SC171 datasheet(20/27 Pages) SEMTECH

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Layout Guideline

Since the SC171 has integrated switches, special consid-

eration should be given to board layout. Let us use the

schematic shown above as an example. The board level

layout is illustrated in the following four layers.

As shown on the top layer layout, U1 is the switching

regulator SC171. C1 and C11 serve as the decoupling ca-

pacitor for the buck converter power train. C11, with a

value between 1nF and 10nF, is the high frequency filter-

ing capacitor. It is recommended to put C1 and C11 as

close as possible to the SC171 to get the best decoupling

performance, with C11 closest. C1, with a value of 10uF,

should be placed no more than 3mm away from the VIN

pin. L1 is the output filtering inductor. C2, C3 and C4 are

the output filtering capacitors. C5 is the boostrap capaci-

tor. Pin 10 (VDD) is the input bias power for the internal

circuits. It is recommended to get the power from VIN

through an RC filtering network consisted of R1, C6 and

C10. The value of R1 can be between 3.01W and 10W and

the capacitance of C10 should be above 1mF. C6, with a

value of 1nF, is the high frequency filtering capacitor. The

locations of C6 and C10 should be as close as possible to

pins 9 and 10, with C6 closest, to get the best possible

filtering result. R2 is the on-time programming resistor.

R2 should be located as close as possible to pin 8 and it

should return to analog ground. The EN/PSV pin is a tri-

state pin. Pull EN/PSV high to enable the part with power

save mode enabled. Connect EN/PSV to AGND to disable

the switching regulator. Leave EN/PSV floating to enable

the IC in forced continuous conduction mode.

Since there are two integrated MOSFETs inside the SC171

that will dissipate a lot of power, to help spread the heat

out of the IC more efficiently, there is a thermal pad un-

derneath the SC171 serving as a heat sink. To enlarge the

heat sinking area, a large copper plane under the thermal

pad as shown on the top layer is recommended.

On inner layer 2, a large analog ground plane (AGND) on

the right hand side is connected to the thermal pad un-

derneath the SC171 using vias. Thus the heat generated

inside the SC171 can be spread through the vias to the

BST

VIN

PGND EN/PSV

TON

AGND

VDD

PGOOD

EN/PSV

VIN+

VIN-

VO+

VO-

SC171

C10

C11

Schematic for layout illustration

SC171

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