SC4524C 查看數據表（PDF） - Semtech Corporation

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SC4524C

28V 2A Step-Down Switching Regulator

Semtech Corporation 

SC4524C

Applications Information (Cont.)

Therefore, the procedure of the voltage loop design for

the SC4524C can be summarized as:

(1) Plot the converter gain, i.e. control to feedback transfer

function.

(2) Select the open loop crossover frequency, FC, between

10% and 20% of the switching frequency. At FC, find the

required compensator gain, AC. In typical applications with

ceramic output capacitors, the ESR zero is neglected and

the required compensator gain at FC can be estimated by

AC

=

− 20 x logGCARS x



2πFC CO x

VFB

VO



(3) Place the compensator zero, FZ1, between 10% and

20% of the crossover frequency, FC.

(4) Use the compensator pole, FP1, to cancel the ESR zero, FZ.

(5) Then, the parameters of the compensation network

can be calculated by

AC

R7

=

0 20

gm

C5

=



2 π FZ

R7

C8

=



2 πFP

R7

Then the compensator parameters are

.4

R7 =

0 20

0.3 x 0−3

=  2 .4 k

C5

=

2π



x6 x03 x 2.4

x03

=

0.8 nF

C8 =



2 πx600 x03 x2.4 x 03

=2 pF

Select R7=12.4k, C5=1nF, and C8=22pF for the design.

Compensator parameters for various typical applications

are listed in Table 5. A MathCAD program is also available

upon request for detailed calculation of the compensator

parameters.

Thermal Considerations

For the power transistor inside the SC4524C, the

conduction loss PC, the switching loss PSW, and bootstrap

circuit loss PBST, can be estimated as follows:

PC = D x VCESATx IO

PSW =



x

2

tS

x VIN x IO x FSW

PB S T

= D x VBSTx

IO

40

where gm=0.3mA/V is the EA gain of the SC4524C.

Example: Determine the voltage compensator for an

800kHz, 12V to 3.3V/2A converter with 22uF ceramic

output capacitor.

Choose a loop gain crossover frequency of 80kHz, and

place voltage compensator zero and pole at FZ1=16kHz

(20% of FC), and FP1=600kHz. From the equation in step

(2), the required compensator gain at FC is

$&





˜ ORJ¨¨©§ 

˜



 ˜

˜

ʌ

˜ 



˜

˜



˜

˜





¸¸¹·

G%

where VBST is the BST supply voltage and tS is the equivalent

switching time of the NPN transistor (see Table 4).

Input Voltage

12V

24V

28V

Load Current

1A

2A

12.5ns 15.3ns

22ns 25ns

25.3ns 28ns

Table 4. Typical switching time

In addition, the quiescent current loss is

PQ = VIN x 2mA

15

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