV. Thus, at no load, the minimum output

voltage will be given by:

OUT(MIN_NO_LOAD) = 1.075 ― 0.024 = 1.051V

This leaves 53.5mV for transient response, giving:

53.5mV = 21.4mΩ and,

ESR(MAX) = ————

2.5A

2.5 • 1 • 10¯6 = 47μF

BULK(MIN) = ———————

53.5 •10¯3

Instead of 2 x 100μF, 12m

Ω capacitors, we can use 1 x

47μF, 15m

Ω capacitor.

Applications Information (Cont.)

1.075V

1.2V +/-5% IN

1.05V @ 2.5A

0.1uF

11.0k

100uF, 25mOhm POSCAP

10.0k

RDROOP

20mOhm

GND

DRV

ADJ

PGD

SC339

SC339 датащи(PDF) 9 Page - Semtech Corporation

SC339 датащи(HTML) 9 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

датащи скачать

ссылки URL