AAT2153IVN-0.6-T1 - AnalogicTech

AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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General Description

The AAT2153 SwitchReg is a 2.5A step-down converter

with an input voltage range of 2.7V to 5.5V and an

adjustable output voltage from 0.6V to VIN. The 1.4MHz

switching frequency enables the use of small external

components. The small footprint and high efficiency make

the AAT2153 an ideal choice for portable applications.

The AAT2153 delivers 2.5A maximum output current

while consuming only 42µA of no-load quiescent current.

Ultra-low RDS(ON) integrated MOSFETs and 100% duty

cycle operation make the AAT2153 an ideal choice for

high output voltage, high current applications which

require a low dropout threshold.

The AAT2153 provides excellent transient response and

high output accuracy across the operating range. No

external compensation components are required.

The AAT2153 maintains high efficiency throughout the

load range. The AAT2153’s unique architecture produces

reduced ripple and spectral noise. Over-temperature and

short-circuit protection safeguard the AAT2153 and sys-

tem components from damage.

The AAT2153 is available in a Pb-free, space-saving

16-pin 3x3mm QFN package. The product is rated over

an operating temperature range of -40°C to +85°C.

Features

• 2.5AMaximumOutputCurrent

• InputVoltage:2.7Vto5.5V

• OutputVoltage:0.6VtoVIN

• Upto95%Efficiency

• LowNoiseLightLoadMode

• 42µANoLoadQuiescentCurrent

• NoExternalCompensationRequired

• 1.4MHzSwitchingFrequency

• 100%DutyCycleLow-DropoutOperation

• InternalSoftStart

• Over-TemperatureandCurrentLimitProtection

• <1µAShutdownCurrent

• 16-Pin3x3mmQFNPackage

• TemperatureRange:-40°Cto+85°C

Applications

• CellularPhones

• DigitalCameras

• HardDiskDrives

• MP3Players

• PDAsandHandheldComputers

• PortableMediaPlayers

• USBDevices

Typical Application

2.2µH

22µF

10µF

2.5V3.3V

187k

59k

LX 14

N/C

VCC

N/C

LX 13

PGND 3

FB 4

LX 15

PGND 2

PGND 1

SGND

N/C 16

AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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Pin Descriptions

Pin # Symbol Function

1, 2, 3 PGND Main power ground return pin. Connect to the output and input capacitor return. (See board layout rules.)

4FB Feedbackinputpin.Foranadjustableoutput,connectanexternalresistivedividertothispin.Forxed

outputvoltageversions,FBistheoutputpinoftheconverter.

5 SGND Signal ground. Connect the return of all small signal components to this pin. (See board layout rules.)

6, 8, 16 N/C Not internally connected.

7 EN Enable input pin. A logic high enables the converter; a logic low forces the AAT2153 into shutdown mode

reducingthesupplycurrenttolessthan1µA.Thepinshouldnotbeleftoating.

9VCC Biassupply.Suppliespowerfortheinternalcircuitry.Connecttoinputpower.

10, 11, 12 VP Input supply voltage for the converter power stage. Must be closely decoupled to PGND.

13, 14, 15 LX Connect inductor to these pins. Switching node internally connected to the drain of both high- and low-side

MOSFETs.

EP Exposed paddle (bottom); connect to PGND directly beneath package.

Pin Configuration

QFN33-16

(Top View)

N/C

PGND

N/C

SGND

N/C

AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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Absolute Maximum Ratings1

Symbol Description Value Units

VCC, VPVCC, VP to GND 6 V

VLX LXtoGND -0.3 to VP + 0.3 V

VFB FBtoGND -0.3 to VCC + 0.3 V

VEN EN to GND -0.3 to -6 V

TJOperating Junction Temperature Range -40 to150 °C

Thermal Characteristics

Symbol Description Value Units

qJA Maximum Thermal Resistance 50 °C/W

qJC Maximum Thermal Resistance 4.2 °C/W

PDMaximum Power Dissipation (TA = 25°C)2, 3 2.0 W

Recommended Operating Conditions

Symbol Description Value Units

TAAmbient Temperature Range -40 to 85 °C

1. Stresses above those listed in Absolute Maximum Ratings may cause damage to the device. Functional operation at conditions other than the operating conditions specified is

not implied. Only one Absolute Maximum Rating should be applied at any one time.

2. Mountedonademoboard(FR4,instillair).ExposedpadmustbemountedtoPCB.

3. Derate 20mW/°C above 25°C.

AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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Electrical Characteristics1

VIN = 3.6V; TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C.

Symbol Description Conditions Min Typ Max Units

VIN Input Voltage 2.7 5.5 V

VOUT Output Voltage Range 0.6 VIN V

VUVLO UVLOThreshold

VIN Rising 2.7 V

Hysteresis 250 mV

VIN Falling 1.8 V

VOUT Output Voltage Tolerance IOUT = 0A to 2.5A, VIN = 2.7V to 5.5V -3.0 3.0 %

IQQuiescent Current NoLoad 42 90 µA

ISHDN Shutdown Current VEN = GND 1.0 µA

ILIM CurrentLimit 2.8 3.5 A

RDS(ON)H High Side Switch On-Resistance 0.120 W

RDS(ON)L LowSideSwitchOn-Resistance 0.085 W

DVLOADREG LoadRegulation ILOAD = 0A to 2.5A 0.5 %

DVLINEREG/DVIN LineRegulation VIN = 2.7V to 5.5V 0.2 %/V

VFB

Feedback Threshold Voltage Accuracy

(Adjustable Version) NoLoad,TA = 25°C 0.591 0.60 0.609 V

IFB FBLeakageCurrent VOUT = 1.0V 0.2 µA

FOSC Internal Oscillator Frequency 1.12 1.4 1.68 MHz

TSStart-Up Time From Enable to Output Regulation;

CFF = 100pF 150 µs

TSD Over-Temperature Shutdown Threshold 140 °C

THYS Over-Temperature Shutdown Hysteresis 15 °C

VIL EnableThresholdLow 0.6 V

VIH Enable Threshold High 1.4 V

IEN EnableLeakageCurrent VIN = VEN = 5.5V -1.0 1.0 µA

1. The AAT2153 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correla-

tion with statistical process controls.

AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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Typical Characteristics

Efficiency vs. Output Current

(VOUT = 3.3V)

Output Current (mA)

Efficiency (%)

100

0110 100 1000 10000

VIN = 5.0V

VIN = 4.5V

VIN = 4.2V

Load Regulation

(VOUT = 3.3V)

Output Current (mA)

Load Regulation (%)

-0.6

-0.4

-0.2

-1.0

-0.8

0.0

0.2

0.4

0.6

0.8

1.0

0.11 10 100 1000 10000

VIN = 5.0V

VIN = 4.5V

VIN = 4.2V

Efficiency vs. Output Current

(VOUT = 1.8V)

Output Current (mA)

Efficiency (%)

0110 100 1000 10000

100

VIN = 4.2V

VIN = 3.6V

VIN = 2.7V

Load Regulation

(VOUT = 1.8V)

Output Current (mA)

Load Regulation (%)

-0.6

-0.4

-0.2

-1.0

-0.8

0.0

0.2

0.4

0.6

0.8

1.0

0.11 10 100 1000 10000

VIN = 4.2V

VIN = 3.6V

VIN = 2.7V

Efficiency vs. Output Current

(VOUT = 1.2V)

Output Current (mA)

Efficiency (%)

0110 100 1000 10000

100

VIN = 4.2V

VIN = 3.6V

VIN = 2.7V

Load Regulation

(VOUT = 1.2)

Output Current (mA)

Load Regulation (%)

0.11 10 100 1000 10000

-0.6

-0.4

-0.2

-1.0

-0.8

0.0

0.2

0.4

0.6

0.8

1.0

VIN = 4.2V

VIN = 3.6V

VIN = 2.7V

AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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Typical Characteristics

Quiescent Current vs. Input Voltage

(VOUT = 1.8V; No Load)

Input Voltage (V)

Quiescent Current (µA)

2.72.9 3.13.3 3.53.7 3.94.1 4.34.5 4.74.9 5.15.3 5.5

85°C

25°C

-40°C

Output Voltage vs. Temperature

(VOUT = 1.8V; IOUT = 2.5A)

Temperature (°C)

Output Voltage Error (%)

VIN = 2.7V VIN = 3.6V

VIN = 4.2V

-40-30 -20-10 0102030405060708090

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Output Voltage vs. Input Voltage

(VOUT = 1.8V; IOUT = 1A)

Input Voltage (V)

Output Voltage (V)

VIN = 2.7V VIN = 3.6V

VIN = 4.2V

1.73

1.74

1.75

1.76

1.77

1.78

1.79

1.80

1.81

1.82

2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5

85°C

25°C

-40°C

Switching Frequency vs. Temperature

(VOUT = 1.8V; IOUT = 2.5A)

Temperature (°C)

Switching Frequency Variation (%)

-40-30 -20-10 0102030405060708090

-6

-5

-4

-3

-2

-1

Load Transient Response

(VOUT = 1.8V)

Time (100µs/div)

Output Voltage (AC coupled)

(top)(mV)

Output Current

(bottom) (A)

VIN = 2.7V VIN = 3.6V

VIN = 4.2V

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Load Transient Response

(VOUT = 1.8V; CFF = 100pF)

Time (100µs/div)

Output Voltage (AC coupled)

(top)(mV)

Output Current

(bottom) (A)

VIN = 2.7V VIN = 3.6V

VIN = 4.2V

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

AAT2153

2.5A Low Noise Step-Down ConverterSwitchRegTM

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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Typical Characteristics

Load Transient Response

(IOUT = 1A to 2.5A; VOUT = 1.8V; R1 = 0Ω; COUT = 2x22µF)

Time (100µs/div)

Output Voltage (top) (V)

Load Current (bottom) (A)

1.6

1.7

1.8

1.9

2.0

Load Transient Response

(IOUT = 1A to 2.5A; VOUT = 1.8V; R1 = 10Ω; COUT = 22µF)

Time (100µs/div)

Output Voltage (top) (V)

Load Current (bottom) (A)

1.6

1.7

1.8

1.9

2.0

Load Transient Response

(IOUT = 250mA to 2.5A; VOUT = 1.8V; R1 = 0Ω; COUT = 2x22µF)

Time (100µs/div)

Output Voltage (top) (V)

Load Current (bottom) (A)

1.6

1.7

1.8

1.9

2.0

Load Transient Response

(IOUT = 250mA to 2.5A; VOUT = 1.8V; R1 = 10Ω; COUT = 22µF)

Time (100µs/div)

Output Voltage (top) (V)

Load Current (bottom) (A)

1.6

1.7

1.8

1.9

2.0

Load Transient Response

(IOUT = 100mA to 2.5A; VOUT = 1.8V; R1 = 0Ω; COUT = 2x22µF)

Time (100µs/div)

Output Voltage (top) (V)

Load Current (bottom) (A)

1.6

1.7

1.8

1.9

2.0

Load Transient Response

(IOUT = 100mA to 2.5A; VOUT = 1.8V; R1 = 10Ω; COUT = 22µF)

Time (100µs/div)

Output Voltage (top) (V)

Load Current (bottom) (A)

1.6

1.7

1.8

1.9

2.0

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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Typical Characteristics

Load Transient Response

(IOUT = 10mA to 2.5A; VOUT = 1.8V; R1 = 0Ω; COUT = 2x22µF)

Time (100µs/div)

Output Voltage (top) (V)

Load Current (bottom) (A)

1.6

1.7

1.8

1.9

2.0

Load Transient Response

(IOUT = 10mA to 2.5A; VOUT = 1.8V; R1 = 10Ω; COUT = 22µF)

Time (100µs/div)

Output Voltage (top) (V)

Load Current (bottom) (A)

1.6

1.7

1.8

1.9

2.0

Line Transient Response

(VOUT = 1.8V; IOUT = 1.5A; CFF = 100pF)

Time (100µs/div)

Input Voltage

(top) (V)

Output Voltage (AC coupled)

(bottom) (V)

VIN = 2.7V VIN = 3.6V

VIN = 4.2V

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Line Regulation

(VOUT = 1.8V; IOUT = 1A)

Input Voltage (V)

VOUT Error (%)

VIN = 2.7V VIN = 3.6V

VIN = 4.2V

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5

Enable Soft Start

(VIN = 3.6V; VOUT = 1.8V; IOUT = 2.5A; CFF = 100pF)

Time (100µs/div)

VOUT

(1V/div)

(2V/div)

IIN

(1A/div)

Enable Soft Start

(VIN = 3.6V; VOUT = 1.8V; IOUT = 2.5A; CFF = 1nF)

Time (100µs/div)

VOUT

(1V/div)

(2V/div)

IIN

(1A/div)

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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Typical Characteristics

Heavy Load Switching Waveform

(VIN = 3.6V; VOUT = 1.8V; IOUT = 2.5A; R1 = 10Ω; COUT = 22µF)

Time (400ns/div)

Output Voltage (AC Coupled)

(top) (mV)

Inductor Ripple Current

(bottom) (A)

-10

2.2

2.4

2.6

2.8

Heavy Load Switching Waveform

(VIN = 3.6V; VOUT = 1.8V; IOUT = 2.5A; R1 = 0Ω; COUT = 2x22µF)

Time (400ns/div)

Output Voltage (AC Coupled)

(top) (mV)

Inductor Ripple Current

(bottom) (A)

-10

2.2

2.4

2.6

2.8

Light Load Switching Waveform

(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 0pF)

Time (5µs/div)

Output Voltage (AC coupled)

(top)(mV)

Inductor Ripple Current

(bottom) (A)

VIN = 2.7V VIN = 3.6V

VIN = 4.2V

-240

-200

-160

-120

-80

-40

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Light Load Switching Waveform

(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 0pF)

Time (200µs/div)

Output Voltage (AC coupled)

(top)(mV)

Inductor Ripple Current

(bottom) (A)

VIN = 2.7V VIN = 3.6V

VIN = 4.2V

-240

-200

-160

-120

-80

-40

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Light Load Switching Waveform

(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 100pF)

Time (5µs/div)

Output Voltage (AC coupled)

(top)(mV)

Inductor Ripple Current

(bottom) (A)

-240

-200

-160

-120

-80

-40

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Light Load Switching Waveform

(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 100pF)

Time (500µs/div)

Output Voltage (AC coupled)

(top)(mV)

Inductor Ripple Current

(bottom) (A)

-240

-200

-160

-120

-80

-40

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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2.5A Low Noise Step-Down ConverterSwitchRegTM

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Functional Description

The AAT2153 is a high performance 2.5A monolithic

step-down converter operating at a 1.4MHz switching

frequency. It minimizes external component size, opti-

mizes efficiency over the complete load range, and pro-

duces reduced ripple and spectral noise. Apart from the

small bypass input capacitor, only a small L-C filter is

required at the output. Typically, a 3.3µH inductor and a

22µF ceramic capacitor are recommended for a 3.3V

output (see table of recommended values).

At dropout, the converter duty cycle increases to 100%

and the output voltage tracks the input voltage minus

the RDS(ON) drop of the P-channel high-side MOSFET (plus

the DC drop of the external inductor). The device inte-

grates extremely low RDS(ON) MOSFETs to achieve low

dropout voltage during 100% duty cycle operation. This

is advantageous in applications requiring high output

voltages (typically > 2.5V) at low input voltages.

The integrated low-loss MOSFET switches can provide

greaterthan95%efficiencyatfullload.Lightloadopera-

tion maintains high efficiency, low ripple and low spectral

noiseevenatlowercurrents(typically<150mA).

In battery-powered applications, as VIN decreases, the

converter dynamically adjusts the operating frequency

prior to dropout to maintain the required duty cycle and

provide accurate output regulation. Output regulation is

maintained until the dropout voltage, or minimum input

voltage, is reached. At 2.5A output load, dropout voltage

headroom is approximately 200mV.

The AAT2153 typically achieves better than ±0.5% out-

put regulation across the input voltage and output load

range. A current limit of 3.5A (typical) protects the IC

and system components from short-circuit damage.

Typical no load quiescent current is 42µA.

Thermal protection completely disables switching when

the maximum junction temperature is detected. The

Functional Block Diagram

VPVCC

ENSGND PGND

LOGIC

0.6V REF

Temp.

Sensing

OSC

OP. AMP

FB DH

CMP

1MΩ

AAT2153

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junction over-temperature threshold is 140°C with 15°C

of hysteresis. Once an over-temperature or over-current

fault condition is removed, the output voltage automati-

cally recovers.

Peak current mode control and optimized internal com-

pensation provide high loop bandwidth and excellent

response to input voltage and fast load transient events.

Soft start eliminates output voltage overshoot when the

enable or the input voltage is applied. Under-voltage

lockout prevents spurious start-up events.

Control Loop

The AAT2153 is a peak current mode step-down con-

verter. The current through the P-channel MOSFET (high

side) is sensed for current loop control, as well as short-

circuit and overload protection. A fixed slope compensa-

tion signal is added to the sensed current to maintain

stability for duty cycles greater than 50%. The peak cur-

rent mode loop appears as a voltage-programmed cur-

rent source in parallel with the output capacitor.

The output of the voltage error amplifier programs the

current mode loop for the necessary peak switch current

to force a constant output voltage for all load and line

conditions. Internal loop compensation terminates the

transconductance voltage error amplifier output. The

reference voltage is internally set to program the con-

verter output voltage greater than or equal to 0.6V.

Soft Start/Enable

Soft start limits the current surge seen at the input and

eliminates output voltage overshoot. When pulled low,

the enable input forces the AAT2153 into a low-power,

non-switching state. The total input current during shut-

down is less than 1µA.

Current Limit and

Over-Temperature Protection

For overload conditions, the peak input current is limit-

ed. To minimize power dissipation and stresses under

current limit and short-circuit conditions, switching is

terminated after entering current limit for a series of

pulses. Switching is terminated for seven consecutive

clock cycles after a current limit has been sensed for a

series of four consecutive clock cycles.

Thermal protection completely disables switching when

internal dissipation becomes excessive. The junction

over-temperature threshold is 140°C with 15°C of hys-

teresis. Once an over-temperature or over-current fault

conditions is removed, the output voltage automatically

recovers.

Under-Voltage Lockout

Internal bias of all circuits is controlled via the VCC

input. Under-voltage lockout (UVLO) guarantees suffi-

cient VIN bias and proper operation of all internal cir-

cuitry prior to activation.

3.0µH

2x22µF

10µF

0.1µF

C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3

C3 Murata 22µF 6.3V GRM21BR60J226ME39L X5R 0805

L1 see Table 2

R1 and C2 are an optional noise filter for internal V

R6, C4, C5-C7 are not populated

C8 100pF to 1nF feed-forward capacitor for enhanced transient response

VOUT+

VIN+

LX 14

N/C

VCC

N/C

LX 13

PGND 3

FB 4

LX 15

PGND 2

PGND 1

SGND

N/C 16

AAT2153

59.0k

100K

Enable LX

GNDGND

VOUT(V) R3 (kW)

0.8 19.6

0.9 29.4

1.0 39.2

1.1 49.9

1.2 59.0

1.3 68.1

1.4 78.7

1.5 88.7

1.8 118

2.0 137

2.5 187

3.3 267

Figure 1: AAT2153 Evaluation Schematic.

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Component Selection

Inductor Selection

The step-down converter uses peak current mode con-

trol with slope compensation to maintain stability for

duty cycles greater than 50%. The output inductor value

must be selected so the inductor current down slope

meets the internal slope compensation requirements.

The inductor should be set equal to the output voltage

numeric value in µH. This guarantees that there is suf-

ficient internal slope compensation.

Manufacturer’s specifications list both the inductor DC

current rating, which is a thermal limitation, and the

peak current rating, which is determined by the satura-

tion characteristics. The inductor should not show any

appreciable saturation under normal load conditions.

Some inductors may meet the peak and average current

ratings yet result in excessive losses due to a high DCR.

Always consider the losses associated with the DCR and

its effect on the total converter efficiency when selecting

an inductor.

The 3.3µH CDRH4D28 series Sumida inductor has a

49.2mW worst case DCR and a 1.57A DC current rating.

At full 2.5A load, the inductor DC loss is 97mW which

gives less than 1.5% loss in efficiency for a 2.5A, 3.3V

output.

Input Capacitor

Selecta10µFto22µFX7RorX5Rceramiccapacitorfor

the input. To estimate the required input capacitor size,

determine the acceptable input ripple level (VPP) and solve

for C. The calculated value varies with input voltage and

is a maximum when VIN is double the output voltage.



· 1 -



VIN

CIN =

VIN



- ESR · FS



VPP



· 1 - = for VIN = 2 · VO



VIN

CIN(MIN) = 1



- ESR · 4 · FS



VPP

Always examine the ceramic capacitor DC voltage coef-

ficient characteristics when selecting the proper value.

Forexample,thecapacitanceofa10µF,6.3V,X5Rceram-

ic capacitor with 5.0V DC applied is actually about 6µF.

Some examples of DC bias voltage versus capacitance for

different package sizes are shown in Figure 2.

Capacitance vs. DC Bias Voltage

DC Bias Voltage (V)

Capacitance (F)

0.01.0 2.03.0 4.05.0 6.07.0

000.0E+0

5.0E+6

10.0E+6

15.0E+6

20.0E+6

25.0E+6

1206 Package

0805 Package

Figure 2: Capacitance vs. DC Bias Voltage for

Different Package Sizes.

ThemaximuminputcapacitorRMScurrentis:



IRMS = IO · · 1 -



VIN

The input capacitor RMS ripple current varies with the

input and output voltage and will always be less than or

equal to half of the total DC load current.



· 1 - = D · (1 - D) = 0.52 =



VIN

for VIN = 2 · VO

RMS(MAX)

The term



· 1 -



VIN

appears in both the input voltage

ripple and input capacitor RMS current equations and is

a maximum when VO is twice VIN. This is why the input

voltage ripple and the input capacitor RMS current ripple

are a maximum at 50% duty cycle.

The input capacitor provides a low impedance loop for

theedgesofpulsedcurrentdrawnbytheAAT2153.Low

ESR/ESL X7R and X5R ceramic capacitors are ideal for

this function. To minimize stray inductance, the capaci-

tor should be placed as closely as possible to the IC. This

keeps the high frequency content of the input current

localized, minimizing EMI and input voltage ripple.

The proper placement of the input capacitor (C1) can be

seenintheevaluationboardlayoutintheLayoutsection

of this datasheet (see Figure 3).

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A laboratory test set-up typically consists of two long

wires running from the bench power supply to the evalu-

ation board input voltage pins. The inductance of these

wires, along with the low-ESR ceramic input capacitor,

can create a high Q network that may affect converter

performance. This problem often becomes apparent in

the form of excessive ringing in the output voltage dur-

ing load transients. Errors in the loop phase and gain

measurements can also result.

Since the inductance of a short PCB trace feeding the

input voltage is significantly lower than the power leads

from the bench power supply, most applications do not

exhibit this problem.

In applications where the input power source lead induc-

tance cannot be reduced to a level that does not affect

the converter performance, a high ESR tantalum or alu-

minum electrolytic should be placed in parallel with the

low ESR/ESL bypass ceramic capacitor. This dampens

the high Q network and stabilizes the system.

Output Capacitor

The output capacitor limits the output ripple and pro-

vides holdup during large load transitions. A 10µF to

22µF X5R or X7R ceramic capacitor typically provides

sufficient bulk capacitance to stabilize the output during

largeloadtransitionsandhastheESRandESLcharac-

teristics necessary for low output ripple.

The output voltage droop due to a load transient is dom-

inated by the capacitance of the ceramic output capacitor.

During a step increase in load current, the ceramic output

capacitor alone supplies the load current until the loop

responds. Within two or three switching cycles, the loop

responds and the inductor current increases to match the

load current demand. The relationship of the output volt-

age droop during the three switching cycles to the output

capacitancecanbeestimatedby:

COUT =

3 · ∆ILOAD

VDROOP · FS

Once the average inductor current increases to the DC

load level, the output voltage recovers. The above equa-

tion establishes a limit on the minimum value for the

output capacitor with respect to load transients.

The internal voltage loop compensation also limits the

minimum output capacitor value to 10µF. This is due to

its effect on the loop crossover frequency (bandwidth),

phase margin, and gain margin. Increased output capac-

itance will reduce the crossover frequency with greater

phase margin.

Adjustable Output Resistor Selection

The output voltage on the AAT2153 is programmed with

external resistors R3 and R4. To limit the bias current

required for the external feedback resistor string while

maintaining good noise immunity, the minimum sug-

gestedvalueforR4is59kW. Although a larger value will

further reduce quiescent current, it will also increase the

impedance of the feedback node, making it more sensi-

tive to external noise and interference. Table 1 summa-

rizes the resistor values for various output voltages with

R4settoeither59kW for good noise immunity or 221kW

for reduced no load input current.

The external resistor R3, combined with an external

100pF feed forward capacitor (C8 in Figure 1), delivers

enhanced transient response for extreme pulsed load

applications and reduces ripple in light load conditions.

The addition of the feed forward capacitor typically

requires a larger output capacitor C3-C4 for stability. The

external resistors set the output voltage according to the

followingequation:





VOUT = 0.6V 1 +









VOUT

VREF

R3 = - 1 R4

VOUT (V)

R4 = 59kW

R3 (kW)

R4 = 221kW

R3 (kW)

0.8 19.6 75

0.9 29.4 113

1.0 39.2 150

1.1 49.9 187

1.2 59.0 221

1.3 68.1 261

1.4 78.7 301

1.5 88.7 332

1.8 118 442

1.85 124 464

2.0 137 523

2.5 187 715

3.0 237 887

3.3 267 1000

Table 1: AAT2153 Resistor Values for Various

Output Voltages.

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Thermal Calculations

There are three types of losses associated with the

AAT2153step-downconverter:switchinglosses,conduc-

tion losses, and quiescent current losses. Conduction

losses are associated with the RDS(ON) characteristics of the

power output switching devices. Switching losses are

dominated by the gate charge of the power output switch-

ing devices. At full load, assuming continuous conduction

mode(CCM),asimplifiedformofthelossesisgivenby:

PTOTAL

2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])

VIN

+ (tsw · FS · IO + IQ) · VIN

IQ is the step-down converter quiescent current. The

term tsw is used to estimate the full load step-down con-

verter switching losses.

For the condition where the step-down converter is in

dropout at 100% duty cycle, the total device dissipation

reducesto:

PTOTAL

2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])

VIN

+ (tsw · FS · IO + IQ) · VIN

Since RDS(ON), quiescent current, and switching losses all

vary with input voltage, the total losses should be inves-

tigated over the complete input voltage range.

Given the total losses, the maximum junction tempera-

ture can be derived from the qJA for the QFN33-16 pack-

age, which is 50°C/W.

TJ(MAX) = PTOTAL · ΘJA + TAMB

Layout

Thesuggested PCBlayoutfor the AAT2153isshownin

Figures 3 and 4. The following guidelines should be used

to help ensure a proper layout.

1. The input capacitor (C1) should connect as closely as

possible to VP and PGND.

2. C2andL1shouldbeconnectedascloselyaspossi-

ble.TheconnectionofL1totheLXpinshouldbeas

short as possible.

3. The feedback trace or FB pin should be separate

from any power trace and connect as closely as pos-

sible to the load point. Sensing along a high-current

load trace will degrade DC load regulation.

4. The resistance of the trace from the load return to

PGND should be kept to a minimum. This will help to

minimize any error in DC regulation due to differ-

ences in the potential of the internal signal ground

and the power ground.

5. Connect unused signal pins to ground to avoid

unwanted noise coupling.

Figure 3: AAT2153 Evaluation Board Figure 4: AAT2153 Evaluation Board

Top Side Layout. Bottom Side Layout.

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Design Example

Specifications

VO 3.3V @ IO=2.5A,PulsedLoadDILOAD = 2.4A

VIN 2.7V to 4.2V (3.6V nominal)

FS 1.4MHz

TAMB 85°C in QFN33-16 Package

Output Inductor

L = VO (µH) = 3.3µH; see Table 2.

ForWurthinductor74477890033.3µHDCR=30mW max.

3.3

3.3V

∆I =

· 1 - = · 1 - = 153m

L · F

3.3µH · 1.4MHz

4.2V

= I

+ ∆I = 2.5A + 0.077A = 2.577A

= I

· DCR = 2.5A

· 30mΩ = 188mW

Output Capacitor

VDROOP = 0.2V

13.3V · (4.2V - 3.3V)

3.3µH · 1.4MHz · 4.2V

RMS(MAX)

IL · FS · VIN(MAX)

= ·

3 · ∆ILOAD

VDROOP · FS

3 · 2.4A

0.2V · 1.4MHz

COUT== = 25.7µF; use 2x22µF

· = 44mArms

·

(VOUT) · (VIN(MAX) - VOUT)=

Pesr = esr · IRMS2 = 5mΩ · (44mA)2 = 9.8µW

Input Capacitor

Input Ripple VPP = 50mV

CIN == = 11.9µF; use 2x10µF

- ESR · 4 · FS

VPP

IO1 + IO2

- 5mΩ · 4 · 1.2MHz

50mV

1.4A

RMS(MAX)

P = esr · IRMS

2 = 5mΩ · (1.25A)2 = 6.25mW

== 1.25Arms

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AAT2153 Losses

Total losses can be estimated by calculating the dropout (VIN = VO) losses where the power MOSFET RDS(ON) will be at

the maximum value. All values assume an 85°C ambient temperature and a 120°C junction temperature with the QFN

50°C/W package.

PLOSS = IO

2 · RDS(ON)H = 2.5A2 · 0.12Ω = 750mW

TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 750mW = 122.5°C

The total losses are also investigated at the nominal lithium-ion battery voltage (3.6V). The simplified version of the

RDS(ON) losses assumes that the N-channel and P-channel RDS(ON) are equal.

PTOTAL = IO

2 · RDS(ON) + [(tsw · FS · IO + IQ) · VIN]

= 2.5A2 · 120mΩ + [(5ns · 1.4MHz · 2.5A + 70µA) · 3.6V] = 813mW

TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 813mW = 125.6°C

VOUT (V)

Inductance

(µH) Part Number Manufacturer Size (mm)

Rated Current

(A)

ISAT

(A)

DCR

(mW)

3.3 3.0 CDRH5D28RHPNP Sumida 6x6x3 2.4 31.8

3.3 3.3 7447789003 Wurth 7x7x3 3.42 4.7 30.0

2.5 2.5 CDRH5D28NP Sumida 6x6x3 2.6 4.2 24.0

1.8 1.8 CDRH4D28 Sumida 6x6x2 2.5 46.0

1.5 1.4 CDRH5D14HPNP Sumida 6x6x1.5 2.8 5.0 40.3

1.2 1.2 CDRH4D28 Sumida 5x5x3 2.56 3.9 23.6

1.0 1.0 CDRH5D14NP Sumida 6x6x1.5 3.6 3.3 25.6

0.8 0.9 CDRH5D14HPNP Sumida 6x6x1.5 3.5 5.0 27.5

0.6 0.6 CDRH5D14HPNP Sumida 6x6x1.5 3.9 6.0 22.5

Table 2: Surface Mount Inductors.

Manufacturer Part Number Value Voltage Temp. Co. Case

Murata GRM21BR60J106KE19 10µF 6.3V X5R 0805

Murata GRM21BR60J226ME39 22µF 6.3V X5R 0805

Murata GRM31CR60J226KE19 22µF 6.3V X5R 1206

Table 3: Surface Mount Capacitors.

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supportthiswarranty.Specictestingofallparametersofeachdeviceisnotnecessarilyperformed.AnalogicTechandtheAnalogicTechlogoaretrademarksofAdvancedAnalogicTechnologiesIncorporated.Allother

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Ordering Information

Package Marking1Part Number (Tape and Reel)2

QFN33-16 5QXYY AAT2153IVN-0.6-T1

All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means semiconductor

products that are in compliance with current RoHS standards, including the requirement that lead not exceed

0.1% by weight in homogeneous materials. For more information, please visit our website at

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Package Information

QFN33-163

3.000 ± 0.050

Pin 1 Dot By Marking

1.700 ± 0.050

0.400 ± 0.050

1.700 ± 0.050

3.000 ± 0.050

0.500 ± 0.050

0.850 ± 0.050

Pin 1 Identification

C0.3

0.025 ± 0.025

0.214 ± 0.036

0.230 ± 0.050

Top View Bottom View

Side View

All dimensions in millimeters.

1. XYY=assemblyanddatecode.

2. Sample stock is generally held on part numbers listed in BOLD.

3. The leadless package family, which includes QFN, TQFN, DFN, TDFN and STDFN, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing

process. A solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder connection.