1
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
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General Description
The AAT2158 SwitchReg is a 1.5A step-down converter
with an input voltage range of 2.4V 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 AAT2158 an ideal choice for portable applications.
The AAT2158 delivers 1.5A 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 AAT2158 an ideal choice for
high output voltage, high current applications which
require a low dropout threshold.
The AAT2158 provides excellent transient response and
high output accuracy across the operating range. No
external compensation components are required.
The AAT2158 maintains high efficiency throughout the
load range. The AAT2158’s unique architecture produces
reduced ripple and spectral noise. Over-temperature and
short-circuit protection safeguard the AAT2158 and sys-
tem components from damage.
The AAT2158 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
• 1.5A Maximum Output Current
• Input Voltage: 2.4V 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
L1
22µF
C3
10µF
C1
2.5V3.3V
187k
R3
59k
R4
LX 14
N/C
6
EN
7
VCC
9
VP
10
N/C
8
LX 13
PGND 3
VP
12
VP
11
FB 4
LX 15
PGND 2
PGND 1
SGND
5
N/C 16
AAT2158
U1
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2
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
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 fi 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.
7EN
Enable input pin. A logic high enables the converter; a logic low forces the AAT2158 into shutdown mode
reducing the supply current to less than 1μA. The pin should not be left fl oating.
9 VCC 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)
VP
VP
VP
N/C
PGND
PGND
PGND
1
2
3
4
N/C
SGND
EN
16
15
14
13
5
6
7
8
12
11
10
9
N/C
VC
C
LX
LX
LX
FB
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AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Absolute Maximum Ratings1
Symbol Description Value Units
VCC, VPVCC, VP to GND 6V
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
JA Maximum Thermal Resistance 50 °C/W
JC 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.
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4
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
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 2.4 5.5 V
VOUT Output Voltage Range 0.6 VIN V
VUVLO UVLO Threshold
VIN Rising 2.4 V
Hysteresis 250 mV
VIN Falling 1.8 V
VOUT Output Voltage Tolerance IOUT = 0A to 1.5A, VIN = 2.4V 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 1.8 A
RDS(ON)H High Side Switch On-Resistance 0.120
RDS(ON)L Low Side Switch On-Resistance 0.085
VLOADREG Load Regulation ILOAD = 0A to 1.5A 0.5 %
VLINEREG/VIN Line Regulation VIN = 2.4V 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 150 μs
TSD Over-Temperature Shutdown Threshold 140 °C
THYS Over-Temperature Shutdown Hysteresis 15 °C
EN
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 AAT2158 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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Typical Characteristics
Efficiency vs. Output Current
(VOUT = 3.3V)
Output Current (mA)
Efficiency (%)
30
40
50
60
70
80
90
100
0 1 10 100 1000 10000
VIN = 3.6V
VIN = 4.2V VIN = 5.0V
Load Regulation
(VOUT = 3.3V)
Output Current (mA)
VOUT Error (%)
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
0.1 1 10 100 1000 1000
0
VIN = 5.0V
VIN = 4.2V
VIN = 3.6V
Efficiency vs. Output Current
(VOUT = 1.8V)
Output Current (mA)
Efficiency (%)
30
40
50
60
70
80
90
100
0 1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V VIN = 4.2V
Load Regulation
(VOUT = 1.8V)
Output Current (mA)
VOUT Error (%)
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
0.1 1 10 100 1000 1000
0
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
Efficiency vs. Output Current
(VOUT = 1.2V)
Output Current (mA)
Efficiency (%)
30
40
50
60
70
80
90
100
0 1 10 100 1000 1000
0
VIN = 2.7V
VIN = 3.6V VIN = 4.2V
Load Regulation
(VOUT = 1.2V)
Output Current (mA)
VOUT Error (%)
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
0.1 1 10 100 1000 1000
0
VIN = 4.2V
VIN = 2.7V
VIN = 3.6V
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6
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Typical Characteristics
Quiescent Current vs. Input Voltage
(VOUT = 1.8V; No Load)
Input Voltage (V)
Quiescent Current (µA)
20
30
40
50
60
70
80
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
10
0
85°C
25°C
-40°C
Output Voltage vs. Temperature
(VOUT = 1.8V; IOUT = 1A)
Temperature (°C)
Output Voltage Error (%)
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
Output Voltage vs. Input Voltage
(VOUT = 1.8V; IOUT = 1A)
Input Voltage (V)
Output Voltage (V)
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 = 1A)
Temperature (°C)
Switching Frequency (MHz)
1.42
1.44
1.46
1.48
1.5
1.52
1.54
1.56
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
Load Transient Response
(VOUT = 1.8V)
Time (100µs/div)
Output Voltage (AC coupled)
(top)(mV)
Output Current
(bottom) (A)
-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)
-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
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AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Typical Characteristics
Line Transient Response
(VOUT = 1.8V; IOUT = 1.5A; CFF = 100pF)
Time (100µs/div)
Input Voltage
(top) (V)
Output Voltage (AC coupled)
(bottom) (V)
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 (%)
-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 = 1.5A)
Time (100µs/div)
VOUT
(1V/div)
EN
(2V/div)
IIN
(500mA/div)
Heavy Load Switching Waveform
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1.5A)
Time (500ns/div)
Output Voltage (AC Coupled)
(top) (mV)
Inductor Ripple Current
(bottom) (A)
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
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)
-240
-200
-160
-120
-80
-40
0
40
80
-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)
-240
-200
-160
-120
-80
-40
0
40
80
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
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8
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Typical Characteristics
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
40
80
-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 (50µs/div)
Output Voltage (AC coupled)
(top)(mV)
Inductor Ripple Current
(bottom) (A)
-240
-200
-160
-120
-80
-40
0
40
80
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
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AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Functional Description
The AAT2158 is a high performance 1.5A 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 oper-
ation maintains high efficiency, low ripple and low spec-
tral 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 1.5A output load, dropout voltage
headroom is approximately 200mV.
The AAT2158 typically achieves better than ±0.5% out-
put regulation across the input voltage and output load
range. A current limit of 2.0A (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
LX
FB
DH
DL
CMP
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10
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
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 AAT2158 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 AAT2158 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
L1
22µF
C3
10µF
C1
100
R1
0.1µF
C2
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C3 Murata 22µF 6.3V GRM21BR60J226ME39L X5R 0805
VOUT+
VIN+
L1 Sumida CDRH5D28-3R0NC (see Table 2)
R1 and C2 are an optional noise filter for internal VCC.
R6, C4, C5-C7 are not populated
C8 100pF feed forward
LX 14
N/C
6
EN
7
VCC
9
VP
10
N/C
8
LX 13
PGND 3
VP
12
VP
11
FB 4
LX 15
PGND 2
PGND 1
SGND
5
N/C 16
AAT2158
U1
R3
C8
59.0k
R4
100K
R2
Enable LX
GNDGND
VOUT(V) R3 (kΩ)
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: AAT2158 Evaluation Schematic.
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AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
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
49.2m worst case DCR and a 1.57A DC current rating.
At full 1.5A load, the inductor DC loss is 97mW which
gives less than 1.5% loss in efficiency for a 1.5A, 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 volt-
age and is a maximum when VIN is double the output
voltage.
⎛⎞
· 1 -
⎝⎠
VO
VIN
CIN =
VO
VIN
⎛⎞
- ESR · FS
⎝⎠
VPP
IO
⎛⎞
· 1 - = for VIN = 2 · VO
⎝⎠
VO
VIN
VO
VIN
1
4
CIN(MIN) = 1
⎛⎞
- ESR · 4 · FS
⎝⎠
VPP
IO
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
ceramic capacitor with 5.0V DC applied is actually about
6μF.
The maximum input capacitor RMS current is:
⎛⎞
IRMS = IO · · 1 -
⎝⎠
VO
VIN
VO
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 =
⎝⎠
VO
VIN
VO
VIN
1
2
for VIN = 2 · VO
IO
RMS(MAX)
I2
=
The term
⎛⎞
· 1 -
⎝⎠
VO
VIN
VO
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 AAT2158. 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 2).
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the eval-
uation 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.
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12
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
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 · ΔILOAD
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 AAT2158 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 59k. 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 59k for good noise immunity or 221k
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:
⎛⎞
⎝⎠
R3
R4
VOUT = 0.6V 1 +
or
⎛⎞
⎝⎠
⎡⎤
⎣⎦
VOUT
VREF
R3 = - 1 R
4
VOUT (V)
R4 = 59k
R3 (k)
R4 = 221k
R3 (k)
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: AAT2158 Resistor Values for Various
Output Voltages.
The typical circuit shown in the AAT2158 evaluation
schematic is intended to be general purpose and suitable
for most applications. An additional example schematic
is shown in Figure 2 according to the design guidelines
for cases where transient load steps are more severe
and the restriction on output voltage deviation is more
stringent. To handle these cases some simple adjust-
ments can be made.
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AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
U1 AAT2158 AnalogicTech, Hi-Voltage Buck, TDFN34-16
C1 Cap, MLC, 10μF/6.3V,X7R, 0805
C3 Cap, MLC, 22μF/6.3V, X7R,0805
C4 Cap, MLC, 22μF/6.3V, X7R, 0805
C8 Cap, MLC, 1nF/6.3V, 0402
R1-R4 Carbon film resistor, 0402
L1 TDK, VLS3012T-1R5N1R7, 1.5μH, ISAT = 1.9A, DCR = 68mΩ
1.0 -1.5μH
L1
1
2
3
EN
C2
open
C8
1nF
(optional)
C3
22μF
C4
22μF
C1
10μF
VP
12
N/C 6
LX 14
FB 4
VP
11
LX 15
LX 13
EN
7N/C 16
PGND
1
VP
10
VCC
9
N/C 8
PGND
2
PGND
3
SGND 5
AAT2158
U1
1.2V/1.5
A
VOUT
2.4V - 5.5V
VIN
R1
0
R2
100K
R3
100K
R4
100K
Figure 2: AAT2158 Alternate Application Schematic.
The schematic 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 22μF output capacitor C4,
increasing the value of the feed forward capacitor C8 to
1nF, and eliminating the RC filter network R1 and C2.
Thermal Calculations
There are three types of losses associated with the
AAT2158 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. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the losses is
given by:
PTOTAL
IO
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
IO
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 JA for the QFN33-16 pack-
age, which is 50°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
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14
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Layout
The suggested PCB layout for the AAT2158 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: AAT2158 Evaluation Board Figure 4: AAT2158 Evaluation Board
Top Side Layout. Bottom Side Layout.
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AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Design Example
Specifications
VO 3.3V @ 1.4A, Pulsed Load ILOAD = 1.4A
VIN 2.7V to 4.2V (3.6V nominal)
FS 1.2MHz
TAMB 85°C in QFN33-16 Package
Output Inductor
L1 = VO(μH) = 3.3μH; see Table 2.
For Sumida inductor CDRH4D28 3.3μH DCR = 49.2m max.
V
O
V
O1
3.3
V
3.3V
ΔI
1
=
⋅ 1 - = ⋅ 1 - = 179m
A
L1 ⋅ F
S
V
IN
3.3µH ⋅ 1.2MHz
4.2V
I
PK1
= I
O1
+ ΔI
1
= 1.4A + 0.089A = 1.49A
2
P
L1
= I
O1
2
⋅ DCR = 1.4A
2
⋅ 49.2mΩ = 96mW
⎛
⎝⎞
⎠
⎛
⎝⎞
⎠
Output Capacitor
VDROOP = 0.2V
1
23
1 3.3V · (4.2V - 3.3V)
3.3µH · 1.2MHz · 4.2V
23
RMS(MAX)
IL · FS · VIN(MAX)
= ·
·
3 · ΔILOAD
VDROOP · FS
3 · 1.4A
0.2V · 1.2MHz
COUT = = = 17.5µF; use 22µF
· = 52mArms
·
(VOUT) · (VIN(MAX) - VOUT)=
Pesr = esr · IRMS2 = 5mΩ · (52mA)2 = 13.3µW
Input Capacitor
Input Ripple VPP = 50mV
CIN = = = 6.8µF; use 10µF
1
⎛⎞
- ESR · 4 · FS
⎝⎠
VPP
IO1 + IO2
1
⎛⎞
- 5mΩ · 4 · 1.2MHz
⎝⎠
50mV
1.4A
IO
RMS(MAX)
I
P = esr · IRMS
2 = 5mΩ · (0.7A)2 = 2.45mW
2
= = 0.7Arms
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16
AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
AAT2158 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 = IO1
2 · RDS(ON)H = 1.4A2 · 0.16Ω = 0.31W
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 310mW = 101°
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]
= 1.4A2 · 152mΩ + [(5ns · 1.2MHz · 1.4A + 50μA) · 3.6V] = 328mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 328mW = 101°
C
VOUT (V)
Inductance
(μH) Part Number Manufacturer Size (mm)
Rated
Current (A) IRMS (A)
ISAT
(A) DCR ()
3.3 3.0 CDRH5D28 Sumida 6x6x3 2.4 24.0
3.3 3.3 CDRH4D28 Sumida 5x5x3 1.57 49.2
2.5 2.2 CDRH4D28 Sumida 5x5x3 2.04 31.3
1.8 1.8 CDRH4D28 Sumida 5x5x3 2.2 27.5
1.5 1.8 CDRH4D28 Sumida 5x5x3 2.2 27.5
1.2 1.2 CDRH4D28 Sumida 5x5x3 2.56 23.6
1.0 1.0 SD3114-1.0 Cooper 3.1x3.1x1.45 1.67 2.07 0.042
0.8 1.0 SD3114-1.0 Cooper 3.1x3.1x1.45 1.67 2.07 0.042
0.6 1.0 SD3114-1.0 Cooper 3.1x3.1x1.45 1.67 2.07 0.042
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
Table 3: Surface Mount Capacitors.
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AAT2158
1.5A Low-Noise,Step-Down Converter
DATA SHEET
Copyright © 2012 Skyworks Solutions, Inc. All Rights Reserved.
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Ordering Information
Package Marking1Part Number (Tape and Reel)2
QFN33-16 XSXYY AAT2158IVN-0.6-T1
Skyworks Green™ products are compliant with
all applicable legislation and are halogen-free.
For additional information, refer to Skyworks
Definition of Green™, document number
SQ04-0074.
Package Information3
QFN33-16
3.000
±
0.050
Pin 1 Dot By Marking
1.250
±
0.050
0.400
±
0.100
1.250
±
0.050
3.000
±
0.050
0.500
±
0.050
0.900
±
0.100
Pin 1 Identification
C0.3
0.025
±
0.025
0.214
±
0.036
0.230
±
0.050
Top Vi e w Bottom View
Side View
1
13
5
9
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.
