AMS4123 datasheet(13/18 Pages) ADMOS

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AMS4123

3A 20V Step-Down Converter + 1A LDO

3/5/2010

www.advanced-monolithic.com

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Typical bootstrap capacitor values are in the 220nF to

470nF range. Insufficient values will not be able to

provide sufficient base drive current to the power

switch during the on time. Values less than 220nF are

not recommended. This will result in excessive losses

and reduced efficiency.

Optional Snubber

To reduce high frequency ringing at the switching

node a snubber network is suggested. The values

typically selected are 470pF ceramic in series with a

10Ω resistor. The power dissipation of the 10Ω

resistor is about 32mW for a 15V input with a 300kHz

switching frequency.

PR1=C3·Vin

2·Fsw

Vin is the maximum input voltage and Fsw is the

switching frequency. The snubber capacitor must be

rated to withstand the input voltage.

Step-Down Converter Input Capacitor

The low esr ceramic capacitor required at the input to

filter out high frequency noise as well as switching

frequency ripple. Placement of the capacitor is critical

for good high frequency noise rejection. See the PCB

layout guidelines section for details. Switching

frequency ripple is also filtered by the ceramic bypass

input capacitor. Given a desired input voltage ripple

(Vripple) limit, the required input capacitor can be

estimated with:

Dmax=

Vo+Vfwd

Vinmin-V

o+Vfwd

Dmax·Io·(1-Dmax)

Fs·Vripple

2.5V +0.2V

9V-0.3V+0.2V ·2A· 1-

2.5V 0.2V

9V-0.3V+0.2V

300kHz·0.2V

=7μF

For high voltage input converters the duty cycle is

always less than 50% so the maximum ripple is at the

minimum input voltage. The ripple will increase as the

duty cycle approaches 50% where it is a maximum.

Step-Down Converter Feedforward Capacitor

For

optimum

start-up

and

improved

transient

response place a feed-forward capacitor (C6) across

the feedback resistor R2. Typical values range from

220pF to 10nF.

Linear Regulator Output Capacitor

The Linear regulator is stable with a wide range of

ceramic capacitors. The ceramic output capacitor can

range from 1uF to 100uF with either X5R or X7R

temperature coefficient. The actual values selected

within the range will depend on the expected load

transients

and

the

output

voltage

tolerance

requirements during the load transient.

Linear Regulator Input Capacitor

Place a 2.2uF X5R or X7R or equivalent ceramic

bypass capacitor at the LDO input.

Feedback Resistor Selection

The step down converter and LDO both use a 0.6V

reference voltage at the positive terminal of the error

amplifier. To set the output voltage a programming

resistor form the feedback node to ground must first

be selected (R2,R3 of figure 4). A 10kΩ resistor is a

good selection for a programming resistor. A higher

value could result in an excessively sensitive

feedback node while a lower value will draw more

current and degrade the light load efficiency. The

equation for selecting the voltage specific resistor is:

R4=

Vout

Vref

-1

·R3 =

2.5V

0.6V

-1V

·10kΩ=31.67kΩ

Table 2. Feedback Resistor values

Vout (V)

R1,R4 (kΩ)

(R2,R3=10kΩ)

1.8

20.0

2.5

31.6

3.3

45.3

5.0

73.2

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