Analogic AAT1121 1.5mhz, 250ma step-down converter Datasheet

AAT1121
1.5MHz, 250mA Step-Down Converter
General Description
Features
The AAT1121 SwitchReg is a 1.5MHz 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 AAT1121 the ideal choice for portable
applications.
•
•
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•
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The AAT1121 delivers 250mA of load current, while
maintaining a low 30µA no load quiescent current.
The 1.5MHz switching frequency minimizes the size
of external components, while keeping switching
losses low. The AAT1121 feedback and control
delivers excellent load regulation and transient
response with a small output inductor and capacitor.
•
•
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The AAT1121 is available in a Pb-free, 8-pin,
2x2mm TDFN or STDFN package and is rated
over the -40°C to +85°C temperature range.
•
SwitchReg™
VIN Range: 2.7V to 5.5V
VOUT Range: 0.6V to VIN
250mA Max Output Current
Up to 96% Efficiency
30µA Typical Quiescent Current
1.5MHz 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 TDFN22-8 or STDFN22-8
Package
Temperature Range: -40°C to +85°C
Applications
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Bluetooth™ Headsets
Cellular Phones
Digital Cameras
Handheld Instruments
Portable Music Players
USB Devices
Typical Application
VIN
VO = 1.8V
AAT1121
VP
LX
VIN
C1
4.7µF
EN
GND
1121.2007.03.1.2
FB
PGND
250mA
L1
3.0μH
R1
118kΩ
R2
59kΩ
C2
4.7µF
1
AAT1121
1.5MHz, 250mA Step-Down Converter
Pin Descriptions
Pin #
Symbol
1
VP
Input power pin; connected to the source of the P-channel MOSFET.
Connect to the input capacitor.
2
VIN
Input bias voltage for the converter.
3
GND
Non-power signal ground pin.
4
FB
Feedback input pin. Connect this pin to an external resistive divider for
adjustable output.
5
N/C
No connect.
6
EN
Enable pin. A logic high enables normal operation. A logic low shuts down
the converter.
7
LX
Switching node. Connect the inductor to this pin. It is connected internally to
the drain of both high- and low-side MOSFETs.
8
PGND
Input power return pin; connected to the source of the N-channel MOSFET.
Connect to the output and input capacitor return.
EP
Function
Exposed paddle (bottom): connect to ground directly beneath the package.
Pin Configuration
TDFN22-8/STDFN22-8
(Top View)
VP
VIN
GND
FB
2
1
8
2
7
3
6
4
5
PGND
LX
EN
N/C
1121.2007.03.1.2
AAT1121
1.5MHz, 250mA 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
Thermal Resistance2
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.
1121.2007.03.1.2
3
AAT1121
1.5MHz, 250mA 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
UVLO Threshold
VOUT
Output Voltage Tolerance2
VOUT
IQ
ISHDN
ILIM
Output Voltage Range
Quiescent Current
Shutdown Current
P-Channel Current Limit
High-Side Switch On Resistance
Low-Side Switch On Resistance
LX Leakage Current
Line Regulation
Feedback Threshold Voltage Accuracy
FB Leakage Current
Oscillator Frequency
RDS(ON)H
RDS(ON)L
ILXLEAK
ΔVLinereg/ΔVIN
VFB
IFB
FOSC
TS
TSD
THYS
VEN(L)
VEN(H)
IEN
Startup Time
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
Enable Threshold Low
Enable Threshold High
Input Low Current
Conditions
Min
Typ
2.7
VIN Rising
Hysteresis
VIN Falling
IOUT = 0 to 250mA,
VIN = 2.7V to 5.5V
Max
Units
5.5
2.6
V
V
mV
V
%
250
2.0
-3.0
3.0
0.6
No Load
EN = GND
VIN
1.5
V
µA
µA
mA
Ω
Ω
µA
%/V
V
µA
MHz
100
µs
140
15
°C
°C
V
V
µA
30
1.0
600
0.59
0.42
VIN = 5.5V, VLX = 0 to VIN
VIN = 2.7V to 5.5V
VIN = 3.6V
VOUT = 1.0V
1.0
0.591
From Enable to Output
Regulation
0.2
0.600
0.609
0.2
0.6
VIN = VEN = 5.5V
1.4
-1.0
1.0
1. The AAT1121 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
1121.2007.03.1.2
1121.2006.10.1.2
AAT1121
1.5MHz, 250mA Step-Down Converter
Typical Characteristics
Efficiency vs. Load
DC Load Regulation
(VOUT = 1.2V; L = 1.5µH)
(VOUT = 1.2V; L = 1.5µH)
100
1.0
Efficiency (%)
Output Error (%)
VIN = 2.7V
90
VIN = 3.6V
80
70
VIN = 4.2V
60
VIN = 5.0V
50
0.5
VIN = 3.6V
0.0
VIN = 2.7V
-0.5
VIN = 4.2V
40
30
0.1
1
10
100
-1.0
1000
0.1
1
Output Current (mA)
(VOUT = 1.8V; L = 3.3µH)
100
1.0
VIN = 2.7V
VIN = 3.6V
Output Error (%)
Efficiency (%)
90
80
70
VIN = 4.2V
60
50
40
0.1
1
10
100
0.5
VIN = 3.6V
0.0
VIN = 2.7V
-0.5
-1.0
0.1
1000
VIN = 4.2V
1
Output Current (mA)
10
100
1000
Output Current (mA)
Efficiency vs. Load
DC Load Regulation
(VOUT = 3.0V; L = 4.7µH)
(VOUT = 3.0V; L = 4.7µH)
100
1.0
VIN = 3.6V
Output Error (%)
Efficiency (%)
1000
DC Load Regulation
(VOUT = 1.8V; L = 3.3µH)
VIN = 4.2V
80
70
VIN = 5.0V
60
50
40
100
Output Current (mA)
Efficiency vs. Load
90
10
0.5
VIN = 4.2V
VIN = 3.6V
0.0
VIN = 5.0V
-0.5
-1.0
0.1
1
10
Output Current (mA)
1121.2007.03.1.2
100
1000
0.1
1
10
100
1000
Output Current (mA)
5
AAT1121
1.5MHz, 250mA Step-Down Converter
Typical Characteristics
Soft Start
Line Regulation
(VOUT = 1.8V)
5.0
0.6
0.5
VEN
IOUT = 0mA
0.4
3.0
2.0
1.0
0.8
0.0
0.6
VO
0.4
0.2
0.0
IL
Accuracy (%)
4.0
Inductor Current
(bottom) (A)
Enable and Output Voltage
(top) (V)
(VIN = 3.6V; VOUT = 1.8V;
IOUT = 250mA; CFF = 100pF)
0.3
IOUT = 50mA
0.2
IOUT = 150mA
0.1
0.0
-0.1
IOUT = 10mA
IOUT = 250mA
-0.2
-0.3
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Time (100µs/div)
Input Voltage (V)
Output Voltage Error vs. Temperature
Switching Frequency Variation
vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; IOUT = 250mA)
(VIN = 3.6V; VOUT = 1.8V)
3.0
2.0
8.0
Variation (%)
Output Error (%)
10.0
1.0
0.0
-1.0
6.0
4.0
2.0
0.0
-2.0
-4.0
-6.0
-2.0
-8.0
-3.0
-40
-10.0
-20
0
20
40
60
80
100
-40
-20
0
Temperature (°°C)
80
100
50
VOUT = 1.8V
1.0
Supply Current (µA)
Frequency Variation (%)
60
No Load Quiescent Current vs. Input Voltage
2.0
0.0
-1.0
-2.0
VOUT = 3.0V
-3.0
2.7
3.1
3.5
3.9
4.3
Input Voltage (V)
6
40
Temperature (°°C)
Frequency Variation vs. Input Voltage
-4.0
20
4.7
5.1
5.5
45
40
35
85°C
30
25°C
25
-40°C
20
15
10
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
1121.2007.03.1.2
AAT1121
1.5MHz, 250mA Step-Down Converter
Typical Characteristics
P-Channel RDS(ON) vs. Input Voltage
N-Channel RDS(ON) vs. Input Voltage
750
1000
120°C
700
100°C
700
600
25°C
500
85°C
550
500
450
25°C
350
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
300
2.5
3.0
Input Voltage (V)
3.5
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
Load Transient Response
Load Transient Response
(10mA to 250mA; VIN = 3.6V; VOUT = 1.8V;
COUT = 4.7µF; CFF = 100pF)
(10mA to 250mA; VIN = 3.6V; VOUT = 1.8V; COUT = 4.7µF)
1.7
IO
250mA
1.6
ILX
10mA
0.2
0.0
-0.2
Time (25µs/div)
1.9
Output Voltage
(top) (V)
VO
1.8
2.0
1.8
VO
1.7
1.6
250mA
IO
10mA
ILX
0.2
0.0
-0.2
Load and Inductor Current
(bottom) (200mA/div)
1.9
Load and Inductor Current
(bottom) (200mA/div)
2.0
Output Voltage
(top) (V)
100°C
600
400
400
300
120°C
650
85°C
800
RDS(ON)L (mΩ
Ω)
RDS(ON)H (mΩ
Ω)
900
Time (25µs/div)
Line Response
(VOUT = 1.8V @ 250mA; CFF = 100pF)
1.90
1.80
VO
1.75
1.70
5.0
4.5
VIN
4.0
Input Voltage
(bottom) (V)
Output Voltage
(top) (V)
1.85
3.5
3.0
Time (25µs/div)
1121.2007.03.1.2
7
AAT1121
1.5MHz, 250mA Step-Down Converter
Typical Characteristics
Output Ripple
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA)
(VIN = 3.6V; VOUT = 1.8V; IOUT = 250mA)
VO
-20
0.04
0.03
0.02
0.01
IL
0.00
20
0
VO
-20
0.3
0.2
IL
0.1
0.0
-0.01
Time (2µs/div)
8
Inductor Current
(bottom) (A)
0
40
Output Voltage
(AC Coupled) (top) (mV)
20
Inductor Current
(bottom) (A)
Output Voltage
(AC Coupled) (top) (mV)
40
Time (200ns/div)
1121.2007.03.1.2
AAT1121
1.5MHz, 250mA Step-Down Converter
Functional Block Diagram
FB
VP
VIN
Err
Amp
DH
Voltage
Reference
LX
Logic
EN
INPUT
DL
PGND
GND
Functional Description
The AAT1121 is a high performance 250mA,
1.5MHz monolithic step-down converter designed
to operate with an input voltage range of 2.7V to
5.5V. The converter operates at 1.5MHz, 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
250mA current capability while maintaining over
1121.2007.03.1.2
90% efficiency at full load. Light load efficiency is
maintained at greater than 80% down to 1mA of
load current.
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.
9
AAT1121
1.5MHz, 250mA Step-Down Converter
Control Loop
The AAT1121 is a 250mA current mode step-down
converter. 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 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
AAT1121 into a low-power, non-switching state. The
total input current during shutdown is less than 1µA.
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 140°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
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
AAT1121 is 0.45A/µsec. 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=
0.75 ⋅ VO 0.75 ⋅ 1.8V
A
=
= 0.45
L
3.0µH
µsec
This is the internal slope compensation for the
AAT1121. When externally programming to 3.0V,
the calculated inductance is 5.0µH.
L=
0.75 ⋅ VO
=
m
= 1.67
0.75 ⋅ VO
µsec
≈ 1.67 A ⋅ VO
A
0.45A µsec
µsec
⋅ 3.0V = 5.0µH
A
In this case, a standard 4.7µH value is selected.
For most designs, the AAT1121 operates with an
inductor value of 1µH to 4.7µH. Table 1 displays
inductor values for the AAT1121 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.
1121.2007.03.1.2
AAT1121
1.5MHz, 250mA 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
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.
VO ⎛
V ⎞
· 1- O =
VIN ⎝
VIN ⎠
D · (1 - D) =
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 =
VO ⎛
V ⎞
· 1- O
VIN ⎝
VIN ⎠
⎛ VPP
⎞
- ESR · FS
⎝ IO
⎠
VO ⎛
V ⎞
1
· 1 - O = for VIN = 2 × VO
VIN ⎝
VIN ⎠
4
CIN(MIN) =
1
⎛ VPP
⎞
- ESR · 4 · FS
⎝ IO
⎠
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:
IRMS = IO ·
1121.2007.03.1.2
VO ⎛
V ⎞
· 1- O
VIN ⎝
VIN ⎠
1
2
for VIN = 2 x VO
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.
0.52 =
IRMS(MAX) =
VO
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
AAT1121. 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.
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.
11
AAT1121
1.5MHz, 250mA Step-Down Converter
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
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 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 · F · 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.
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 ⎠
⎝
⎠
With enhanced transient response for extreme
pulsed load application, an external feed-forward
capacitor, (C3 in Figure 1), can be added.
R2 = 59kΩ
R2 = 221kΩ
VOUT (V)
R1 (kΩ)
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.
12
1121.2007.03.1.2
AAT1121
1.5MHz, 250mA Step-Down Converter
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dissipation reduces to:
Thermal Calculations
There are three types of losses associated with
the AAT1121 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 =
2
O
I
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.
Given the total losses, the maximum junction temperature can be derived from the θJA for the
TDFN22-8 package which is 50°C/W.
· (RDSON(H) · VO + RDSON(L) · [VIN - VO])
VIN
TJ(MAX) = PTOTAL · ΘJA + TAMB
+ (tsw · F · IO + IQ) · VIN
IQ is the step-down converter quiescent current.
The term tsw is used to estimate the full load stepdown converter switching losses.
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
AAT1121
C2
4.7μF
R1
Adj.
C3
(optional)
100pF
R2
59kΩ
GND
GND
Figure 1: AAT1121 Schematic.
1121.2007.03.1.2
13
AAT1121
1.5MHz, 250mA Step-Down Converter
Layout
The suggested PCB layout for the AAT1121 is
shown in Figures 2, 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 (Pin 1), PGND (Pin 8),
and GND (Pin 3)
2. C2 and L1 should be connected as closely as
possible. The connection of L1 to the LX pin
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.
3. 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
Figure 2: AAT1121 Evaluation Board
Top Side Layout.
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.
4. 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.
5. A high density, small footprint layout can be
achieved using an inexpensive, miniature, nonshielded, high DCR inductor.
Figure 3: Exploded View of AAT1121
Evaluation Board Top Side Layout.
Figure 4: AAT1121 Evaluation Board
Bottom Side Layout.
14
1121.2007.03.1.2
AAT1121
1.5MHz, 250mA Step-Down Converter
Step-Down Converter Design Example
Specifications
VO
= 1.8V @ 250mA, Pulsed Load ΔILOAD = 200mA
VIN
= 2.7V to 4.2V (3.6V nominal)
FS
= 1.5MHz
TAMB
= 85°C
1.8V Output Inductor
L1 = 1.67
µsec
µsec
⋅ VO2 = 1.67
⋅ 1.8V = 3µH
A
A
(use 3.0µH; see Table 1)
For Sumida inductor CDRH2D09-3R0, 3.0µH, DCR = 150mΩ.
ΔIL1 =
⎛
VO
V ⎞
1.8V
1.8V ⎞
⎛
⋅ 1- O =
⋅ 1= 228mA
L1 ⋅ F ⎝
VIN ⎠ 3.0µH ⋅ 1.5MHz ⎝
4.2V ⎠
IPKL1 = IO +
ΔIL1
= 250mA + 114mA = 364mA
2
PL1 = IO2 ⋅ DCR = 250mA2 ⋅ 150mΩ = 9.375mW
1.8V Output Capacitor
VDROOP = 0.1V
COUT =
3 · ΔILOAD
3 · 0.2A
=
= 4µF (use 4.7µF)
0.1V · 1.5MHz
VDROOP · FS
IRMS =
(VO) · (VIN(MAX) - VO)
1
1.8V · (4.2V - 1.8V)
·
= 66mArms
=
3.0µH
· 1.5MHz · 4.2V
·
V
L1
·
F
2· 3
2· 3
S
IN(MAX)
1
·
Pesr = esr · IRMS2 = 5mΩ · (66mA)2 = 21.8µW
1121.2007.03.1.2
15
AAT1121
1.5MHz, 250mA Step-Down Converter
Input Capacitor
Input Ripple VPP = 25mV
CIN =
IRMS =
1
⎛ VPP
⎞
- ESR · 4 · FS
⎝ IO
⎠
=
1
= 1.38µF (use 4.7µF)
⎛ 25mV
⎞
- 5mΩ · 4 · 1.5MHz
⎝ 0.2A
⎠
IO
= 0.1Arms
2
P = esr · IRMS2 = 5mΩ · (0.1A)2 = 0.05mW
AAT1121 Losses
PTOTAL =
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN -VO])
VIN
+ (tsw · F · IO + IQ) · VIN
=
0.22 · (0.59Ω · 1.8V + 0.42Ω · [4.2V - 1.8V])
4.2V
+ (5ns · 1.5MHz · 0.2A + 30µA) · 4.2V = 26.14mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 26.14mW = 86.3°C
16
1121.2007.03.1.2
AAT1121
1.5MHz, 250mA Step-Down Converter
Output Voltage
VOUT (V)
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ1
R1 (kΩ)
L1 (µH)
—
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
2
0.6
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
Table 3: Evaluation Board Component Values.
Manufacturer
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Taiyo Yuden
Taiyo Yuden
Taiyo Yuden
Taiyo Yuden
FDK
FDK
FDK
FDK
Part Number
Inductance
(µH)
Max DC
Current (mA)
DCR
(mΩ)
Size (mm)
LxWxH
Type
CDRH2D09-1R5
CDRH2D09-2R2
CDRH2D09-2R5
CDRH2D09-3R0
CDRH2D09-3R9
CDRH2D09-4R7
CDRH2D09-5R6
CDRH2D11-1R5
CDRH2D11-2R2
CDRH2D11-3R3
CDRH2D11-4R7
NR3010
NR3010
NR3010
NR3010
MIPWT3226D-1R5
MIPWT3226D-2R2
MIPWT3226D-3R0
MIPWT3226D-4R2
1.5
2.2
2.5
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
730
600
530
470
450
410
370
900
780
600
500
1200
1100
870
750
1200
1100
1000
900
88
115
135
150
180
230
260
54
78
98
135
80
95
140
190
90
100
120
140
3.0x3.0x1.0
3.0x3.0x1.0
3.0x3.0x1.0
3.0x3.0x1.0
3.0x3.0x1.0
3.0x3.0x1.0
3.0x3.0x1.0
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.
1. For reduced quiescent current, R2 = 221kΩ.
2. R2 is opened, R1 is shorted.
1121.2007.03.1.2
17
AAT1121
1.5MHz, 250mA Step-Down Converter
Manufacturer
Murata
Murata
Part Number
Value
(µF)
Voltage
Rating
Temp.
Co.
Case
Size
GRM118R60J475KE19B
GRM188R60J106ME47D
4.7
10
6.3
6.3
X5R
X5R
0603
0603
Table 5: Surface Mount Capacitors.
18
1121.2007.03.1.2
AAT1121
1.5MHz, 250mA Step-Down Converter
Ordering Information
Output Voltage
Package
Marking1
Part Number (Tape and Reel)2
0.6V
0.6V
TDFN22-8
STDFN22-8
RWXYY
RWXYY
AAT1121IPS-0.6-T1
AAT1121IES-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/pbfree.
Package Information3
TDFN22-8
0.600 ± 0.050
0.1 REF
(optional)
0.450 ± 0.050
C0.3
Bottom View
2.000 ± 0.050
0.350 ± 0.100
Detail "A"
1.270 ± 0.050
2.000 ± 0.050
Index Area
Pin 1 Indicator
(optional)
0.230 ± 0.050
Top View
0.050 ± 0.050
0.229 ± 0.051
0.850 MAX
4x
Detail "A"
Side View
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on all 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.
1121.2007.03.1.2
19
AAT1121
1.5MHz, 250mA Step-Down Converter
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.
© Advanced Analogic Technologies, Inc.
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rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. Except as provided in AnalogicTech’s terms and conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent,
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20
1121.2007.03.1.2