CM3706A 查看數據表（PDF） - Champion Microelectronic

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CM3706A

1.5MHz, 600mA Synchronous Step-Down Regulator

Champion Microelectronic 

CM3706A

1.5MHz, 600mA Synchronous Step-Down Regulator

APPLICATIONS INFORMATION

Setting the Output Voltage

Figure 2 above shows the basic application circuit with

CM3706A adjustable output version. The external resistor

sets the output voltage according to the following equation:

VOUT

=

0.6V ⎜⎛1+

⎝

R2 ⎟⎞

R1 ⎠

R1=316KΩ ; R2= 316KΩ ; VOUT =1.2V

R1=316KΩ ; R2= 470KΩ ; VOUT =1.5V

R1=316KΩ ; R2= 630KΩ ; VOUT =1.8V

R1=316KΩ ; R2= 1MΩ ; VOUT =2.5V

R1=316KΩ ; R2= 1.42MΩ ; VOUT =3.3V.

Input Capacitor Selection

The input capacitor reduces the surge current drawn from the

input and switching noise from the device. The input capacitor

impedance at the switching frequency shall be less than input

source impedance to prevent high frequency switching current

passing to the input. A low ESR input capacitor sized for

maximum RMS current must be used Ceramic capacitors with

X5R or X7R dielectrics are highly recommended because of

their low ESR and small temperature coefficients. A 4.7µF

ceramic capacitor for most applications is sufficient.

Inductor Selection

For most designs, the CM3706A operates with inductors of

1µH to 4.7µH. Low inductance values are physically smaller

but require faster switching, which results in some efficiency

loss. The inductor value can be derived from the following

equation:

( ) L = VOUT × VIN − VOUT

VIN × ΔI L × fOSC

Where ΔI L is inductor Ripple Current.

Large

value

inductors lower ripple current and small value inductors result

in high ripple currents. Choose inductor ripple current

approximately 35% of the maximum load current

600mA, or ΔI L =210mA.

IN

2.5V TO

5.5V

L1

4

3

2.2uH

VIN

SW

OUT

CM3706A

R2

1%634k

1 RUN GND VFB 5

Cin

4.7uF

2

R1

Cout

10uF

C1

316K

C3

1%

22PF

C2

Figure2 Application Circuit with

adjustable output versions

For output voltages above 2.0V, when light-load efficiency is

important, the minimum recommended inductor is 2.2µH.

For optimum voltage-positioning load transients, choose an

inductor with DC series resistance in the 50mΩ to 150mΩ

range. For higher efficiency at heavy loads, or minimal load

regulation (but some transient overshoot), the resistance

should be kept below 100mΩ. The DC current rating of the

inductor should be at least equal to the maximum load current

plus half the ripple current to prevent core saturation

(600mA+105mA).

Output Capacitor Selection

The output capacitor is required to keep the output voltage

ripple small and to ensure regulation loop stability. The output

capacitor must have low impedance at the switching frequency.

Ceramic capacitors with X5R or X7R dielectrics are

recommended due to their low ESR and high ripple current.The

output ripple VOUT is determined by:

ΔVOUT

≤

VOUT × (VIN − VOUT

VIN × fOSC × L

)

×

⎜⎜⎝⎛

ESR

+

8

×

f

1

osc

×

C3

⎟⎟⎠⎞

Input Zener Selection

In USB device application, we will add zener. The zener

can be reduced the surge voltage when device plug in and plug

out. Application circuit is show in Figure3.

2008/03/07 Rev. 1.0

Champion Microelectronic Corporation

Page 9

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