9

 2004 Semtech Corp.

www.semtech.com

SC338(A)

POWER MANAGEMENT

allowable voltage droop. Therefore with dI = 2.5A,

t = 1µs, and dV = 13.1mV:

F

191

10

1

.

13

10

1

5

.

2

C

3

6

)

MIN

(

BULK

µ

=

•

•

•

=

−

−

So if we use 1% V

>100µF, 12m

Ω POSCAPs for output capacitance (which

assumes that local ceramic bypass capacitors will

absorb the balance of the (6 - 5.3)m

Ω ESR

requirement - otherwise 10m

Ω capacitors should be

used).

If we use 0.1% set resistors, then the total DC error

becomes ±2.85% = ±29.9mV, leaving ±2.15% = 22.6mV

for the ESR spike. In this case:

Ω

=

=

m

0

.

9

A

5

.

2

mV

6

.

22

R

)

MAX

(

ESR

and

F

111

10

6

.

22

10

1

5

.

2

C

3

6

)

MIN

(

BULK

µ

=

•

•

•

=

−

−

So for 0.1% resistors we could use 2 x 100µF, 18m

Ω

POSCAPs for output capacitance, or 1 x >100µF, 10m

Ω

POSCAP.

Obviously this is a very severe example, since the output

voltage is so low and therefore the allowable window is

very small. See solution 2 below for an alternate

solution. 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

SC338(A) is 5V±5%, the minimum available gate drive

is:

V

3

.

3

)

1025

.

1

4

.

4

(

V

GS

=

−

=

So a MOSFET rated for V

an R

Ω

=

−

=

−

=

m

36

5

.

2

)

05

.

1

14

.

1

(

I

)

V

V

(

R

)

MAX

(

OUT

OUT

)

MIN

(

IN

)

MAX

)

ON

(

DS

Obviously, if a 12V rail is available to power the SC338(A),

the number of FET options increases dramatically.

Applications Infomation (Cont.)

Solution 2: using passive droop.

or similar

1.075V

1.2V +/-5% IN

1.05V @ 2.5A

C1

0.1uF

R1

11.0k

C3

100uF, 25mOhm POSCAP

U1

SC338(A)

1

2

3

4

5

6

7

8

9

10

DRV1

ADJ1

EN1

PGD1

GND

PGD2

EN2

ADJ2

DRV2

IN

R2

10.0k

RDROOP

20mOhm

Q1

IRF7311

4

1

2

3

5

6

7

8

Passive droop allows us to use almost the full output

tolerance window for transients, hence making the

output capacitor selection simpler and (hopefully)

cheaper. 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

Thus at no load, V

and at I

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

±3.75% = 39mV. Thus at no load, the minimum output

voltage will be given by:

V

036

.

1

039

.

0

075

.

1

V

)

LOAD

_

NO

_

MIN

(

OUT

=

−

=

This leaves 38.5mV for transient response, giving:

Ω

=

=

m

4

.

15

A

5

.

2

mV

5

.

38

R

)

MAX

(

ESR

and

F

65

10

5

.

38

10

1

5

.

2

C

3

6

)

MIN

(

BULK

µ

=

•

•

•

=

−

−

Instead of 2 x 100µF, 12m

Ω capacitors, we can use 1 x

100µF, 15m

Ω capacitor.

Layout Guidelines

The advantages of using the SC338(A) to drive external

MOSFETs are a) that the bandgap reference and control

circuitry are in a die that does not contain high power

dissipating devices and b) that the device itself does not