AMS4123 [ADMOS]

Dual Threshold Enable;


型号：	AMS4123
厂家：	ADVANCED MONOLITHIC SYSTEMS
描述：	Dual Threshold Enable
文件：	总18页 (文件大小：590K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

AMS4123

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

General Description

Features

The AMS4123 combines a 3A Step-Down converter

with a 1A LDO in a single SO-8 exposed paddle

package. Both the LDO and Step-Down converter

are low ESR, ceramic capacitor output, stable. The

Step-Down converter is internally compensated with

internal soft-start to minimize the number of external

components. An Enable pin provides built-in

externally programmable power-up sequencing. The

Step-Down converter enable threshold is 2.0V and

the LDO enable threshold is 2.5V. It also has hiccup

current limit and thermal protection. Thermal

protection shuts down both the Step-Down converter

and LDO when the die temperature exceeds 135°C.

•

Step-Down Converter + LDO in SO-8EP

Internally Compensated

Up to 95% Efficiency

Low ESR Ceramic Output Capacitor Stable

Soft Start

Under-Voltage Lockout

Dual Threshold Enable

300 kHz Switching Frequency

Hiccup Current Limit

Over-Temperature Shutdown

Ultra-Low Dropout LDO 350mV @ 1A

Up to 3A Step-Down Output Current

Up to 1A LDO Output Current

Excellent Light Load Efficiency

Both regulators are adjustable using

a 0.6V

reference for low output voltage settings. The LDO

has options for fixed output voltages from 0.6V to 5V

in 100mV steps. The LDO external input can be

powered from the Step-Down converter output, for

improved efficiency, or from any voltage source that

is less than or equal to the device supply voltage

(Vin). With a dropout voltage of less than 350mV at

1A, the AMS4123 LDO makes the perfect solution

for a low noise 1.8V power source developed from

2.5V Step-Down converter output. The AMS4123 is

a complete solution for LCD TV power requirements

when combined with the AMS4122 (2A Dual

Switching Regulator in SO-8).

Applications

•

Audio Power Amplifiers

Portable (Notebook) Computers

Point of Regulation for High Performance

Electronics

•

Consumer Electronics

DVD, Blue-ray DVD writers

LCD TVs and LCD monitors

Distributed Power Systems

Battery Chargers

Pre-Regulator for Linear Regulation

Typical Application

Vin 4.5V to 20V

220nF

10uH

AMS4123

2.5V at 2A

SW out

Vin

2.5V

LDOin

BST

22uF

100uF

10uF

1.8V at1A

B340LB

LDOout FB SW

20.0k

FB LDO

2.2uF

10.0k

31.6k

R1 and R4 Voltage Options

1.8V 20.0k

Enable

2.5V 31.6k

3.3V 45.3k

4.7nF

5.0V 73.2k

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AMS4123

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

Pin Description

Pin #

Symbol

Description

Step-Down converter switching node that connects the internal power switch to the

output inductor.

The bootstrap capacitor tied to this pin is used as the bias source for the drive to the

internal power switch. Use a 220nF or greater capacitor from the BST to the SW

pin.

Input Power. Supplies bias to the IC and is also the power input to the step-down

converter main power switch. Bypass Vin with low impedance ceramic with

sufficient capacitance to minimize switching frequency ripple as well as high

frequency noise.

BST

Vin

Enable. A voltage greater than 2V at this pin enables the switching regulator. 2.5V

enables the LDO section.

Step-Down Converter Feedback input. A resistor network of two resistors is used to

set-up the output voltage connected between V_{SW out}and GND. The node between

the two resistors is connected to Feedback Switch pin.

LDO Feedback input. A resistive voltage divider is used to set the output voltage

connected between the LDO output and GND. The node between the two resistors

is connected to FB LDO pin.

FB SW

FB LDO

LDO Input. Connect to the output of the Step-Down converter. LDO IN can also be

powered from any power supply as long as it is 2V less than Vin.

LDO Output pin.

Ground paddle to be connected to PCB ground plane. This is also the ground for

internal voltage reference.

LDO in

LDO out

GND (PADDLE)

Pin Configuration

8L SOIC

SO Package (S)

Top View

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

Absolute Maximum Ratings ⁽¹⁾

Recommended Operating Conditions ⁽²⁾

V_INSupply Voltage………………...………..….-0.3V to 23V

LDO_INSupply Voltage…………………..……...-0.3V to 20V

LDO_OUTOutput Voltage………………….….....-0.3V to 20V

BST Boot Strap Voltage………………....…. -0.3V to 27V

FBLDO,FBSW feedback pins………....……-0.3V to +12V

EN Enable Voltage………………………..….-0.3V to +20V

Storage Temperature Range……………...-65⁰C to 150⁰C

Lead Temperature…………………..……….….…… 260⁰C

Junction Temperature………...………………..…… 150⁰C

Input Voltage………………………………….………..4.5V to 20V

Ambient Operating Temperature…… …………….-40⁰C to 85⁰C

Thermal Information

(3)

8L SOIC EP θ_JA

…………………………………….…...45⁰C/W

θ_JC...........................................................10⁰C/W

Maximum Power Dissipation…………………………...….…….2W

Electrical Characteristics T_A= 25 °C and V_IN=12V (unless otherwise noted).

Parameter

Symbol

V_in

V_FBLDO

V_FBSW

Conditions

Min.

Typ.

Max.

Units

V_in

4.5

LDO Feedback Voltage

I_LDO=0A

I_sw=0A

tbd

0.586

0.596

tbd

Switcher Feedback Voltage

tbd

V_{LDO out}=0.6V to 5V in

100mV increments

LDO Output Voltage tolerance

V_{LDO Out}

-1.5

1.5

Step-Down Converter Bias

Current

V_LDOin=V_EN=5V

V_FBSW= 1.5V

V_LDOin=V_EN=5V

I_QSW

1.4

1.3

1.9

2.0

LDO+SW Bias Current

I_QSW+LDO

V_FBLDO=V_FBSW= 1.5V

LDO Bias Current

I_QLDO

I_Vinsd

V_EN= 5V; V_FBLDO= 1.5V

V_EN=0V

400

μA

Shutdown Supply Current

SW NPN Saturation Voltage

Converter Current Limit

V_SAT

I_LIMSW

I_LIMLDO

V_DO

I_{SW out}=1A

0.66

4.2

V_{SW out}=5V

LDO Current Limit

1.1

V_{LDO in}=5V; C_o=2.2μF

LDO Dropout Voltage

V_LDOin=V_LDOout-0.1V, Io=1A

350

ΔV_{LDO Out}

V_{LDO Out}

LDO Load Regulation

LDO Line Regulation

I_LDO= 0 to1A

0.5

0.1

ΔV_{LDO Out}

V_{LDO Out}

V_LDOin= V_LDOout+0.5V to 20V,

V_in=20V

Oscillator Frequency

Maximum Duty Cycle

Minimum Duty Cycle

Converter Enable Threshold

Enable Hysteresis

F_OSC

260

300

340

kHz

D_MAX

D_MIN

V_FB=0V

V_FB=1.5V

V_{EN SW}

V_ENHYS

V_{EN LDO}

I_EN

2.0

100

2.5

0.7

4.2

2.1

LDO Enable Threshold

Enable Pull-up Current

Under Voltage Lockout

2.55

V_EN= 0V

V_inrising

μA

V_UVLO

Under Voltage Lockout

Hysteresis

V_{UVLO HYS}

200

Total Power dissipation

Thermal Shutdown

P_D

Note (4)

2.5

T_SD

145

°C

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AMS4123

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

Notes:

Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device.

Operation outside of the recommended operating conditions is not guaranteed.

Measured on approximately 1” square of 1 oz. copper.

The total power dissipation for SO-8 EDP package is recommended to 2.5W rated at 25⁰C ambient temperature. The thermal resistance Junction to Case

is 45⁰C/W. Total power dissipation for the switching regulator and the LDO should be taken in consideration when calculating the output current capability

of each regulator.

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AMS4123

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

Typical Characteristics

Efficiency V_{SW out}=5V, L=10µH,

B340LB Schottky

Load Regulation V_{SW out}=5V, L=10µH

1.0

100

V_in=12V

0.6

V_in=12V

0.2

V_in=23V

-0.2

-0.6

-1.0

V_in=23V

0.01

0.1

0.01

0.1

Output Current (A)

Efficiency V_{SW out}=3.3V,

L=10µH, B340LB Schottky

Load Regulation V_{SW out}=3.3V, L=10µH

1.0

0.6

100

V_in=12V

V_in=23V

V_in=12V

0.2

V_in=23V

-0.2

-0.6

-1.0

0.01

0.1

0.01

0.1

Output Current (A)

Efficiency V_{sw out}=2.5V, L=10µH,

B340LB Schottky

Load Regulation V_{sw out}=2.5V, L=10 µH

1.0

0.6

100

V_in=12V

V_in=23V

0.2

-0.2

-0.6

-1.0

V_in=23V

V_in=12V

0.01

0.1

0.01

0.1

Output Current (A)

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

Typical Characteristics

Output Voltage Error vs. Input Voltage

No Load Input Current vs. Input Voltage

_{sw out}= 2.5V, V_{LDO out}= 1.8V

_{SW out =}V_{LDO in}= 2.5V, V_{LDO out}=1.8V

0.50

0.25

3.2

2.4

1.6

0.8

0.0

V _{LDO out}

0.00

V_{sw out}

-0.25

-0.50

I_LDO=0.6A

I_sw=1.6A

Input Voltage (V)

Input Voltage V_in(V)

Switching Frequency vs. Input Voltage

_{sw out}= 2.5V, V_{LDO out}= 1.8V

LDO Dropout Voltage vs. Load Current

_{LDO in}= V_{LDO out}- 0.1V

308

304

300

296

292

0.4

0.3

0.2

0.1

V_{LDO out}programmed for 1.8V

V_in= 12V

0.2

0.4

0.6

0.8

Input Voltage (V)

LDO Output Current (V)

V_{LDO Out}Load Regulation

_{SW out}= V_{LDO in}=2.5V, V_{LDO Out}=1.8V

Feedback Voltage Temperature

Variation

1.5

0.61

0.60

0.59

0.58

0.57

FB_SW

V_in=12V

V_in=15V

FB_LDO

0.5

I_LDO=I_sw=0

V_{LDO out}=1.8V, V_{SW out}=2.5V

V_in=20V

0.8 1

-50

-10

110

150

0.2

0.4

0.6

Output Current (A)

Ambient Temperature (ºC)

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

Typical Characteristics

Step-Down Converter Output Ripple

V_{SW out}=2.5V, I_{SW out}=1.8A, V_in=12V

LDO 200mA to 800mA Transient Response,

V_{LDO in}=3.3V, Co=2.2µF, V_{LDO out}= 1.8V, V_in=12V

V_{SW out}

20mVac

/div

V_{LDO out}

100mVac

/div

1A/div

I_{LDO out}

500mA

/div

V_SW

5V/div

2 µsec/div

1 µsec/div

Step-Down Converter Load Transient

No Load to 2A,V_{SW out}=2.5V, V_in=12V

Step-Down Converter Load Transient

200mA to 2A, V_{sw out}= 2.5V, V_in=12V

V_{SW out}

100mVac

/div

V_{SW out}

100mVac

/div

I_{SW out}

500mA

/div

I_{SW out}

1A/div

2 msec/div

40 µsec/div

LDO Transient Response

No Load to 1A, V_{LDO in}=V_{sw out}=2.5V,

Step-Down Converter Load Transient

200mA to 1.2A, V_{sw out}= 2.5V,V_in=12V

V_{SW out}

200mVac/

div

V_{SW out}

100mVac

/div

V_{LDO out}

100mVac

/div

I_{SW out}

500mA

/div

I_{LDO out}

1A/div

20 µsec/div

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AMS4123

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

Typical Characteristics

Start-Up Response V_in=12V

Switching Frequency Temperature

Variation V_{SW out}=2.5V, V_in=12V

V_{SW out}

1V /div

300

V_{LDO out}

1V /div

280

260

240

220

2A/div

V_en

5V /div

-45

-10

130

Ambient Temperature (ºC)

400 µsec/div

Start-Up Response

Enable=V_in=12V

Feedback Voltage Temperature

Variation

•

0.8

0.0

FB_SW

V_{LDO out}

1V /div

FB_LDO

-0.8

-1.6

-2.4

I_LDO=I_sw=0

V_{LDO out}=1.8V, V_{SW out}=2.5V

2A/div

V_in

10V /div

-50

-10

110

150

Ambient Temperature (ºC)

2 msec/div

Start-Up Response V_in=20V

Step-Down Converter Power Switch

Saturation Voltage V_in=12V

1.2

V_{SW out}

1V /div

0.9

0.6

0.3

V_{LDO out}

1V /div

2A/div

Tamb = 25⁰ C

Mounted on Eval. Board

V_en

5V /div

0.7

1.4

2.1

2.8

3.5

Current (A)

1 msec/div

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

Typical Characteristic

LDO Ground Current

V_{LDO in =}3.3V, V_{LDO out}=1.8V

LDO Current Limit

V_{LDO in =}3.3V, V_{LDO out}=1.8V

1.4

1.3

1.2

1.1

0.9

0.8

0.7

0.6

Voltage Mode Load V_{LDO out}= 1.68V

200

400

600

800

1000

Load Current (mA)

V_inInput Voltage (V)

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

Functional Block Diagram

Vin

UVLO

BST

Reg.

4.2V / 3.8V

Internal

Vcc

Regulator

Vcc

3.3V

Isense

Vref

0.6V

EAout

BST

SET

CLR

Level

Shift

300kHz

Oscillator

SW out

FB SW

EAout

Vref

0.6V

Switching

Regulator

Shutdown

PVin

LDO In

2.0V

Vref

Shutdown

Comparators

2.5V

LDO Out

Pgnd

Paddle

FB LDO

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

Device Summary

Fault Protection

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.

Short circuit and over-temperature shutdown disable

the converter and LDO in the event of an overload

condition.

The peak current mode step-down converter has

internal compensation and is stable with a wide range

of 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.

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

2.5V

ꢀ

ꢁ

L= 15V-2.5V ·

=9.4µH

15V·2A·0.4·300kHz

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:

(Vo+Vfwd)·(1-D)

Ipp=

L·Fs

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.

Using the maximum input voltage values the ripple is:

ꢀ

ꢁ

(2.5V+0.2V)· 1-0.23

Ipp=

=0.7A

10μH·300kHz

Under Voltage Lockout

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 in the DC 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 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.

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

voltage and is a maximum at the maximum input

voltage.

Step-Down Converter Output Capacitor

The optimum solution for the switching regulator is to

use a large bulk capacitor for large load transients in

parallel with a smaller, low ESR, X5R or X7R ceramic

capacitor to minimize the switching frequency ripple.

(Vo+Vfwd)·(1-Dmin)

Ipkmax=Io+

2·L·Fs

High Frequency Ripple

Dmin=

The following equation determines the required low

ESR ceramic output capacitance for a given inductor

current ripple (Ipp).

Vinmax

The duty cycle can be more accurately estimated by

including the drops of the external Schottky diode and

the internal power switch:

Ipp

0.7A

=15μF

Fs·8·dV 300kHz·8·20mV

Vo+Vfwd

Dmin=

Large Signal Transient

For applications with large load transients an

additional capacitor may be required to keep the

output voltage within the limits required during large

load transients.

Vinmax-Vo+Vfwd

2.5V+0.2V

Dmin=

=0.23

15V-0.3V+0.2V

In this case the required capacitance can be

examined for the load application and load removal.

For full load to no load transient the required

capacitance is

Vfwd is the diode freewheeling diode drop and Vsw is

the collector to emitter drop of the internal power

switch.

With a good estimate of the duty cycle (D) the

inductor peak current can be determined:

L·Io²

Vos²-Vo²(2.7V)²-(2.5V)²

10μH·(2A)²

Cbulk=

=36μF

(2.5V+0.2V)·(1-0.23)

For the application of a load pulse the capacitance

required form hold up depends on the time it takes for

the power supply loop to build up the inductor current

to match the load current. For the AMS4123 this can

be estimated to be less than 10 µsec or about three

clock cycles.

Ipkmax=2A+

=2.35A

2·10µH·300kHz

There are a wide range 2 and 3 Amp, shielded and

non-shielded inductors available. Table 1 lists a few.

Table 1. Inductor Selection Guide

Dimensions (mm)

Io·t 2A·10μsec

Cbulk=

=100μF

0.2V

Series

Coilcraft

Type

For applications that do not have any significant load

transient requirements a ceramic capacitor alone is

typically sufficient.

Non-

Shielded

Non-

DO3316P

DO3308

9.4

5.2

3.0

Shielded

Boot Strap Capacitor

Sumida

An external capacitor is required for the high side

switch drive. The capacitor is biased during the off

time while the switch node is at ground by way of the

freewheeling diode. During the on time portion of the

switching cycle the switch node is tied to the input

voltage by way of the internal power switch. The boot

strap capacitor is always referenced to the switch

node so the charge stored in the capacitor during the

off time is then used to drive the internal power switch

during the on time.

CDRH6D26

Shielded

2.8

5.2

Non-

Shielded

CDH74

7.3

Coiltronics

SD8328

Shielded

8.3

9.5

3.0

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

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.

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.

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.

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:

P_R1=C3·Vin²·Fsw

V_inis the maximum input voltage and F_swis 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:

R4=ꢅ^V_V^o_re^u_f^t-1ꢆ ·R3 =ꢅ₀²_.^.5₆^V_V-1Vꢆ ·10kΩ=31.67kΩ

Table 2. Feedback Resistor values

R1,R4 (kΩ)

Vout (V)

1.8

(R2,R3=10kΩ)

Vo+Vfwd

Dmax=

20.0

31.6

Vinmin-Vo+Vfwd

2.5

3.3

5.0

45.3

73.2

Dmax·Io·(1-Dmax)

Fs·Vripple

2.5V +0.2V

9V-0.3V+0.2V

2.5Vꢄ0.2V

9V-0.3V+0.2V

ꢂ

ꢃ ·2A· ꢂ1-

ꢃ

=7μF

300kHz·0.2V

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.

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AMS4123

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

PCB Layout

The following guidelines should be followed to insure

proper layout.

Ion+ Ioff

1. Vin Capacitor. A low ESR ceramic bypass

capacitor must be placed as close to the IC as

possible.

2. Schottky Diode. During the off portion of the

switching cycle the inductor current flows through

the Schottky diode to the output cap and returns

to the inductor through the output capacitor. The

trace that connects the output diode to the output

capacitor sees a current signal with a very high

di/dt. To minimize the associated spiking and

ringing, the inductance and resistance of this

trace should be minimized by connecting the

diode anode to the output capacitor return with a

short wide trace.

3. Feedback Resistors. The feedback resistors

should be placed as close as possible the IC.

Minimize the length of the trace from the feedback

pin to the resistors. This is a high impedance

node susceptible to interference from external RF

noise sources.

Ion

Ioff

PCB Inductance

Ion

High

di/dt

Ioff

Ion+Ioff

Ion

4. Inductor. Minimize the length of the SW node

trace. This minimizes the radiated EMI associated

with the SW node.

Ioff

5. Ground. The most quiet ground or return potential

available is the output capacitor return. The

inductor current flows through the output

capacitor during both the on time and off time,

hence it never sees a high di/dt. The only di/dt

seen by the output capacitor is the inductor ripple

current which is much less than the di/dt of an

edge to a square wave current pulse. This is the

best place to make a solid connection to the IC

ground and input capacitor. This node is used as

the star ground shown in Figure 1. This method of

grounding helps to reduce high di/dt traces, and

the detrimental effect associated with them, in a

step-down converter. The inductance of these

traces should always be minimized by using wide

traces, ground planes, and proper component

placement.

High di/dt trace reduction

“Star Ground”

Figure 1. Step Down Converter Layout

6. For good thermal performance vias are required

to couple the exposed tab of the SO-8 package to

the PCB ground plane. The via diameter should

be 0.3mm to 0.33mm positioned on a 1.2mm grid.

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AMS4123

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

Output Power and Thermal Limits

The AMS4123 junction temperature, Step-Down

converter and LDO current capability depends on the

internal dissipation and the junction to case thermal

resistance of the SO8 exposed paddle package. This

gives the junction temperature rise above the device

paddle and PCB temperature.

The temperature of the paddle and PCB will be

elevated above the ambient temperature due to the

total losses of the step down converter and losses of

other circuits and or converters mounted to the PCB.

Tjmax=Pd·θjc+Tpcb+Tamb

The losses associated with the AMS4123 overall

efficiency are;

1. Output Diode Conduction Losses

2. Inductor DCR Losses

3. AMS4123 Internal losses

a. Power Switch Forward Conduction

and Switching Losses

b. Quiescent Current Losses

The internal losses contribute to the junction

temperature rise above the case and PCB

temperature.

The junction temperature depends on many factors

and should always be verified in the final application

at the maximum ambient temperature. This will assure

that the device does not enter over-temperature

shutdown when fully loaded at the maximum ambient

temperature.

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AMS4123

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

Figure 2. AMS4123 Evaluation Board Top Side

Figure 3. AMS4123 Evaluation Board Bottom Side

JP1

LDO Input

VLX

VLDOIn

10uH

U1 AMS4123_1

SW LDO Out

470pF

Vout

31.6k

VLDOOut

BST LDO In

Vin FB LDO

Vin

220nF

100uF 16V

22uF

10uF 50V

FB SW

gnd

22uF 35V

10.0k

2.2uF

B340LB

45.3k

10.0k

Enable

gnd

4.7nF

Figure 4. AMS4123 Evaluation Board Schematic

Table 3. Evaluation Board Bill of Materials

Component Value

Manufacturer Manufacturer Part Number

10µH 3.9A

Coilcraft

DO3316P

9.4mm x 13mm x 5.2mm

100µF, 16V, X case General

Purpose Tantalum

Kemet

T491X107M016AS

Taiyo Yuden

TDK

LMK212BJ226MG-T

C3225X5R1A226M

22µF, 10V, X5R, 0805, Ceramic

C3,C6

option

10µF, 50V, X5R, 1210, Ceramic

2.2µF, 10V, X5R, 0805

2.2µF, 10V, X5R, 0603

Taiyo Yuden

Murata

UMK325BJ106KM-T

GRM216R61A225KE24

GRM39X5R225K10H52V

470pF 50V, 20%, X7R, 0603

220nF 25V, 10%, X7R, 0603

4.7nF 50V, 20%, X7R, 0603

22µF 35V Tantalum Case E

10Ω, 0.1W, 0603 5%

Murata

Vishay

GRM188R71H471MA01

GRM188R71E224KA88

GRM188R71H472MA01

293D226X9035E2TE3

CRCW060310R0JNEA

Vishay/Dale

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AMS4123

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

R2,R3

R1,R4

10kΩ, 0.1W, 0603 1%

See table 2

3A, 40V Schottky

Various

Diodes Inc.

AMS

CRCW060310K0FKEA

CRCW0603xxKxFKEA

B340LB

Step-Down Converter / LDO

AMS4123

ORDERING INFORMATION

Package Type

SOIC EDP

AMS4123S

TEMP. RANGE

-25°C to 125°C

PACKAGE DIMENSIONS inches (millimeters) unless otherwise noted.

8 LEAD SOIC PLASTIC PACKAGE (S)

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AMS4123

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

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AMS4123 [ADMOS]

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