Analogic AAT2120 500ma low noise step-down converter Datasheet

PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
General Description
Features
The AAT2120 SwitchReg is a 1.8MHz step-down converter with an input voltage range of 2.7V to 5.5V and
output as low as 0.6V. Its low supply current, small size,
and high switching frequency make the AAT2120 the
ideal choice for portable applications.
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The AAT2120 delivers up to 500mA of load current, while
maintaining a low 45μA no load quiescent current. The
1.8MHz switching frequency minimizes the size of external components, while keeping switching losses low. The
AAT2120 feedback and control delivers excellent load
regulation and transient response with a small output
inductor and capacitor.
The AAT2120 maintains high efficiency throughout the
load range. The AAT2120’s unique architecture produces
reduced ripple and spectral noise. Over-temperature and
short-circuit protection safeguard the AAT2120 and system components from damage.
VIN Range: 2.7V to 5.5V
VOUT Range: 0.6V to VIN
Up to 500mA Output Current
Up to 96% Efficiency
Low Noise Light Load Mode
45μA Typical Quiescent Current
1.8MHz Switching Frequency
Soft-Start Control
Over-Temperature and Current Limit Protection
100% Duty Cycle Low-Dropout Operation
<1μA Shutdown Current
Small External Components
Ultra-Small STDFN22-8 Package
Temperature Range: -40°C to +85°C
Applications
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The AAT2120 is available in a Pb-free, 8-pin, 2x2mm
STDFN package and is rated over the -40°C to +85°C
temperature range.
Bluetooth® Headsets
Cellular Phones
Digital Cameras
Handheld Instruments
Micro Hard Disk Drive
Portable Music Players
USB Devices
Typical Application
VIN
VO = 1.8V
AAT2120
VP
LX
L1
3.0µH
VIN
C1
4.7µF
EN
GND
2120.2008.02.1.2
500mA
FB
PGND
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R1
118kΩ
R2
59kΩ
C2
4.7µF
1
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Pin Descriptions
Pin #
Symbol
1
2
3
4
5
6
VP
VIN
GND
FB
N/C
EN
7
LX
8
PGND
EP
Function
Input power pin; connected to the source of the P-channel MOSFET. Connect to the input capacitor.
Input bias voltage for the converter.
Non-power signal ground pin.
Feedback input pin. Connect this pin to an external resistive divider for adjustable output.
No connect.
Enable pin. A logic high enables normal operation. A logic low shuts down the converter.
Switching node. Connect the inductor to this pin. It is connected internally to the drain of both high- and
low-side MOSFETs.
Input power return pin; connected to the source of the N-channel MOSFET. Connect to the output and
input capacitor return.
Exposed paddle (bottom); connect to ground directly beneath the package.
Pin Configuration
STDFN22-8
(Top View)
VP
VIN
GND
FB
2
1
8
PGND
2
7
LX
3
6
EN
4
5
N/C
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Absolute Maximum Ratings1
Symbol
VIN
VLX
VOUT
VEN
TJ
TLEAD
Description
Input Voltage and Bias Power to GND
LX to GND
FB to GND
EN to GND
Operating Junction Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
Value
Units
6.0
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to 6.0
-40 to 150
300
V
V
V
V
°C
°C
Value
Units
2
50
W
°C/W
Thermal Information
Symbol
PD
θJA
Description
Maximum Power Dissipation (STDFN22-8)
Thermal Resistance2 (STDFN22-8)
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent 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 an FR4 board.
2120.2008.02.1.2
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3
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Electrical Characteristics1
VIN = 3.6V, TA = -40°C to +85°C, unless otherwise noted; typical values are TA = 25°C.
Symbol
Description
VIN
Input Voltage
VUVLO
VOUT
VOUT
IQ
ISHDN
ILIM
RDS(ON)H
RDS(ON)L
ΔVLinereg/ΔVIN
VFB
IFB
FOSC
TS
TSD
THYS
VEN(L)
VEN(H)
IEN
Conditions
Min
Typ
2.7
UVLO Threshold
Output Voltage Tolerance2
Output Voltage Range
Quiescent Current
Shutdown Current
P-Channel Current Limit
High-Side Switch On Resistance
Low-Side Switch On Resistance
Line Regulation
Feedback Threshold Voltage Accuracy
FB Leakage Current
Oscillator Frequency
Startup Time
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
Enable Threshold Low
Enable Threshold High
Input Low Current
VIN Rising
Hysteresis
VIN Falling
IOUT = 0 to 500mA, VIN = 2.7V to 5.5V
Units
5.5
2.3
V
V
mV
V
%
V
μA
μA
mA
Ω
Ω
%/V
V
μA
MHz
μs
°C
°C
V
V
μA
70
1.4
-3.0
0.6
No Load
EN = GND
VIN = 2.7V to 5.5V
VIN = 3.6V
VOUT = 1.0V
Max
45
0.591
700
0.59
0.42
0.2
0.600
3.0
VIN
85
1.0
0.609
0.2
1.8
100
105
15
From Enable to Output Regulation
0.6
VIN = VEN = 5.5V
1.4
-1.0
1.0
1. The AAT2120 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correlation with statistical process controls.
2. Output voltage tolerance is independent of feedback resistor network accuracy.
4
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Typical Characteristics
Efficiency vs. Load
Load Regulation
(VOUT = 3.0V; L = 4.7µH)
(VOUT = 3.0V; L = 4.7µH)
1.0
100
Load Regulation (%)
Efficiency (%)
90
VIN = 3.6V
80
VIN = 4.2V
70
VIN = 5.0V
60
50
40
0.1
1
10
100
0.8
0.6
VIN = 3.6V
0.4
0.0
-0.2
-0.4
VIN = 4.2V
-0.6
-0.8
-1.0
1000
0.1
1
Output Current (mA)
Load Regulation
(VOUT = 1.8V; L = 3.3µH)
(VOUT = 1.8V; L = 3.3µH)
Efficiency (%)
80
VIN = 4.2V
70
60
50
0.8
0.6
10
100
VIN = 4.2V
0.4
0.2
0.0
-0.2
-0.4
VIN = 3.6V
-0.6
1000
0.1
1
Output Current (mA)
100
Efficiency vs. Load
Load Regulation
(VOUT = 1.2V; L = 1.5µH)
(VOUT = 1.2V; L = 1.5µH)
1000
1.0
VIN = 2.7V
VIN = 3.6V
70
VIN = 4.2V
60
VIN = 5.0V
50
40
0.1
0.6
VIN = 3.6V
0.4
0.2
0.0
-0.2
-0.4
VIN = 4.2V
-0.6
VIN = 5.0V
-0.8
-1.0
1
10
100
1000
Output Current (mA)
2120.2008.02.1.2
VIN = 2.7V
0.8
Load Regulation (%)
Efficiency (%)
10
Output Current (mA)
100
80
VIN = 2.7V
-0.8
-1.0
40
1
1000
1.0
VIN = 3.6V
90
90
100
Efficiency vs. Load
VIN = 2.7V
0.1
10
Output Current (mA)
Load Regulation (%)
100
VIN = 5.0V
0.2
0.1
1
10
100
Output Current (mA)
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5
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Typical Characteristics
Line Regulation
Soft Start
(VOUT = 1.8V)
(VIN = 3.6V; VOUT = 1.8V; 500mA)
VO
VEN
1.2
2
1.0
1
0.8
0
0.6
-1
0.4
ILX
-2
0.2
-3
0.0
-4
-0.2
0.30
0.20
Accuracy (%)
Enable and Output Voltage
(top) (V)
3
1.4
Inductor Current (bottom) (A)
4
IOUT = 10mA
0.10
IOUT = 500mA
IOUT = 0mA
0.00
IOUT = 50mA
IOUT = 150mA
-0.10
-0.20
IOUT = 250mA
-0.30
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
Time (100µs/div)
Output Voltage Error vs. Temperature
Frequency Variation vs. Input Voltage
(VIN = 3.6V; VOUT = 1.8V; IOUT = 500mA)
4
Frequency Variation (%)
Output Error (%)
3.0
2.0
1.0
0.0
-1.0
-2.0
-40
-20
0
20
40
60
80
2
1
0
-1
100
VOUT = 3.0V
-2
-3
-4
2.7
-3.0
VOUT = 1.8V
3
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Temperature (°C)
No Load Quiescent Current vs. Input Voltage
Switching Frequency Variation vs. Temperature
(VIN = 3.6V; VOUT = 1.8V)
60
8
55
Supply Current (µA)
10
Variation (%)
6
4
2
0
-2
-4
-6
-8
-10
50
25°C
40
35
-40°C
30
25
20
-40
-20
0
20
40
60
80
100
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Temperature (°C)
6
85°C
45
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Typical Characteristics
P-Channel RDS(ON) vs. Input Voltage
RDS(ON) (mΩ
Ω)
900
750
120°C
700
100°C
700
600
500
300
2.5
600
450
3.5
4.0
4.5
5.0
5.5
300
2.5
6.0
25°C
3.0
3.5
4.0
4.5
5.0
5.5
Load Transient Response
(350mA to 500mA; VIN = 3.6V; VOUT = 1.8V;
COUT = 4.7µF; CFF = 100pF)
VO
1.6
1.4
IO
1.2
1.0
I LX
0.8
0.6
2.0
1.9
1.8
1.7
1.6
VO
IO
1.5
1.4
1.3
1.2
1.1
I LX
1.0
Time (20µs/div)
Load and Inductor Current
(bottom) (100mA/div)
1.8
Output Voltage (top) (V)
Load Transient Response
(1mA to 500mA; VIN = 3.6V; VOUT = 1.8V;
COUT = 4.7µF; CFF = 100pF)
2.0
6.0
Input Voltage (V)
Load and Inductor Current
(bottom) (400mA/div)
Output Voltage (top) (V)
Input Voltage (V)
2.2
100°C
500
350
3.0
120°C
550
400
25°C
400
85°C
650
85°C
800
RDS(ON) (mΩ
Ω)
1000
N-Channel RDS(ON) vs. Input Voltage
Time (25µs/div)
Line Response
(VOUT = 1.8V @ 500mA)
Output Voltage (top) (V)
7.0
1.85
VO
1.80
6.5
6.0
1.75
5.5
1.70
5.0
1.65
4.5
VIN
1.60
4.0
1.55
3.5
1.50
3.0
Input Voltage (bottom) (V)
1.90
Time (25µs/div)
2120.2008.02.1.2
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7
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; IOUT = 500mA)
40
0.14
20
0.12
VO
0.10
-20
0.08
-40
0.06
-60
0.04
-80
ILX
0.02
-100
0.00
-120
-0.02
Time (2µs/div)
8
1.0
40
20
0.9
VO
0
0.8
-20
0.7
-40
ILX
0.6
-60
0.5
-80
0.4
-100
0.3
-120
0.2
Inductor Current
(bottom) (A)
0
Output Voltage (AC Coupled)
(top) (mV)
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA)
Inductor Current
(bottom) (A)
Output Voltage (AC Coupled)
(top) (mV)
Typical Characteristics
Time (200ns/div)
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Functional Block Diagram
FB
VP
VIN
Err
Amp
DH
Voltage
Reference
LX
Logic
EN
INPUT
DL
PGND
GND
Functional Description
The AAT2120 is a high performance 500mA, 1.8MHz
monolithic step-down converter designed to operate with
an input voltage range of 2.7V to 5.5V. The converter
operates at 1.8MHz, which minimizes the size of external
components. Typical values are 3.3μH for the output
inductor and 4.7μF for the ceramic output capacitor.
The device is designed to operate with an output voltage
as low as 0.6V. Power devices are sized for 500mA current capability while maintaining over 90% efficiency at
full load. Low current efficiency is maintained at greater
than 80% down to 1mA of load current.
2120.2008.02.1.2
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 highside MOSFET.
A high-DC gain error amplifier with internal compensation controls the output. It provides excellent transient
response and load/line regulation. Soft start eliminates
any output voltage overshoot when the enable or the
input voltage is applied.
Control Loop
The AAT2120 is a 500mA current mode step-down converter. The current through the P-channel MOSFET (high
side) is sensed for current loop control, as well as short-
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PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
circuit and overload protection. A fixed slope compensation signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak current mode loop appears as a voltage-programmed current 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
error amplifier reference is fixed at 0.6V.
Soft Start / Enable
Soft start increases the inductor current limit point in
discrete steps when the input voltage or enable input is
applied. It limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT2120 into a low-power,
non-switching state. The total input current during shutdown is less than 1μA.
Applications Information
Inductor Selection
The step-down converter uses peak current mode control
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 internal
slope compensation for the adjustable and low-voltage
fixed versions of the AAT2120 is 0.45A/μs. This equates
to a slope compensation that is 75% of the inductor current down slope for a 1.8V output and 3.0μH inductor.
m=
This is the internal slope compensation for the AAT2120.
When externally programming to 3.0V, the calculated
inductance is 5.0μH.
L=
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limited. As load impedance decreases and the output voltage
falls closer to zero, more power is dissipated internally,
raising the device temperature. Thermal protection completely disables switching when internal dissipation
becomes excessive, protecting the device from damage.
The junction over-temperature threshold is 105°C with
15°C of hysteresis.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN power.
Under-voltage lockout (UVLO) guarantees sufficient VIN
bias and proper operation of all internal circuits prior to
activation.
10
0.75 ⋅ VO 0.75 ⋅ 1.8V
A
=
= 0.45
L
3.0µH
µs
0.75 ⋅ VO
µs
0.75 ⋅ VO
≈ 1.67 A ⋅ VO
=
m
A
0.45A µs
= 1.67
µs
⋅ 3.0V = 5.0µH
A
In this case, a standard 4.7μH value is selected.
For most designs, the AAT2120 operates with an inductor
value of 1μH to 4.7μH. Table 1 displays inductor values
for the AAT2120 with different output voltage options.
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 saturation 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.
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Output Voltage (V)
L1 (μH)
1.0
1.2
1.5
1.8
2.5
3.0
3.3
1.5
2.2
2.7
3.0
3.9
4.7
5.6
Table 1: Inductor Values.
The 3.0μH CDRH2D09 series inductor selected from
Sumida has a 150mΩ DCR and a 470mA DC current
rating. At full load, the inductor DC loss is 9.375mW
which gives a 2.08% loss in efficiency for a 250mA,
1.8V output.
Input Capacitor
Select a 4.7μF to 10μF 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 CIN. The calculated value varies with input voltage and
is a maximum when VIN is double the output voltage.
CIN =
V ⎞
VO ⎛
· 1- O
VIN ⎝
VIN ⎠
⎛ VPP
⎞
- ESR · FS
⎝ IO
⎠
VO ⎛
V ⎞
1
· 1 - O = for VIN = 2 · VO
VIN ⎝
VIN ⎠
4
CIN(MIN) =
1
Always examine the ceramic capacitor DC voltage coefficient 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:
for VIN = 2 · VO
IRMS(MAX) =
VO
0.52 =
1
2
IO
2
⎛
V ⎞
· 1- O
The term VIN ⎝ 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 AAT2120. 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 Figure 2.
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the evaluation 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 during load transients. Errors in the loop phase and gain
measurements can also result.
In applications where the input power source lead inductance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or aluminum electrolytic should be placed in parallel with the
low ESR, ESL bypass ceramic. This dampens the high Q
network and stabilizes the system.
Output Capacitor
VO ⎛
V ⎞
· 1- O
VIN ⎝
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.
2120.2008.02.1.2
D · (1 - D) =
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.
⎛ VPP
⎞
- ESR · 4 · FS
⎝ IO
⎠
IRMS = IO ·
VO ⎛
V ⎞
· 1- O =
VIN ⎝
VIN ⎠
The output capacitor limits the output ripple and provides holdup during large load transitions. A 4.7μF to
10μF X5R or X7R ceramic capacitor typically provides
sufficient bulk capacitance to stabilize the output during
large load transitions and has the ESR and ESL characteristics necessary for low output ripple. For enhanced
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PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
transient response and low temperature operation application, a 10μF (X5R, X7R) ceramic capacitor is recommended to stabilize extreme pulsed load conditions.
The output voltage droop due to a load transient is dominated 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 voltage 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 equation 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 4.7μF. This is due to
its effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capacitance will reduce the crossover frequency with greater
phase margin.
The maximum output capacitor RMS ripple current is
given by:
IRMS(MAX) =
1
VOUT · (VIN(MAX) - VOUT)
L · FS · VIN(MAX)
2· 3
·
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than
a few degrees rise in hot-spot temperature.
With enhanced transient response for extreme pulsed
load application, an external feed-forward capacitor, (C3
in Figure 1), can be added.
VOUT (V)
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ
R1 (kΩ)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 2: Adjustable Resistor Values for
Step-Down Converter.
Thermal Calculations
There are three types of losses associated with the
AAT2120 step-down converter: switching losses, conduction 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 conduction
mode (CCM), a simplified form of the losses is given by:
PTOTAL =
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])
VIN
+ (tsw · FS · IO + IQ) · VIN
Adjustable Output Resistor Selection
Resistors R1 and R2 of Figure 1 program the output to
regulate at a voltage higher than 0.6V. To limit the bias
current required for the external feedback resistor string
while maintaining good noise immunity, the suggested
value for R2 is 59kΩ. Decreased resistor values are necessary to maintain noise immunity on the FB pin, resulting in increased quiescent current. Table 2 summarizes
the resistor values for various output voltages.
⎛ VOUT ⎞
⎛ 3.3V ⎞
R1 = V
-1 · R2 = 0.6V - 1 · 59kΩ = 267kΩ
⎝ REF ⎠
⎝
⎠
12
IQ is the step-down converter quiescent current. The
term tsw is used to estimate the full load step-down converter switching losses.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
PTOTAL = IO2 · RDSON(H) + IQ · VIN
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be investigated over the complete input voltage range.
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
3.
Given the total losses, the maximum junction temperature can be derived from the θJA for the STDFN22-8
package which is 50°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
Layout
The suggested PCB layout for the AAT2120 in an
STDFN22-8 package is shown in Figures 2, 3, and 4. The
following guidelines should be used to help ensure a
proper layout.
1.
2.
4.
The input capacitor (C1) should connect as closely
as possible to VP (Pin 1), PGND (Pin 8), and GND
(Pin 3)
C2 and L1 should be connected as closely as possible. The connection of L1 to the LX pin (Pin 7) should
be as short as possible. Do not make the node small
by using narrow trace. The trace should be kept
wide, direct and short.
5.
The feedback pin (Pin 4) should be separate from
any power trace and connect as closely as possible
to the load point. Sensing along a high-current load
trace will degrade DC load regulation. Feedback
resistors should be placed as closely as possible to
the FB pin (Pin 4) to minimize the length of the high
impedance feedback trace. If possible, they should
also be placed away from the LX (switching node)
and inductor to improve noise immunity.
The resistance of the trace from the load return to
PGND (Pin 8) and GND (Pin 3) should be kept to a
minimum. This will help to minimize any error in DC
regulation due to differences in the potential of the
internal signal ground and the power ground.
A high density, small footprint layout can be achieved
using an inexpensive, miniature, non-shielded, high
DCR inductor.
U1
1
VIN
2
3
4
C1
4.7μF
VP
PGND
VIN
LX
GND
EN
FB
N/C
8
7
LX
L1
+VOUT
6
5
AAT2120
C2
4.7μF
R1
Adj.
C3
(optional)
100pF
R2
59kΩ
GND
GND
Figure 1: AAT2120 Schematic.
2120.2008.02.1.2
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13
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Figure 2: AAT2120 Evaluation Board
Top Side Layout.
Figure 3: Exploded View of AAT2120
Evaluation Board Top Side Layout.
Figure 4: AAT2120 Evaluation Board
Bottom Side Layout.
14
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Step-Down Converter Design Example
Specifications
VO
VIN
FS
TAMB
= 1.8V @ 250mA, Pulsed Load ∆ILOAD = 200mA
= 2.7V to 4.2V (3.6V nominal)
= 1.8MHz
= 85°C
1.8V Output Inductor
L1 = 1.67
µs
µs
⋅ VO2 = 1.67
⋅ 1.8V = 3µH (use 3.0μH; see Table 1)
A
A
For Sumida inductor CDRH2D09-3R0, 3.0μH, DCR = 150mΩ.
⎛
VO
1.8V
1.8V⎞
⎛ V ⎞
⋅ 1- O =
⋅ 1= 190mA
L1 ⋅ FS ⎝ VIN⎠ 3.0µH ⋅ 1.8MHz ⎝
4.2V⎠
ΔIL1 =
IPKL1 = IO +
ΔIL1
= 250mA + 95mA = 345mA
2
PL1 = IO2 ⋅ DCR = 250mA2 ⋅ 150mΩ = 9.375mW
1.8V Output Capacitor
VDROOP = 0.1V
COUT =
3 · ΔILOAD
3 · 0.2A
=
= 3.3µF (use 3.3µF)
VDROOP · FS
0.1V · 1.8MHz
IRMS =
(VO) · (VIN(MAX) - VO)
1
1.8V · (4.2V - 1.8V)
·
= 66mArms
=
L1 · FS · VIN(MAX)
2 · 3 3.0µH · 1.8MHz · 4.2V
2· 3
1
·
Pesr = esr · IRMS2 = 5mΩ · (66mA)2 = 21.8µW
Input Capacitor
Input Ripple VPP = 25mV
CIN =
IRMS =
⎛ VPP
⎝ IO
1
1
=
= 1.16µF (use 4.7µF)
⎞
⎛ 25mV
⎞
- 5mΩ · 4 · 1.8MHz
- ESR · 4 · FS
⎠
⎝ 0.2A
⎠
IO
= 0.1Arms
2
P = esr · IRMS2 = 5mΩ · (0.1A)2 = 0.05mW
2120.2008.02.1.2
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15
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
AAT2120 Losses
PTOTAL =
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN -VO])
VIN
+ (tsw · FS · IO + IQ) · VIN
=
0.22 · (0.59Ω · 1.8V + 0.42Ω · [4.2V - 1.8V])
4.2V
+ (5ns · 1.8MHz · 0.2A + 45µA) · 4.2V = 19.71mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 26.14mW = 86°C
16
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Output Voltage
VOUT (V)
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ1
R1 (kΩ)
L1 (μH)
0.62
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
—
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
—
75
113
150
187
221
261
301
332
442
464
523
715
1000
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.2
2.7
3.0/3.3
3.0/3.3
3.0/3.3
3.9/4.2
5.6
Table 3: Evaluation Board Component Values.
Manufacturer
Part Number
Inductance
(μH)
Max DC
Current (mA)
DCR
(mΩ)
Size (mm)
LxWxH
Type
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Taiyo Yuden
Taiyo Yuden
Taiyo Yuden
Taiyo Yuden
FDK
FDK
FDK
FDK
CDRH2D14-1R5
CDRH2D14-2R2
CDRH2D14-2R7
CDRH2D14-3R3
CDRH2D14-3R9
CDRH2D14-4R7
CDRH2D14-5R6
CDRH2D11-1R5
CDRH2D11-2R2
CDRH2D11-3R3
CDRH2D11-4R7
NR3010
NR3010
NR3010
NR3010
MIPWT3226D-1R5
MIPWT3226D-2R2
MIPWT3226D-3R0
MIPWT3226D-4R2
1.5
2.2
2.7
3.3
3.9
4.7
5.6
1.5
2.2
3.3
4.7
1.5
2.2
3.3
4.7
1.5
2.2
3
4.2
1800
1500
1350
1200
1100
1000
950
900
780
600
500
1200
1100
870
750
1200
1100
1000
900
50
75
85
100
110
135
150
54
78
98
135
80
95
140
190
90
100
120
140
3.0x3.0x1.55
3.0x3.0x1.55
3.0x3.0x1.55
3.0x3.0x1.55
3.0x3.0x1.55
3.0x3.0x1.55
3.0x3.0x1.55
3.2x3.2x1.2
3.2x3.2x1.2
3.2x3.2x1.2
3.2x3.2x1.2
3.0x3.0x1.0
3.0x3.0x1.0
3.0x3.0x1.0
3.0x3.0x1.0
3.2x2.6x0.8
3.2x2.6x0.8
3.2x2.6x0.8
3.2x2.6x0.8
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Chip shielded
Chip shielded
Chip shielded
Chip shielded
Table 4: Suggested Inductors and Suppliers.
Manufacturer
Part Number
Value (μF)
Voltage Rating
Temp. Co.
Case Size
Murata
Murata
GRM118R60J475KE19B
GRM188R60J106ME47D
4.7
10
6.3
6.3
X5R
X5R
0603
0603
Table 5: Surface Mount Capacitors.
1. For reduced quiescent current, R2 = 221kΩ.
2. R2 is opened, R1 is shorted.
2120.2008.02.1.2
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PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Ordering Information
Output Voltage
Package
Marking1
Part Number (Tape and Reel)2
0.6V
STDFN22-8
YDXYY
AAT2120IES-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
http://www.analogictech.com/about/quality.aspx.
1. XYY = assembly and date code.
2. Sample stock is generally held on all part numbers listed in BOLD.
18
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2120.2008.02.1.2
PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
Package Information1
STDFN22-8
Index Area
(D/2 x E/2)
0.80 ± 0.05
Detail "A"
1.45 ± 0.05
2.00 ± 0.05
2.00 ± 0.05
Top View
Bottom View
Side View
Pin 1 Indicator
(optional)
0.45 ± 0.05
0.23 ± 0.05
0.05 ± 0.05
0.15 ± 0.025
0.55 ± 0.05
0.35 ± 0.05
Detail "A"
All dimensions in millimeters.
1. 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.
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Phone (408) 737-4600
Fax (408) 737-4611
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