© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

400mA SmartLDO

MOSFET

SC1532

January 3, 2000

8

APPLICATIONS INFORMATION (Cont.)

Inserting VIN1 = 5.5V, VO = 3.234V, I

VIN2 = 3.6V, I

Using this figure, we can calculate the maximum ther-

mal impedance allowable to maintain T

ambient temperature of 55°C:

This is readily achievable using pcb copper area to aid

in conducting the heat away from the device (see Fig-

ure 1 below).

VIN1 Source Impedance

In order to ensure seamless transitions between sup-

plies with VIN1 rising and falling, it is recommended

that the source impedance of VIN1 is less than

0.125

Ω. This is because as the output current

switches from VIN1 to VIN2 and visa-versa, the sup-

plies can “chatter” if:

In general, this can be avoided by minimizing supply

trace lengths and resistances. In circumstances where

the source impedance is causing supply “chattering”,

increasing the value of the VIN1 input capacitor should

solve the problem by reducing the instantaneous drop

or jump in VIN1 as the supplies are switched.

Layout Considerations

While layout for linear devices is generally not as criti-

cal as for a switching application, careful attention to

detail will ensure reliable operation. See Figure 1 below

for a sample layout.

1) Attaching the part (pins 5 to 8) to a larger copper

footprint will enable better heat transfer from the de-

vice, especially on PCBs where there are internal

ground and power planes.

2) Place the input and output capacitors close to the

device for optimal transient response.

W

/

C

75

936

.

0

55

125

P

T

T

R

)

MAX

(

D

)

MAX

(

A

)

MAX

(

J

)

MAX

)(

A

J

(

TH

°

=

−

=

−

=

−

Fig. 1: Suggested pcb layout based upon Application Circuit on Page 1.

NOTES:

(1) All vias go to ground plane.

(2) Copper area on pins 5 thru 8 is recommended, 0.5” x 0.5” area only is shown. Connect to the ground plane

with a via or vias.

HYST

SOURCE

O

V

R

I

>

•

W

936

.

0

P

)

MAX

(

D

=

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