AAT2522IRN-1-T1 - AnalogicTech

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

2522.2009.06.1.0 1

www.analogictech.com

General Description

The AAT2522 SwitchReg is a dual high-current 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 compact footprint and high

efficiency make the AAT2522 an ideal choice for portable

applications.

The AAT2522 delivers load currents up to 3.0A maximum

output current per regulator. Ultra-low RDS(ON) integrated

MOSFETs and 100% duty cycle operation make the

AAT2522 an ideal choice for high output-voltage, high

current applications which require a low dropout thresh-

old.

The AAT2522 provides excellent transient response and

high output accuracy across the operating range. The

AAT2522’s unique architecture requires no external com-

pensation components, and produces reduced ripple and

spectral noise. Over-temperature and short-circuit pro-

tection safeguard the AAT2522 and system components

from damage.

The AAT2522 is available in a Pb-free, space-saving

16-pin 3mm x 4mm TDFN package. The product is rated

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

Features

• Dual 3.0A Peak Output Current Regulators

• 2.7V to 5.5V Input Voltage Range

• Adjustable Output Voltage (0.6V to VIN)

• 100% Duty-Cycle, Low-Dropout Operation

• Integrated 120m High-Side Power MOSFET

• Integrated 85m Low-Side Power MOSFET

• Low Noise Light Load Mode

• No External Compensation Required

• Very Low 90A No-Load Operating Current

• <1A Shutdown Current

• Up to 95% Efficiency

• 1.4MHz Switching Frequency

• Overload and Short-Circuit Protection

• Over-Temperature Protection

• Internal Voltage Ramped Soft-Start

• Temperature Range: -40°C to +85°C

• 16-pin 3mm x 4mm TDFN Package

Applications

• Digital Cameras and Camcorders

• Netbooks and Nettops

• Portable DVD and Media Devices

• Power-Over-Ethernet

• Set-Top Boxes

Typical Application

IN1

VCC1

AGND1

LX1

FB1

PGND1

IN2

VCC2

PGND2

AGND2

LX2

FB2

EN1

EN2

VIN

2.7V to 5.5V

VOUT1

0.6V to VIN

VOUT2

0.6V to VIN

ON/OFF

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

2 2522.2009.06.1.0

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Pin Descriptions

Pin # Name Function

1 VCC2 Bias supply input for regulator #2.

2 EN2

Enable input for regulator #2. A logic high enables the second regulator of the AAT2522. A logic

low forces regulator #2 into shutdown mode, placing the second output into a high-impedance

state and reducing the VCC2 quiescent current to less than 1A.

3 IN2 Power supply input for regulator #2.

4 EN1

Enable input for regulator #1. A logic high enables the primary regulator of the AAT2522. A logic

low forces regulator #1 into shutdown mode, placing the primary output into a high-impedance

state and reducing the VCC1 quiescent current to less than 1A.

5 VCC1 Bias supply input for regulator #1.

6-7 IN1 Power supply input for regulator #1.

8 LX1

Inductor switching node for regulator #1. LX1 is the drain of the internal high-side P-channel and

low-side N-channel MOSFETs. Externally connected to the power inductor as shown in the "Typical

Application" drawing on page 1 of this datasheet.

9-10 PGND1 Power ground for regulator #1. PGND1 is internally connected to the source of the internal low-side

N-channel MOSFET.

11 FB1

Feedback input for regulator #1. FB1 senses the output voltage for regulation control. Connect a

resistive divider network from the output to FB1 to AGND1 to set the output voltage accordingly.

The FB1 regulation threshold is 0.8V.

12 AGND1 Analog ground for regulator #1. AGND1 is internally connected to the analog ground of the control

circuitry.

13 LX2

Inductor switching node for regulator #2. LX2 is the drain of the internal high-side P-channel and

low-side N-channel MOSFETs. Externally connected to the power inductor as shown in the "Typical

Application" drawing on page 1 of this datasheet.

14 PGND2 Power ground for regulator #2. PGND2 is internally connected to the source of the internal low-side

N-channel MOSFET.

15 FB2

Feedback input for regulator #2. FB2 senses the output voltage for regulation control. Connect a

resistive divider network from the output to FB2 to AGND2 to set the output voltage accordingly.

The FB2 regulation threshold is 0.6V.

16 AGND2 Analog ground for regulator #2. AGND2 is internally connected to the analog ground of the control

circuitry.

EP AGND Substrate analog ground.

Pin Configuration

TDFN34-16

(Top View)

IN2

EN1

VCC1

VCC2

EN2

IN1

LX1

PGND2

LX2

AGND1

AGND2

AGND

FB2

FB1

PGND1

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

2522.2009.06.1.0 3

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Absolute Maximum Ratings

TA = 25°C unless otherwise noted.

Symbol Description Value Units

VIN1, VIN2 IN1 to PGND1, IN2 to PGND2 -0.3 to +6 V

VCC1, VCC2 VCC1 to AGND1, VCC2 to AGND2 -0.3 to +6 V

IINP1, IINP2 INPx RMS Current Capability ±3.0 A

VLX1 LX1 to PGND1 -0.3 to (VIN1 + 0.3) V

VLX2 LX2 to PGND2 -0.3 to (VIN2 + 0.3) V

ILX1, ILX2 LX RMS Current Capability ±5.0 A

VEN1, VEN2 EN1 to AGND1, EN2 to AGND2 -0.3 to VIN V

VFB1 FB1 to AGND1 -0.3 to (VFB1 + 0.3) V

VFB2 FB2 to AGND2 -0.3 to (VFB2 + 0.3) V

VGND AGND1 to PGND1, AGND2 to PGND2 -0.3 to +0.3 V

Thermal Characteristics

Symbol Description Value Units

TAAmbient Temperature Range -40 to +85 °C

TJOperating Junction Temperature Range -40 to +150 °C

TLEAD Maximum Soldering Temperature (at leads, 10 sec.) 300 °C

Power SO-10 Thermal Impedance

JA Maximum Junction-to-Ambient Thermal Resistance 50 °C/W

PDMaximum Power Dissipation 2 W

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

4 2522.2009.06.1.0

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1. Specified by design.

Electrical Characteristics

CIN = 10F, C OUT = 22F, L = 1.5H. VIN1 = VIN2 = 3.6V, IN1 = VCC1 = EN1, IN2 = VCC2 = EN2, AGND = PGND. TA

= -40°C to 85°C, unless otherwise noted. Typical values are at TA = 25°C.

Symbol Description Conditions Min Typ Max Units

VIN Input Voltage Range 2.7 5.5 V

VOUT Output Voltage Range 0.6 VIN V

Output Voltage Tolerance IOUT = 0 to 3A, VIN = 2.7V to 5.5V -3.0 +3.0 %

VFB FB Regulation Threshold No Load, TA = 25°C 591 600 609 mV

IQNo Load Supply Current Including IN1, VCC1, IN2, and VCC2 supply

currents; No Load Current; not switching 90 180 A

ISHDN Shutdown Current EN = GND 1.0 A

IFB FB Leakage Current VFB = 1.0V 200 nA

VOUT(LOAD) Load Regulation 0A to 3A Load 0.5 %

VOUT/VIN Line Regulation VIN = 2.7V to 5.5V 0.2 %/V

fOSC Oscillator Frequency 1.40 MHz

tSS Soft-Start Period 150 s

Protection Features

VUVLO Input Under-Voltage Lockout VIN Rising, Hysteresis = 0.25V 2.7 V

TSHDN

Over-Temperature Shutdown

Threshold Hysteresis = 15°C 140 °C

MOSFETs

RDS(ON)HI

High-Side P-Channel MOSFET

On-Resistance 120 m

ILIMPK

High-Side P-Channel MOSFET

Current Limit 3.61A

RDS(ON)LO Low-Side N-Channel On-Resistance 85 m

Logic Input/Output Pins

VEN EN Input Logic Threshold 0.6 1.4 V

IEN EN Input Current 0V, VIN -1.0 +1.0 A

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

2522.2009.06.1.0 5

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Typical Characteristics

Step-Down Converter Efficiency vs. Load

(VOUT = 1.2V; L = 1.2µH)

Output Current (mA)

Efficiency (%)

0.1 1 10 100 1000 10000

100

VIN = 2.7V

VIN = 3.0V

VIN = 3.3V

VIN = 3.6V

VIN = 4.2V

VIN = 5.0V

VIN = 5.5V

0.1 1 10 100

Output Error (%)

Output Current (mA)

Step-Down Converter DC Regulation

(VOUT = 1.2V; L = 1.2µH)

1000 1000

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0 VIN = 2.7V

VIN = 3.0V

VIN = 3.3V

VIN = 3.6V

VIN = 4.2V

VIN = 5.0V

VIN = 5.5V

Step-Down Converter Efficiency vs. Load

(VOUT = 1.8V; L = 1.8µH)

Output Current (mA)

Efficiency (%)

0.1 1 10 100 1000 10000

100

VIN = 2.7V

VIN = 3.0V

VIN = 3.3V

VIN = 3.6V

VIN = 4.2V

VIN = 5.0V

VIN = 5.5V

Step-Down Converter DC Regulation

(VOUT = 1.8V; L = 1.8µH)

Output Current (mA)

Output Error (%)

0.1 1 10 100 1000 10000

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0 VIN = 2.7V

VIN = 3.0V

VIN = 3.3V

VIN = 3.6V

VIN = 4.2V

VIN = 5.0V

VIN = 5.5V

Step-Down Converter Efficiency vs. Load

(VOUT = 2.5V; L = 2.5µH)

Output Current (mA)

Efficiency (%)

0.1 1 10 100 1000 10000

100

VIN = 3.3V

VIN = 3.6V

VIN = 4.2V

VIN = 5.0V

VIN = 5.5V

Step-Down Converter DC Regulation

(VOUT = 2.5V; L = 2.5µH)

Output Current (mA)

Output Error (%)

0.1 1 10 100 1000 10000

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0 VIN = 3.3V

VIN = 3.6V

VIN = 4.2V

VIN = 5.0V

VIN = 5.5V

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

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Typical Characteristics

Step-Down Converter Efficiency vs. Load

(VOUT = 3.3V; L = 3.3µH)

Output Current (mA)

Efficiency (%)

0.1 1 10 100 1000 10000

100

VIN = 3.6V

VIN = 4.2V

VIN = 5.0V

VIN = 5.5V

Step-Down Converter DC Regulation

(VOUT = 3.3V; L = 3.3µH)

Output Current (mA)

Output Error (%)

0.1 1 10 100 1000 10000

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

VIN = 4.2V

VIN = 5.0V

VIN = 5.5V

Step-Down Converter Line Regulation

(VOUT = 1.2V; L = 1.2µH)

Input Voltage (V)

Accuracy (%)

2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

IOUT = 0.10mA

IOUT = 100mA

IOUT = 1A

IOUT = 1.5A

IOUT = 2A

IOUT = 3A

Step-Down Converter Line Regulation

(VOUT = 1.8V; L = 1.8µH)

Input Voltage (V)

Accuracy (%)

2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0 IOUT = 0.10mA

IOUT = 100mA

IOUT = 1A

IOUT = 1.5A

IOUT = 2A

IOUT = 3A

Step-Down Converter Output Voltage Error

vs. Temperature

(VIN = 3.6V; VOUT = 1.2V)

Temperature (°C)

Output Voltage Error (%)

-50 -25 0 25 50 75 100

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

IOUT = 1A

IOUT = 1.5A

IOUT = 2A

IOUT = 3A

Step-Down Converter Output Voltage Error

vs. Temperature

(VIN = 3.6V; VOUT = 1.8V)

Temperature (°C)

Output Voltage Error (%)

-50 -25 0 25 50 75 100

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

IOUT = 1A

IOUT = 1.5A

IOUT = 3A

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

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Typical Characteristics

Step-Down Converter Output Voltage Error

vs. Temperature

(VIN = 3.6V; VOUT = 2.5V)

Temperature (°C)

Output Voltage Error (%)

-50 -25 0 25 50 75 100

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

IOUT = 1A

IOUT = 1.5A

IOUT = 3A

Step-Down Converter Output Voltage Error

vs. Temperature

(VIN = 4.2V; VOUT = 3.3V)

Temperature (°C)

Output Voltage Error (%)

-50 -25 0 25 50 75 100

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

IOUT = 1A

IOUT = 1.5A

IOUT = 2A

IOUT = 3A

No Load Input Current vs. Input Voltage

(VEN1 = VEN2 = VIN)

Input Voltage (V)

Input Current (µA)

2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.52.9 3.3 3.7 4.1 4.5 4.9 5.3

100

110

120

130

85°C

25°C

-40°C

Step-Down Converter Switching Frequency

vs. Input Voltage

(VOUT = 1.8V; IOUT = 1.5A)

Input Voltage (V)

Frequency Variation (%)

2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5

-5

-4

-3

-2

-1

Step-Down Converter Switching Frequency

vs. Temperature

(VIN = 3.6V; VOUT = 1.8V; IOUT = 1.5A)

Temperature (°

Switching Frequency (MHz)

-40 -20 0 20 40 60 80 100

1.20

1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

Step-Down Converter Soft Start

(VIN = 3.6V; VOUT = 1.8V; IOUT = 1.5A; CFF = 100pF)

Time (100µs/div)

Enable Voltage (top) (V)

Output Voltage (middle) (V)

Inductor Current (bottom) (A)

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

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Typical Characteristics

Step-Down Converter Load Transient Response

(IOUT = 0.3A to 3.0A; VIN = 3.6V; VOUT = 1.2V; COUT = 22µF)

Time (100µs/div)

Output Voltage (bottom) (V)

Output Current (top) (A)

0.8

1.0

1.2

1.4

0.3A

3.0A

Step-Down Converter Load Transient Response

(IOUT = 2.25A to 3.0A; VIN = 3.6V; VOUT = 1.2V; COUT = 22µF)

Time (100µs/div)

Output Voltage (bottom) (V)

Output Current (top) (A)

0.8

1.0

1.2

1.4

2.25A

3.0A

Step-Down Converter Load Transient Response

(IOUT = 0.3A to 3.0A; VIN = 4.2V; VOUT = 3.3V; COUT = 22µF)

Time (100µs/div)

Output Voltage (bottom) (V)

Output Current (top) (A)

2.9

3.1

3.3

3.5

3.7 0

0.3A

3.0A

Step-Down Converter Load Transient Response

(IOUT = 2.25A to 3.0A; VIN = 4.2V; VOUT = 3.3V; COUT = 22µF)

Time (100µs/div)

Output Voltage (bottom) (V)

Output Current (top) (A)

2.9

3.1

3.3

3.5

2.25A

3.0A

Step-Down Converter Line Transient Response

(VIN = 3.6V to 4.3V; VOUT = 1.2V; IOUT = 3A)

Time (200µs/div)

Input Voltage (top) (V)

Output Voltage (bottom) (V)

1.0

1.1

1.2

1.3

3.6V

4.3V

Step-Down Converter Line Transient Response

(VIN = 3.6V to 4.3V; VOUT = 1.8V; IOUT = 3A)

Time (100µs/div)

Input Voltage (top) (V)

Output Voltage (bottom) (V)

1.6

1.7

1.8

1.9

2.0

3.6V

4.3V

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

2522.2009.06.1.0 9

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Typical Characteristics

Step-Down Converter Line Transient Response

(VIN = 3.6V to 4.3V; VOUT = 2.5V; IOUT = 3A)

Time (200µs/div)

Input Voltage (top) (V)

Output Voltage (bottom) (V)

2.1

2.3

2.5

2.7

2.9

3.6V

4.3V

Step-Down Converter Output Ripple

(VIN = 3.6V; VOUT = 1.2V; IOUT = 1mA)

Time (100µs/div)

Output Voltage (middle) (V)

LX Voltage (top) (V)

Inductor Current (bottom) (A)

1.18

1.20

1.22

0.0

0.2

0.4

3.6V

Step-Down Converter Output Ripple

(VIN = 3.6V; VOUT = 1.2V; IOUT = 3A)

Time (500ns/div)

Output Voltage (middle) (V)

LX Voltage (top) (V)

Inductor Current (bottom) (A)

1.18

1.20

1.22

2.5

3.0

3.5

3.6V

Step-Down Converter Output Ripple

(VIN = 4.2V; VOUT = 3.3V; IOUT = 1mA)

Time (500ns/div)

Output Voltage (middle) (V)

LX Voltage (top) (V)

Inductor Current (bottom) (A)

3.25

3.30

3.35

0.0

0.2

4.2V

Step-Down Converter Output Ripple

(VIN = 4.2V; VOUT = 3.3V; IOUT = 3A)

Time (500ns/div)

Output Voltage (middle) (V)

LX Voltage (top) (V)

Inductor Current (bottom) (A)

3.25

3.30

3.35

2.9

3.0

3.1

4.2V

Step-Down Converter Short Circuit Protection

(VIN = 5V; VOUT = 1.8V; CFF = 100pF)

Time (100µs/div)

LX Voltage (top) (V)

Output Voltage (middle) (V)

Inductor Current (bottom) (A)

1.8V

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

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Functional Block Diagram

EN1

PGND1

FB1

LX1

0.6V

REF

0.6V

REF

IN1

Control

Logic

Control

Logic

VCC1

AGND1

INP

Slope

Comp

Osc

TSHDN

EN2

PGND2

FB2

LX2

IN2

VCC2

GND2

INP

Osc

TSHDN

Slope

Comp

Functional Description

The AAT2522 dual step-down regulators provide high-

performance operation with a 1.4MHz switching frequen-

cy. The AAT2522 regulators are completely independent,

including separate power supply inputs and enable sig-

nals. The highly integrated controller minimizes the

external component requirements, optimizes efficiency

over the complete load range, and produces reduced

ripple and spectral noise. Apart from the small bypass

input capacitor, only a small LC filter is required at the

output. Typically, a 3.3H inductor and a 22F 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 high-side P-channel MOSFET (plus the

DC drop of the external inductor and PCB layout). The

device integrates 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 85% efficiency at full load (5V Input to 3.3V

Output). Light-load, low-noise operation maintains high

efficiency, low ripple and low spectral noise with low cur-

rent conditions (typically < 150mA).

In battery-powered applications, as VIN decreases, the

converter dynamically adjusts the operating frequency

prior to dropout to maintain the required high duty-cycle

and maintain accurate output regulation. The regulators

will maintain output regulation until either the dropout

voltage limit is exceeded, or the input under-voltage

threshold is reached.

The AAT2522 typically achieves better than ±0.5% out-

put regulation across the input voltage and output load

range. A current limit of 4.0A (typical) protects the IC

and system components from short-circuit damage.

Typical no load quiescent current is 90A.

AAT2522

Dual High-Current, Low-Noise, Step-Down Regulators SwitchRegTM

PRODUCT DATASHEET

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Thermal protection completely disables switching when

the maximum junction temperature is detected. The

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 Scheme

The AAT2522 regulators are peak current-mode, step-

down converters. The controller senses the current

through the high-side P-channel MOSFET for current

loop control, as well as short-circuit and overload protec-

tion. A fixed slope compensation signal is added to the

sensed current to maintain stability for duty cycles

greater than 50%. The resulting peak current-mode loop

appears as a voltage-programmed current source in par-

allel 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 AAT2522 into a low-power

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

down is less than 1A.

Protection Circuitry

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.

Input 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.

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.

Therefore, the inductor should be set equal to the output

voltage numeric value in H. This guarantees that there

is sufficient 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 3.3H CDRH6D38NP series Sumida inductor has a

15m worst case DCR and a 3.5A DC current rating.

With a 3A load, the inductor DCR conduction loss is

135mW, which gives less than 1.4% loss in efficiency for

a 3A, 3.3V output.

Output Capacitor Selection

The output capacitor limits the output ripple and pro-

vides holdup during large load transitions. A 10F to

22F 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 domi-

nated 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

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responds. Within two or three switching cycles, the loop

responds and the inductor current increases to match the

load current demand. The first-order relationship of the

output voltage droop during the three switching cycles to

the output capacitance can be estimated by:

COUT =

3 · ΔIO

VDROOP · fSW

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 10F. 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.

Input Capacitor Selection

Select a 10F to 22F X7R or X5R ceramic capacitor for

the input. To estimate the required input capacitor size,

determine the acceptable input ripple level (VPK-PK) and

solve for CIN. The calculated value varies with input volt-

age and is a maximum when VIN is double the output

voltage (VIN = 2x VO):

CIN = and D =

D · (1 - D) VO

VIN

- ESR · fSW

VPKPK

The peak ripple voltage occurs when VIN = 2x VO (50%

duty cycle), resulting in a minimum output capacitance

recommendation:

CIN(MIN) = 1

- ESR · 4 · fSW

VPKPK

Always examine the ceramic capacitor DC voltage coef-

ficient characteristics when selecting the proper value.

For example, the derated capacitance of a 10F, 6.3V,

X5R ceramic capacitor with 5.0V DC applied is actually

about 6F.

The maximum input capacitor RMS current is:

IRMS = IO · D · (1 - D)

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:

RMS(MAX)

I occurs when VIN = 2 · VO

The term D (1-D) appears in both the input voltage rip-

ple 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 AAT2522. Low

ESR/ESL X7R and X5R ceramic capacitors are ideal for

this function. To minimize stray inductance, the capacitor

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).

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 damp-

ens the high Q network and stabilizes the system.

Adjustable Feedback Network

The output voltage on the AAT2522 is programmed with

external resistors ROUT-FB and RFB-GND. To limit the bias cur-

rent required for the external feedback resistor string

while maintaining good noise immunity. Although a

larger value will further reduce quiescent current, it will

also increase the impedance of the feedback node, mak-

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ing it more sensitive to external noise and interference.

Therefore, the recommended value range for RFB-GND (R3

and R5 in Figure 2) is 100k for good noise immunity or

221k for reduced no load input current.

The external resistor ROUT-FB (R2 and R4 in Figure 2), com-

bined with an external 100pF feed forward capacitor (C5

and C6 in Figure 2), 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

(COUT) for stability. The external resistors set the output

voltage according to the following equation:

VO = 0.6V · 1 + ROUT-FB

RFB-GND

or solving for ROUT-FB

ROUT-FB = - 1 · RFB-GND

0.6V

VOUT (V)

R3 = R5= 100kΩ

R2 = R4 (kΩ)

1.0 65.5

1.2 100

1.5 150

1.8 200

2.2 267

2.5 316

3.3 453

4.2 604

4.6 655

5 806

Table 1: Step-Down Converter Feedback Resistor

Selection for Different Output Voltages.

The typical circuit shown in the AAT2522 evaluation

schematic is intended to be general purpose and suitable

for most applications. In applications where transient

load steps are more severe and the restriction on output

voltage deviation is more stringent. To handle these

cases some simple adjustments can be made. The sche-

matic in Figure 2 shows the configuration for improved

transient response in an application where the output is

stepped down to 1.2V. The adjustments consist of adding

an additional 22F output capacitor, increasing the value

of the feed forward capacitor C6 to 1nF, and adding the

bias RC filter networks R1, C3 and R6, C4 in Figure 2.

Applications Information

Thermal Calculations

There are three types of losses associated with the

AAT2522 step-down converter: switching losses, con-

duction losses, and quiescent current losses. Conduction

losses are associated with the RDS(ON) characteristics of

the power output switching devices:

+ RDS(ON)L · PLOSS(RES) = IO2 · RDS(ON)H · VO

VIN

VIN - VO

VIN

Switching losses are dominated by the gate charge of

the power output switching devices. At full load, assum-

ing continuous conduction mode (CCM), a simplified

form of the switching losses is given by:

PLOSS(SW) = tSW · fSW · IO · VIN

The term tSW is used to estimate the full load step-down

converter switching losses. Finally, the losses associated

with the controller bias requirements are based the

regulator’s quiescent current (IQ):

PLOSS(BIAS) = IQ · VIN

Summing the three power loss terms together provides

the total power loss that the AAT2522 package must dis-

sipate:

PTOTAL = PLOSS(RES) + PLOSS(SW) + PLOSS(BIAS)

For the condition where the step-down converter is in

dropout at 100% duty cycle, the total device dissipation

reduces to:

+ IQ · VIN

PTOTAL = IO2 · RDS(ON)H = VO

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.

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Given the total losses, the maximum junction tempera-

ture can be derived from the JA for the TDFN34-16

package, which is 50°C/W.

TJ(MAX) = PTOTAL · θJA + TAMB

Assuming the operating ambient temperature is 85°C

(the worst case), the maximum power dissipation for the

TDFN34-16 package is determined in the following equa-

tion:

J(MAX) -

AMB

MAX

= = 1.1W

140°C

85°C

50°C/W

The power dissipation varies with the duty cycle and the

output current of the converters. Given the maximum

power dissipation of the TDFN34-16 package at 25°C

and 85°C, the relationship between the maximum allow-

able load for each channel and percent duty cycle are

expressed in Figure 1.

Maximum Output Current vs. Duty Cycle

(VIN = 3.6V)

Maximum Output Current (A)

02040608010 30 50 70 90 100

Duty Cycle (%)

0.5

1.5

2.5

3.5

85°C

25°C

Figure 1: Maximum Allowable Current for

AAT2522 Step-Down Converters.

As illustrated in Figure 1, the load limitation varies with

the percentage of duty cycle and the operating ambient

temperature. The total maximum load for both channels

running at the same time in an 85°C ambient is about

4A (2A per channel). Therefore, if channel 1 is running

at 1A, the maximum allowable load for channel 2 is no

more than 3A to prevent the thermal shutdown. However,

the maximum allowable load for each channel running at

room temperature can increase up to 3A. In high current

applications, the exposed pad needs to be connected to

a thick power ground plane through vias for thermal dis-

sipation.

Layout Recommendations

The suggested PCB layout for the AAT2522 is shown in

Figures 3 and 4. The following guidelines should be used

to help ensure a proper layout.

1. Place the input capacitor (CIN) as closely as possible

to VIN and PGND. Split the input supply tray to

separate the two input capacitors in order to prevent

noise coupling between two channels at heavy load.

2. The output capacitor and inductor should be con-

nected as closely as possible. The inductor connec-

tion to the LX pin should be as short as possible.

3. The feedback trace or FB pin should be separated

from any power trace and connected as closely as

possible 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.

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22μF

100kΩ

3.3V/3A

opt

22μF

U1 AAT2522IRN AnalogicTech, TDFN34-16

C1, C2 Ceramic cap, MLC, 10μF/10V, 0803

C3, C4 Ceramic cap, MLC, 0.1μF/10V, 0402

C5, C6 Ceramic cap, MLC, 100pF/10V, 0402, optional

C7-C9 Ceramic cap, MLC, 22μF/10V, 0805

L1 1.5μH, Sumida, CDRH4D22HPNP-1R5NC, 3.9A, 25mΩ

L2 3.3μH, Sumida, CDRH6D38NP-3R3N, 3.5A, 15mΩ

R1-R6 Carbon film resistor, 0402

1.5μH

453kΩ

100kΩ

opt

EN1

EN2

10μF

3.3μH

L2 VOUT2

1.2V/3

VOUT1

VIN

100Ω

opt

C10

PGND1 10

AGND2 16

EN1

AGND1 12

VCC2

IN1

IN2

VCC1

IN1

PGND1 9

LX1 8

FB1 11

PGND2

EN2

LX2 13

FB2 15

AAT2522 IRN

0.1μF

2.7V-5.5V

100Ω

0.1μF

Figure 2: AAT2522 Evaluation Board Schematic and Bill of Materials.

Figure 3: AAT2522 Evaluation Board Figure 4: AAT2522 Evaluation Board

Top Side Layout. Bottom Side Layout.

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AAT2522 Design Example

Specifications

VOUT1 = 3.3V @ 1A, Pulsed Load ΔILOAD = 1A

VOUT2 = 1.2V @ 2A, Pulsed Load ΔILOAD = 2A

VIN1 = 3.6V

FS = 1.4MHz

TAMB = 85°C in TDFN34-16 Package

Step-Down Converter Output Inductor

The internal slope compensation for the AAT2522 is set to 75% of the inductor current down slope for a 1.8V output

and 1.8H inductor:

0.75 · V

m = = = 0.75

0.75 · 1.8V

1.8μH

μs

For 3.3V and 1.2V outputs, the inductor values are given in the following equations:

0.75 · V

L = = = 3.3μH; use 3.3μH

0.75 · 3.3V

0.75A

μs

0.75 · V

L = = = 1.2μH; use 1.5μH

0.75 · 1.2V

0.75A

μs

For Sumida inductor, CDRH6D38NP-3R3N, 3.3H, ISAT = 3.5A, DCR = 15m.

For Sumida inductor, CDRH4D22HPNP-1R5NC, ISAT = 3.9A, 1.5H, DCR = 25m.

PK1

= I

OUT1

+ ΔI

= 1A + 0.03A = 1.03A

PK2

= I

OUT2

+ ΔI

= 2A + 0.476A = 2.5A

OUT2

1.2

1.2V

ΔI

1 - = · 1 - = 0.476

· F

1.2μH · 1.4MHz

3.6V

OUT1

3.3

3.3V

ΔI

1 - = · 1 - = 0.06A

· F

3.3μH · 1.4MHz

3.6V

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Step-Down Converter Output Capacitor

VDROOP = 0.2V

1 1.2V · (5.5V - 1.2V)

1.5μH · 1.4MHz · 5.5V

RMS(MAX)

I = ·

3 · ΔILOAD

VDROOP · FS

3 · 2A

0.2V · 1.4MHz

COUT = = = 21.4μF; use 22μF

· = 129mARMS

VOUT2 · (VIN(MAX) - VOUT2)

L · FS · VIN(MAX)

PRMS = ESR · IRMS2 = 5mΩ · (129mA)2 = 83μW

Step-Down Converter Input Capacitor

Input Ripple VPP = 50mV

CIN = = = 9μF; use 10μF

- ESR · 4 · FS

VPP

- 5mΩ · 4 · 1.4MHz

50mV

0.2A

IOUT

RMS

P = ESR · (IRMS

2) = 5mΩ · (1A)2 = 5mW

== 1A

AAT2522 Losses

All values assume at 85°C ambient temperature and thermal resistance of 50°C/W in the TDFN34-16 package.

PTOTAL

+ (tsw · FS · IOUT1 + IQ1) · VIN

IOUT1

2 · (RDS(ON)H · VOUT1 + RDS(ON)L · [VIN -VOUT1])

VIN

+ (tsw · FS · IOUT2 + IQ2) · VIN

IOUT2

2 · (RDS(ON)H · VOUT2 + RDS(ON)L · [VIN -VOUT2])

VIN

PTOTAL = 0.58W

+ (5ns · 1.4MHz · 1A + 84μA) · 3.6V

1A2 · (0.12Ω · 3.3V + 0.085Ω · [3.6V - 3.3V])

3.6V

++ (5ns · 1.4MHz · 2A + 84μA) · 3.6V

2A2 · (0.12Ω · 1.2V + 0.085Ω · [3.6V - 1.2V])

3.6V

(

MAX

)

= TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 0.58mW = 114°C

AAT2522

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

2. 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.

Ordering Information

Package Marking Part Number (Tape and Reel)1

TDFN34-16 9BXYY AAT2522IRN-1-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

http://www.analogictech.com/about/quality.aspx.

Package Information

TDFN34-162

3.000

0.050 1.600

0.050

0.050 0.229

0.051

(4x)

0.850 MAX

4.000

0.050

3.300

0.050

Index Area

Detail "A"

Top View Bottom View

Side View

0.350

0.100

0.230

0.0500.450

0.050

Detail "A"

Pin 1 Indicator

(optional)

C0.3

All dimensions in millimeters.

AAT2522

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Advanced Analogic Technologies, Inc.

3230 Scott Boulevard, Santa Clara, CA 95054

Phone (408) 737-4600

Fax (408) 737-4611

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design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to

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