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SC4609EVB

Low Input, MHz Operation, High Efficiency Synchronous Buck

Semtech Corporation 

SC4609

POWER MANAGEMENT

Application Information (Cont.)

For a low voltage and high output current application such

as the 3.3V/1.5V@12A case, the conduction loss is of-

ten dominant and selecting low RDS(ON) MOSFETs will no-

ticeably improve the efficiency of the converter even

though they give higher switching losses.

The gate charge loss portion of the top/bottom MOSFET’s

total power loss is derived from the SC4609. This gate

charge loss is based on certain operating conditions (f ,

s

VGATE, and IO).

The thermal estimations have to be done for both

MOSFETs to make sure that their junction temperatures

do not exceed their thermal ratings according to their

total power losses PTOTAL, ambient temperature TA and their

thermal resistance RθJA as follows:

Figure 4. Compensation network provides 3 poles and

2 zeros.

For voltage mode step down applications as shown in

Figure 4, the power stage transfer function is:

TJ(max)

<

TA

+

PTOTAL

RθJA

Loop Compensation Design

For a DC/DC converter, it is usually required that the

converter has a loop gain of a high cross-over frequency

for fast load response, high DC and low frequency gain

for low steady state error, and enough phase margin for

its operating stability. Often one can not have all these

properties at the same time. The purpose of the loop

compensation is to arrange the poles and zeros of the

compensation network to meet the requirements for a

specific application.

The SC4609 has an internal error amplifier and requires

the compensation network to connect among the COMP

pin and VSENSE pin, GND, and the output as shown in

Figure 4. The compensation network includes C1, C2,

R1, R7, R8 and C9. R9 is used to program the output

voltage according to

VO

=

0.5 ⋅ (1+

R7

R9

)

1+

s

1

GVD (s)

=

VI

1+

s

RC

L1

R

+

⋅ C4

s2L1C4

Where:

R = load resistance and

RC = C4’s ESR.

The compensation network will have the characteristic

as follows:

Where

1+ s 1+ s

GCOMP (s) =

ωI

s

⋅

1+

ωZ1

s

⋅

⋅1+

ωZ2

s

ωP1

ωP2

1

ωI = R7 ⋅ (C1 + C2 )

1

ωZ1 = R1 ⋅ C2

1

ωZ2 = (R7 + R8 ) ⋅ C9

ωP1

=

C1 + C2

R1 ⋅ C1 ⋅ C2

 2006 Semtech Corp.

11

www.semtech.com

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