AMS4123S datasheet(11/18 Pages) ADMOS

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AMS4123

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

3/5/2010

www.advanced-monolithic.com

Phone (925) 443-0722

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Device Summary

The AMS4123 is combines a high voltage 3 Amp fixed

frequency step-down converter combined with a 1

Amp low drop out (LDO) linear regulator on a single

die.

The peak current mode step-down converter has

internal compensation and is stable with a wide range

ceramic,

tantalum,

and

electrolytic

output

capacitors. The step-down converter output voltage is

sensed through an external resistive divider that feeds

the negative input to an internal transconductance

error amplifier. The output of the error amplifier is

connected to the input to a peak current mode

comparator. The inductor current is sensed as it

passes through the power switch, amplified and is

also fed to the current mode comparator. The error

amplifier regulates the output voltage by controlling

the peak inductor current passing through the power

switch so that, in steady state, the average inductor

current equals the load current. The step-down

converter has an input voltage range of 4.5V to 20V

with an output voltage as low as 0.6V.

The LDO operates from an input voltage ranging from

1V to 20V and a typical dropout voltage of 350mV at

1A. The input to the LDO can be supplied by the

output of the Step-Down converter or some other

available power source that must be 2V less than the

input voltage (Vin). The LDO is also stable for a wide

range of ceramic output capacitors ranging from as

low as 1µF.

Enable

The enable input has two levels so that the step-down

converter can be enabled independently of the LDO.

The enable threshold for the step-down converter is

2.0V while the enable threshold for the linear regulator

output is 2.5V typical.

Under Voltage Lockout

The under-voltage lockout (UVLO) feature guarantees

sufficient input voltage (Vin) bias for proper operation

of all internal circuitry prior to activation. The input

voltage (Vin) is internally monitored and the converter

and LDO are enabled when the rising level of Vin

reaches 4.2V. To prevent UVLO chatter 400mV of

hysteresis is built in to the UVLO comparator so that

the step-down converter and LDO are disabled when

VIN drops to 3.8V.

Fault Protection

Short circuit and over-temperature shutdown disable

the converter and LDO in the event of an overload

condition.

Application

Inductor

The step-down converter inductor is typically selected

to limit the ripple current to 40% of the full load output

current. Solve for this value at the maximum input

voltage where the inductor ripple current is greatest.

L= Vin-Vo ·

Vin·Io·0.4·Fs

L= 15V-2.5V ·

2.5V

15V·2A·0.4·300kHz

=9.4µH

For most applications the duty cycle of the AMS4123

step down converter is less than 50% duty and does

not require slope compensation for stability. This

provides some flexibility in the selected inductor

value. Given the above selected value, others values

slightly greater or less may be examined to determine

the effect on efficiency without a detrimental effect on

stability.

With and inductor value selected, the ripple current

can be calculated:

Ipp=

(Vo+Vfwd)·(1-D)

L·Fs

Using the maximum input voltage values the ripple is:

Ipp=

(2.5V+0.2V)· 1-0.23

10μH·300kHz

=0.7A

Once the appropriate value is determined, the

component is selected based on the DC current and

the peak (saturation) current. Select an inductor that

has a DC current rating greater than the full load

current of the application. The DC current rating is

also

reflected

the

resistance

(DCR)

specification of the inductor. The inductor DCR should

limit the inductor loss to less than 2% of the step-

down converter output power.

The peak current at full load is equal to the full load

DC current plus one half of the ripple current. As

mentioned before, the ripple current varies with input

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