8

 2004 Semtech Corp.

www.semtech.com

SC2738

PRELIMINARY

POWER MANAGEMENT

MOSFETs: very low or low threshold N-channel MOSFETs

are required. Selecting FETs rated for V

will depend upon the available drive voltage (6.9V from

12V in or 4.85V from 5V in), the output voltage and

output current. For the device to work under all

operating conditions, a maximum R

ensure that the output will never go into dropout:

Ω

−

=

)

MAX

(

OUT

)

MAX

(

OUT

)

MIN

(

IN

)

MAX

)(

ON

(

DS

I

V

V

R

Note that R

the minimum V

Setting The Output Voltage: the adjust pins connect

directly to the inverting input of the error amplifiers, and

the output voltage is set using external resistors (please

refer to the Typical Application Circuit on page 1).

Using output 1 as an example, the output voltage can

be calculated as follows:











 +

•

=

2

R

1

R

1

5

.

0

V

OUT

The input bias current for the adjust pin is so low that it

can be safely ignored. To avoid picking up noise, it is

recommended that the total resistance of the feedback

chain be less than 100k

Ω.

Please see Table 1 on this page for recommended

resistor values for some standard output voltages. All

resistors are 1%, 1/10W.

The maximum output voltage that can be obtained from

each output is determined by the input supply voltage

and the R

external MOSFET. Assuming that the MOSFET gate

threshold voltage is sufficiently low for the output

voltage chosen and the worst-case drive voltage, V

OUT(MAX)

is given by:

)

MAX

)(

ON

(

DS

)

MAX

(

OUT

)

MIN

(

DRAIN

)

MAX

(

OUT

R

I

V

V

•

−

=

)

V

(

T

U

O

Vk

(

3

R

r

o

1

R

Ω

Ω

Ω

Ω

Ω)k

(

4

R

r

o

2

R

Ω

Ω

Ω

Ω

Ω)

5

0

.

10

.

1

10

.

0

1

2

.

10

.

4

10

.

0

1

5

.

10

.

0

20

.

0

1

5

.

23

.

5

43

.

1

1

3

.

34

.

3

63

.

1

1

Table 1: Recommended Resistor Values For SC2738

Design Example

Goal: 1.5V±5% static and ±7% transient @ up to 4A

from 2.5V±5% and 5V±5%

Total window for DC error and ripple is ±75mV.

Total window for transient is ±105mV.

Since this device is linear, and assuming that it has been

designed to not ever enter dropout, we do not have ripple

on the output.

The DC error for this output is the sum of:

V

Feedback chain tolerance = ±1% = ±15mV

Load regulation = ±0.25% = ±3.8mV

Set resistors per Table 1 should be 20.0k

Ω (top) and

10.0k

Ω (bottom).

Total DC error = ±4.75% = 71.3mV

This leaves ±2.25% = 33.7mV for the load transient ESR

spike, therefore:

Ω

=

=

m

4

.

8

A

4

mV

7

.

33

R

)

MAX

(

ESR

Bulk capacitance required is given by:

F

dV

t

dI

C

)

MIN

(

BULK

µ

•

=

Where dI is the maximum load current step, t is the

maximum regulator response time and dV is the

allowable voltage droop. Therefore with dI = 4A,

t = 1µs, and dV = 33.7mV:

Applications Infomation (Cont.)